Multi-station token pass communication system

• US 4,491,946 A
• Filed: 03/09/1981
• Issued: 01/01/1985
• Est. Priority Date: 03/09/1981
• Status: Expired due to Term

• Current Assignee: Schneider Automation Incorporated (Schneider Electric SE)
• Original Assignee: Gould, Inc. (Transdigm Group Incorporated)
• Inventors: Allen, Bruce S., Bobzin, Jeffery J., Kryskow, J. Michael Jr.
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US06/241,688
• Time in Patent Office: 1,394 Days
• Field of Search: 370/85, 370/92, 370/95, 370/89, 370/60, 370/94, 340/825.04, 340/825.52, 340/825.5, 340/825.51, 455/31, 455/38
• US Class Current: 370/453
• CPC Class Codes: H04L 12/417 with deterministic access, e.g. token passing

Abstract

A data communication system is disclosed that utilizes a synchronized serial bus for communication between multiple stations connected to the bus. A token is employed such that when owned by a station it allows that station to transmit high level messages over the bus and command other stations to transmit high level messages to it if required by the token owner. Fundamental events called primitives are perceived by stations. Each station capable of token ownership embodies the same set of rules, the rules in turn defining the circumstances when perceived events cause a station to change from one state to another.

Fundamental sets of rules, or modules, are used in the communication system to achieve a system for the automatic generation of a token list of stations with periodic ownership of the token, as well as for expansion or contraction of this token list. In addition, automatic change in the token pass handshake protocol is described.

The communication system allows bus interconnection to stations having no potential token access, with communication control provided by master stations which have token ownership capability.

Claims

1. A method of communicating over a communication medium by a plurality of stations interconnected to the medium, comprising the steps of:

(1) generating a list of all stations wanting periodic access to the medium (token list);

(2) each station in the token list capable of sending access type messages for receiving and transferring access to the medium between itself and other stations interconnected to the medium and non-access type messages for transferring all other types of information to stations interconnected to the medium;

(3) each station in the token list knowing from which station (its "FROM" station) it is to receive access transfer enable information and access control information (token) to allow it to obtain access to the medium, as well as to which station (its "TO" station) it is to next pass access transfer enable information and access control information after it has received access to the medium;

(4) each station in the token list, within a first predetermined length of time after it has received access to the medium transmitting non-access messages on the medium or passing access transfer enable information to its TO station, and within a maximum non-access message predetermined length of time after it has received access to the medium, passing access transfer enable information to its TO station;

(5) the TO station, within a second predetermined length of time, acknowledging receipt of the access transfer enable information to the station from which it received the access transfer enable information;

(6) the station passing the access transfer enable information listening to the medium to ascertain if its TO station has sent acknowledge receipt information of the access transfer enable information within the second predetermined length of time, and if acknowledge receipt information has been received, sending access control information to its TO station; and

if the station passing access transfer enable information does not receive from its TO station such acknowledge receipt information within the second predetermined length of time, it resends N number of times (where N is an integer equal to or greater than

     0) access transfer enable information to its TO station; and

(7) if the station passing access transfer enable informationm has not received acknowledge receipt information from its TO station after resending access transfer enable information N times, it then sends a broadcast message to all stations interconnected to the medium requesting identification of the station which is the present station'"'"'s TO station'"'"'s TO station, and upon ascertaining its TO station'"'"'s TO station sending access transfer enable information to that new TO station.

2. A method of communicating over a communication medium as defined in claim 1 further comprising the step of patching out a station from the token list which does not acknowledge receipt of access transfer enable information.

3. A method of communication as defined in claim 2 wherein each station in the list listens to the medium to ascertain when its FROM station has access to the medium so that when this condition occurs and if it determines that its FROM station does not transmit on the medium within the first predetermined length of time, initiating and obtaining its own access to the medium and patching out its FROM station from the token list.

4. A method of communication as defined in claim 1 wherein each station in the list listens to the medium to ascertain when its FROM station has access to the medium so that when this condition occurs and if it determines that its FROM station does not transmit on the medium within the first predetermined length of time, initiating and obtaining its own access to the medium.

5. A method of communicating over a communication medium as defined in claim 4, further comprising the step of each station in the list ascertaining the number of times that it has passed the token to its TO station in an error-free manner and if the number of such passes exceeds a predetermined number, eliminating the token enable information and token acknowledge information for the transfer of the token for that station and only transferring the token to its TO station.

6. A method of communicating over a communication medium as defined in claim 1, wherein the station that passes the token listens to the medium and ascertains if its TO station is transmitting on the medium within a third predetermined length of time, and if its TO station is not so using the medium as perceived by the station that passed the token, then that station reattempts M number of times (where M is an integer equal to or greater than 0) to pass access transfer enable information to its TO station;

and further wherein if the TO station does not acknowledge receipt of this access transfer enable information within the second predetermined length of time or does not transmit on the medium within the third predetermined length of time, sending a broadcast message to all stations asking for identification of the station which is the present station'"'"'s TO station'"'"'s TO station and upon ascertaining its TO station'"'"'s TO station, passing access transfer enable information and access control information to that station in a manner as set forth in steps 3-6 of claim 5.

7. A method of communicating over a medium as defined in claim 6 further comprising the step of patching out a station from the token list which has been skipped over by its FROM station.

8. A method of communicating over a communication medium as defined in claim 1 wherein the step of generating a list of stations wanting periodic access to the medium comprises the following substeps:

(a) each station upon power up, if a token list does not presently exist, entering a state (BUS IS DEAD) for initial generation of the token list;

(b) each station which has entered the BUS IS DEAD state waiting an amount of time proportional to a unique characteristic of the station (BUS REBUILD TIMEOUT

     68), which, for the first station to so timeout, enters a search state (PASSING INITIALIZATION TOKEN state) for determining the next station to enter the search state and if such a station is found, for itself to enter the token list state (IN LIST state) as the first station in that list;

(c) each station enters the PASSING INITIALIZATION TOKEN state in turn entering the token list after determining the next station to enter the PASSING INITIALIZATION TOKEN state; and

(d) continuing step (c) until the next station that enters the PASSING INITIALIZATION TOKEN state is the first station that entered the token list, at which time the initial token list is completed.

9. A method of communicating over a communication medium as defined in claim 8 wherein each station has an address and wherein the unique characteristic of each station for determining its bus rebuild timeout period is the address of the station.

10. A method of communicating over a communication medium as defined in claim 9 wherein

(a) each station on the medium which is active enters a BUS IS DEAD state upon not seeing intelligible information on the medium for a fourth predetermined length of time;

each station which enters the BUS IS DEAD state waiting an amount of time proportional to its address, with the first station to have its time period expire (BUS REBUILD TIMEOUT event

     68) entering the PASSING INITIALIZATION TOKEN state for determining the next station to enter this state and if such a station is found, for itself to enter a token list (IN LIST state) as the first station in the list;

(b) each station that enters the (PASSING INITIALIZATION TOKEN state) in turn entering the token list after determining the next station to enter the PASSING INITIALIZATION TOKEN state; and

(c) continuing substep (b) until the first station to enter the token list is the next station that would enter the PASSING INITIALIZATION TOKEN state, at which time a token list is completed.

11. A method of communicating over a communication medium as defined in claim 10 further wherein some of the stations that interface with the medium cannot enter the token list and wherein all communications by these stations are in response to messages received from stations which have access to the medium.

12. A method of communicating over a communication medium as defined in claim 10 wherein each station in the PASSING INITIALIZATION TOKEN state, if unsuccessful in its search for the next station to receive the initialization token, enters an ONLY ONE state where it remains until any type message activity is seen on the medium or the station receives the initialization token.

13. A method of communicating over a communication medium as defined in claim 12 further wherein a station in the ONLY ONE state if it receives an initialization token from another station which is conducting an initialization search, becomes the station which continues the initialization search and thus is no longer in the ONLY ONE state.

14. A method of communicating over a communication medium as defined in claim 13 wherein a station in the ONLY ONE state enters an OUT OF LIST state if it receives a message from the medium.

15. A method of communicating over a communication medium as defined in claim 12 wherein a station in the ONLY ONE state enters an OUT OF LIST state if it receives a message from the medium.

16. A method of communicating over a communication medium as defined in claim 1 wherein the TO station after receipt of access control information provides a demand window for a fifth predetermined length of time so as to allow stations (demanders) having addresses between the present token owner'"'"'s address and its FROM station'"'"'s address to request entry into the token list, and if such a request is received during the demand window time period, passing the token to the demanding station.

17. A method of communicating over a communication medium as defined in claim 16 wherein the station which demands access to the medium informs the FROM station of the station that passed the token, to change the address of its TO station with its (the demander'"'"'s) address;

whereby the demanding station is patched into the token list and need not demand the token any longer.

18. A method of communicating over a communications medium as defined in claim 17 wherein the station which demands the token, upon receipt of the token initiates a demand window whereby any station having an address between the previous FROM station and the present demand station can demand access to the medium, and if such a demand station is perceived, the station is patched into the token list.

19. A method of communicating over a communications medium as defined in claim 18, wherein each station has an address and it two or more stations demand access to the medium during the same demand window, the present token owner station searches backward for the highest addressed demanding station and passes the token to that station and wherein that station in turn has a demand window which if it perceives multiple demanders, passes the token backward to the highest addressed demander, and wherein this process is continued until the lowest addressed demander is located, which demander then patches itself into the token list by requesting that the first station'"'"'s FROM station change the address of its TO station to the present token owner station.

20. A method of communicating over a communications medium as defined in claim 1 wherein each station in the token list if it does not perceive either passing of the token from one station to another or non-access messages on the medium within a sixth predetermined length of time, enters the BUS IS DEAD state.

21. A method of communicating over a communication medium as defined in claim 1 further wherein some of the stations that interface with the medium cannot enter the token list and wherein all communications by these stations are in response to messages received from stations which have access to the medium.

22. A method of communicating over a communication medium as defined in claim 1 wherein the step of ascertaining a station'"'"'s TO station'"'"'s TO station encompasses the step of the TO station'"'"'s TO station listening to the medium for a message that identifies its FROM station as another station'"'"'s TO station which is not operating properly and for which the identity of its TO station is required, and wherein the TO station'"'"'s TO station in response sends a message to its FROM station'"'"'s FROM station identifying itself as the TO station'"'"'s TO station.

23. A method of communicating over a communication medium as defined in claim 22 wherein the station seeking its TO station'"'"'s TO station sends a broadcast message containing a WHO'"'"'s NEXT command and wherein the TO station'"'"'s TO station, upon receiving such a WHO'"'"'S NEXT command, sends an I'"'"'M NEXT command to its FROM station'"'"'s FROM station so as to identify itself to that station.

24. A method of initializing a first group of stations (token list) forming a perfect intersection or a subset of a second group of stations interfaced with a communication medium, the first group of stations to share periodic access to the communication medium, comprising the steps of:

(1) each station upon power up entering a state (NEW ONE) which upon expiration of a timeout (NEW ONE TIMEOUT

     66) enters a state (BUS IS DEAD) for initial generation of a list (token list) of all stations which want to have periodic access to the medium (token ownership);

(2) each station which has entered the BUS IS DEAD state waiting an amount of time proportional to a unique characteristic of the station (BUS REBUILD TIMEOUT

     68), which, for the first station to timeout, enters a search state (PASSING INITIALIZATION TOKEN) for determining the next station to enter the search state, and if such a station is found, for itself to enter the token list state (IN LIST) as the first station; and

(3) each station that enters the PASSING INITIALIZATION TOKEN state in turn entering the token list after determining the next station to enter the PASSING INITIALIZATION TOKEN state; and

(4) continuing step 3 until the next station to enter the PASSING INITIALIZATION TOKEN state, is the first station that entered the token list, at which time the initial token list is completed;

whereby an initialization of stations to have periodic access to the medium is obtained.

25. A method as defined in claim 24 wherein each station to enter the token list knows the station (FROM station) it is to receive the token as well as to which station (TO station) it is to pass the token.

26. A method as defined in claim 25 wherein each station has an address and wherein the unique characteristic of each station for determining its bus rebuild timeout period is the address of the station.

27. A method as defined in claim 26 wherein each station when in the PASSING INITIALIZATION TOKEN state, attempts to pass access transfer information to the next higher addressed station and if successful (SUCCESSFUL INITIALIZATION PASS event 70) it enters the IN LIST state and if unsuccessful (UNSUCCESSFUL SEARCH event 71) the station enters a state wherein it perceives itself as the only station having access to the medium (ONLY ONE state);

wherein each station when in the ONLY ONE state initializes a timer which upon expiration (ONLY ONE TIMEOUT event

     72) enters back to the ONLY ONE state while making evident to the medium that it is passing the access transfer information to itself so as to give notice to other stations that this station is active; and

repeating the ONLY ONE TIMEOUT event until the station receives an initialization token from a station in the PASSING INITIALIZATION TOKEN state or receives a message (MESSAGE RECEIVED event

     74) at which time it ultimately enters a state (OUT OF LIST) from which it can enter the token list.

28. A method as defined in claim 27 wherein a station entering the OUT OF LIST state enters the token list by perceiving the passing of the token from a station having a lower address than itself to a station having a higher address than itself and when this is perceived entering a state (DEMANDING) so as to demand from the station receiving the token that it is to receive the token instead.

29. A method as defined in claim 28 wherein each station which receives the token after initialization of the token list is completed, has a demand window time period within which stations having addresses greater than the station from which it has received the token or having addresses less than said passing station if said station has the highest address of all stations in the token list may demand the token and if such a demand is received, passing the token to the demanding station.

30. A method as defined in claim 29 wherein the station receiving the demand token informs the station (the FROM station) which originally sent the token to the station (the TO station) from which it received the demand token that it, the demanding station, is the new station (new TO station) to receive the token from this lower addressed station (FROM station) rather than the station (former TO station) which formerly received the token.

31. A method as defined in claim 30 wherein the station which demands the token, upon receipt of the token, initiates a demand window so that any station having an address between the present station and the FROM station can demand access to the medium and if such a demand station is perceived, that station is patched into the token list.

32. A method as defined in claim 31, wherein each station has an address and if two or more stations demand access to the medium during the same demand window, the present token owner station searches backward for the highest addressed demanding station and passes the token to that station and wherein that station in turn has a demand window and if it perceives multiple demanders, passes the token backward to the highest addressed demander, and wherein this process is continued until the demander having an address closest to the first station'"'"'s FROM station is located, which demander then patches itself into the token list by requesting that the first station'"'"'s FROM station change the address of its TO station to the present token owner station.

33. A method of communicating over a communication medium by a plurality of stations interconnected to the medium, comprising the steps of:

(1) generating a list of all stations wanting periodic access to the medium (TOKEN LIST);

(2) each station in the token list knowing from what station (its FROM station) it is to receive access transfer information to allow it to obtain access to the medium, as well as to what station (its TO station) it is to next pass access transfer information after it has received access to the medium;

(3) each station in the list listening to the medium to ascertain when its FROM station has access to the medium so that when this condition occurs and if it (the station to next receive access) determines that its FROM station does not transmit on the medium within a predetermined length of time, initiating and obtaining its own access to the medium.

34. A method as defined in claim 33 wherein each station in the list after having access to the medium in turn passing access information to its TO station and if the TO station does not appropriately respond to the transfer of access transfer information, the station which sent the access transfer information determining the TO station'"'"'s TO station so as to pass access transfer information to the TO station'"'"'s TO station.

35. A method as defined in claim 34 wherein each station which has access to the medium, if such station determines that its TO station is not appropriately responding to the transfer of access information, in addition to passing access transfer information to its TO station'"'"'s TO station further patches out its original TO station from the TOKEN LIST.

36. A method as defined in claim 35 wherein the station which determines that its FROM station is not operating properly determines its new FROM station when this new FROM station patches out its previous TO station by determining its TO station'"'"'s TO station.

37. An access method for passing access to a communication medium by a plurality of addressable stations that interface with the medium, comprising the steps of:

(1) each station if it is active, entering a BUS IS DEAD state upon not seeing intelligible information on the medium for a predetermined length of time;

(2) each station which has entered the BUS IS DEAD state waiting an amount of time proportional to its address, with the first station to have its time period expire (BUS REBUILD TIMEOUT event

     68) entering a PASSING INITIALIZATION TOKEN (search) state for determining the next station to enter this search state and if such a station is found, for itself to enter the token list (IN LIST state) as the first station in the list;

(3) each station that enters the PASSING INITIALIZATION TOKEN state in turn entering the token list after determining the next station to enter the PASSING INITIALIZATION TOKEN state; and

(4) continuing step 3 until the first station to enter the token list is the next station that would enter the PASSING INITIALIZATION TOKEN state, at which time the initial token list is completed;

whereby an initialization of stations to have periodic access to the medium is obtained upon initial start up or whenever intelligible information is not seen on the medium by a station for a predetermined length of time.

38. An access method as defined in claim 37 wherein each station when in the PASSING INITIALIZATION TOKEN state, if unsuccessful in its search for the next station to receive the initialization token, enters on ONLY ONE state where it remains until message activity is seen on the medium or the station receives the initialization token.

39. An access method as defined in claim 38 wherein each station when in the ONLY ONE state, periodically passes the token to itself so as to inform all other stations interfaced with the medium that it is active on the medium and further wherein this time period is substantially greater than the time out period for entering the BUS IS DEAD state so that a station in the ONLY ONE state does not burden the medium with token passes to itself in the event that the station in the ONLY ONE state is truly faulty.

40. An access method as defined in claim 37 wherein the first station to enter the search state sets a search stop bit which is the means by which step 4 is ended with completion of the initial token list.

48. An access method as defined in claim 40 wherein each station in the token list if it does not hear messages on the medium within a third predetermined length of time, enters a state (BUS IS DEAD) from which regeneration of a token list can be obtained.

49. An access method as defined in claim 48 further wherein each station which enters the BUS IS DEAD state waits an amount of time proportional to a unique characteristic of the station, which for the first station to time out, enters a search state for determining the next station to enter the search state and if such a station is found, for itself to enter the token list as the first station in that list;

and further wherein each station which enters the search state in turn enters the token list upon determining the next station to enter the search state, and continuing the search until the first station to enter the token list is the next station that would enter the search state;

at which time the token list is completed.

50. An access method as defined in claim 49 wherein each station has a unique address and wherein the unique characteristic for determining the first station to enter the search state is the address of the station.

58. A masterless communication system as defined in claim 50 wherein the means for generating a list of stations wanting periodic access to the medium comprises the following submeans:

(a) submeans for entering a state (BUS IS DEAD) for initial generation of the token list whenever a token list does not exist upon power up;

(b) each station which has entered the BUS IS DEAD state having means for waiting an amount of time proportional to a unique characteristic of the station (BUS REBUILD TIMEOUT

     68), which, for the first station to so timeout, has means for entering a search state (PASSING INITIALIZATION TOKEN state) for determining the next station to enter the search state and if such a station is found, for itself to enter the token list state (IN LIST state) as the first station in that list;

(c) each station that enters the PASSING INITIALIZATION TOKEN state having means for in turn entering the token list after determining the next station to enter the PASSING INITIALIZATION TOKEN state; and

(d) continuing the function recited by the submeans in element c until the next station that enters the PASSING INITIALIZATION TOKEN state is the first station that entered the token list, at which time the initial token list is completed.

59. A masterless communication system as defined in claim 58 wherein each station has a search stop bit and further wherein the first station to enter the search state has means for setting this bit, and further wherein each station has means for sensing the state of said bit in the station that it determines is to next enter the search state so as to end the initial token list upon perception of a set search stop bit in said next station.

60. A masterless communication system as defined in claim 59 wherein each station has an address and wherein the unique characteristic of each station for determining its bus rebuild timeout period is the address of the station.

61. A masterless communication system as defined in claim 60 wherein

(a) each station on the medium which is active has means for entering a BUS IS DEAD state upon not seeing intelligible information on the medium for a fourth predetermined length of time;

each station which enters the BUS IS DEAD state having means for waiting an amount of time proportional to its address, with the first station to have its time period expire (BUS REBUILD TIMEOUT event

     68) having means for entering the PASSING INITIALIZATION TOKEN state for determining the next station to enter this state and if such a station is found, for itself to enter a token list (IN LIST state) as the first station in the list;

(b) each station that enters the (PASSING INITIALIZATION TOKEN state) having means for in turn entering the token list after determining the next station to enter the PASSING INITIALIZATION TOKEN state; and

(c) continuing the function recited by the submeans in element b until the first station to enter the token list is the next station that would enter the PASSING INITIALIZATION TOKEN state, at which time a token list is completed.

62. A masterless communication system as defined in claim 61, further wherein some of the stations that connect to the medium cannot control the transmission of information on the medium and wherein all communications by these stations are in response to messages received from stations which have access to the medium.

63. A masterless communication system as defined in claim 62 wherein each station in the PASSING INITIALIZATION TOKEN state, if unsuccessful in its search for the next station to receive the initialization token, has means for entering an ONLY ONE state where it remains until any type message activity is seen on the medium or the station receives the initialization token.

64. A masterless communication system as defined in claim 63 further wherein a station in the ONLY ONE state if it receives an initialization token from another station which is conducting an initialization search, has means for becoming the station which continues the initialization search and thus is no longer in the ONLY ONE state.

65. A masterless communication system as defined in claim 64 wherein a station in the ONLY ONE state has means for entering an OUT OF LIST state if it receives a message from the medium.

66. A masterless communication system as defined in claim 65 wherein a station in the ONLY ONE state has means for entering an OUT OF LIST state if it receives a message from the medium.

41. An access method for transferring access to a communication medium by a plurality of station interfaced to the medium wherein no single station failure can incapacitate or cause the reinitialization of the remaining stations, comprising steps of:

(1) generating a list of all stations which have periodic access to the medium (TOKEN LIST);

(2) each station in the token list knowing from which station (its "FROM" station) it is to receive its token as well as to which station (its "TO" station) it is to pass the token after it has received the token;

(3) each station in the token list, when it has the token, being able to use and command use of the medium by respectively generating non-access messages or commanding other stations to generate non-access messages to the exclusion of all other stations for a maximum non-access message predetermined length of time after receipt of the token;

(4) each station having the token in turn initiating the passing of the token to its TO station within the non-access message predetermined length of time;

(5) each station in the token list listening to the medium to determine when its FROM station has been passed the token, and when the token has been passed to its FROM station, that observing station waiting a first predetermined length of time to determine if its FROM station has generated any type of message on the medium and if no message is perceived by the observing station, the observing station takes the token from its FROM station;

(6) each station (the observing station) which passes the token to its TO station listening to the medium to determine if its TO station has generated any type of message on the medium within a second predetermined length of time different from the first predetermined length of time, and if no message is perceived by the observing station from its TO station, the observing station generates a broadcast message on to the medium requesting identification of its TO station'"'"'s TO station, and upon ascertaining its TO station'"'"'s TO station sending the token to its TO station'"'"'s TO station;

whereby each station in the token list by knowing from which station it has received the token and to which station it is to pass the token eliminates the need for reinitializing the token list upon the perceived malfunctioning of any station in the token list.

42. An access method as defined in claim 41 wherein any station which is perceived as not properly using the token or failing to accept the token is patched out of the token list by the remaining stations in the token list.

43. An access method as defined in claim 42, wherein each station when it receives the token and before initiation of non-access messages, listens to the medium during a predetermined demand window time period so that any station between the present token owning station and the station which sent the token to the token owning station (the FROM station) may demand the token (demanding station) from the present token owning station and further wherein the demander station is patched into the token list.

44. An access method as defined in claim 43 wherein if two or more stations demand access to the medium during the same demand window, the present token owning station searches backward for the demanding station closest to itself and passes the token to that station and wherein that station in turn presents a demand window before it initiates non-access messages so that it can accept demanders between itself and the previous FROM station.

45. An access method as defined in claim 44 wherein each station in the token list if it does not hear at least one message on the medium within a third predetermined length of time, enters a state (BUS IS DEAD) from which regeneration of a token list can be obtained.

46. An access method as defined in claim 45 further wherein each station which enters the BUS IS DEAD state waits an amount of time proportional to a unique characteristic of the station, which for the first station to time out, enters a search state for determining the next station to enter the search state and if such a station is found, for itself to enter the token list as the first station in that list;

and further wherein each station which enters the search state in turn enters the token list upon determining the next station to enter the search state, and continuing the search until the first station to enter the token list is the next station that would enter the search state;

at which time the token list is completed.

47. An access method as defined in claim 46 wherein each station has a unique address and wherein the unique characteristic for determining the first station to enter the search state is the address of the station.

51. A masterless communication system of the type comprising three or more stations connected to a medium, wherein each station that is able to control the transmission of information on the medium comprises:

(A) means for initiating the generation of a list of all stations wanting periodic access to the medium (token list);

(B) means for sending access type messages for receiving and transferring access to the medium between itself and other stations connected to the medium and non-access type messages for transferring all other types of information to stations connected to the medium;

(C) means for knowing from which station (its "FROM" station) it is to receive access transfer enable information and access control information (token) to allow it to obtain access to the medium, as well as to which station (its "TO" station) it is to next pass access transfer enable information and access control information after it has received access to the medium;

(D) means for generating a first timeout, said means enabled after the station has received access to the medium,

(E) means for initiating the transmission of non-access messages on the medium or passing access transfer enable information to its TO station before said first timeout;

(F) means for generating a maximum non-access message timeout, said means enabled after the station has received access to the medium;

(G) means for passing access transfer enable information to its TO station within said maximum non-access message timeout;

(H) means for generating a second timeout, said means enabled whenever a station passes access transfer enable information to its TO station;

(I) means for acknowledging receipt of the access transfer enable information by the station receiving same to the station from which it was received, said acknowledgement generated within said second predetermined length of time;

(J) means for listening to the medium ascertain if its TO station has sent acknowledge receipt information of the access transfer enable information within the second predetermined length of time;

(K) means for sending access control information to its TO station if acknowledge receipt information has been received within said second timeout;

(L) means for resending up to N number of times (where N is an integer equal to or greater than

     0) access transfer enable information to its TO station if the station passing access transfer enable information does not receive from its TO station such acknowledge receipt information within the second timeout;

(M) means for sending a broadcast message to all stations interconnected to the medium requesting identification of the station which is the present station'"'"'s TO station'"'"'s TO station if the station passing access transfer enable information has not received acknowledge receipt information from its TO station after resending access transfer enable information N times; and

(N) means, responsive to ascertainment of its TO station'"'"'s TO station for sending access transfer enable information to that new TO station.

52. A masterless communication system as defined in claim 51 wherein each station that is able to control the transmission of information on the medium further comprises means for patching out a station from the token list which does not acknowledge receipt of access transfer enable information.

53. A masterless communication system as defined in claim 52 wherein each station in the token list has means for listening to the medium to ascertain when its FROM station has access to the medium so that when this condition occurs and if it determines that its FROM station does not transmit on the medium within the first predetermined length of time, initiating and obtaining its own access to the medium and patching out its FROM station from the token list.

54. A masterless communication system as defined in claim 51 wherein each station in the token list has means for listening to the medium to ascertain when its PROM station has access to the medium so that when this condition occurs and if it determines that its FROM station does not transmit on the medium within the first predetermined length of time, initiating and obtaining its own access to the medium.

55. A masterless communication system as defined in claim 54, wherein each station in the list further comprises means for ascertaining the number of times that it has passed the token to its TO station in an error-free manner and if the number of such passes exceeds a predetermined number, eliminating the token enable information and token acknowledge information for the transfer of the token for that station and only transferring the token to its TO station.

56. A masterless communication system as defined in claim 51, wherein the station that passes the token has means for listening to the medium to ascertain if its TO station is transmitting on the medium within a third predetermined length of time, and if its TO station is not so using the medium as perceived by the station that passed the token, then that station further having means for reattempting M number of times (where M is an integer equal to or greater than 0) to pass access transfer enable information to its TO station;

and further wherein if the TO station does not acknowledge receipt of this access transfer enable information within the second predetermined length of time or does not transmit on the medium within the third predetermined length of time, having means for sending a broadcast message to all stations asking for identification of the station which is the present station'"'"'s TO station'"'"'s TO station and upon ascertaining its TO station'"'"'s TO station, passing access transfer enable information and access control information to that station by the means set forth in elements G through I of claim 55.

57. A masterless communication system as defined in claim 56 wherein each station in the token list further comprises means for patching out a station from the token list which has been skipped over by its FROM station.

67. A masterless communication system as defined in claim 51 wherein the TO station after receipt of access control information has means for providing a demand window for a fifth predetermined length of time so as to allow stations (demanders) having address between the present token owner'"'"'s address and its FROM station'"'"'s address to request entry into the token list, and if such a request is received during the demand window time period, having means for passing the token to the demanding station.

68. A masterless communication system as defined in claim 67, wherein the station which demands access to the medium has means for informing the FROM station of the station that passed the token, to change the address of its TO station with its (the demander'"'"'s) address;

whereby the demanding station is patched into the token list and need not demand the token any longer.

69. A masterless communication system as defined in claim 68 wherein the station which demands the token, upon receipt of the token, has means for initiating a demand window whereby any station having an address between the previous FROM station and the present demand station can demand access to the medium, and if such a demand station is perceived, the station is patched into the token list.

70. A masterless communication system as defined in claim 69 wherein each station has an address and if two or more stations demand access to the medium during the same demand window, the present token owner station has means for searching backward for the highest addressed demanding station and passes the token to that station and wherein that station in turn has means for generating a demand window which, if it perceives multiple demanders, passes the token backward to the highest addressed demander, and wherein this backward passing of the token is continued until the lowest addressed demander is located, which demander has means for patching itself into the token list by requesting that the first station'"'"'s FROM station change the address of its TO station to the present token owner station.

71. A masterless communication system as defined in claim 51 wherein each station in the token list if it does not perceive either passing of the token from one station to another or non-access messages on the medium within a sixth predetermined length of time, has means for entering the BUS IS DEAD state.

72. A masterless communication system as defined in claim 51 wherein the means for ascertaining a station'"'"'s TO station'"'"'s TO station encompasses the TO station'"'"'s TO station having means for listening to the medium for a message that identifies its FROM station as another station'"'"'s TO station which is not operating properly and for which the identity of its TO station is required, and wherein the TO station'"'"'s TO station in response has means for sending a message to its FROM station'"'"'s FROM station to identify itself as the TO station'"'"'s TO station.

73. A masterless communication system as defined in claim 72 wherein the station seeking its TO station'"'"'s TO station has means for sending a broadcast message containing a WHO'"'"'S NEXT command and wherein the TO station'"'"'s TO station upon receiving such a WHO'"'"'S NEXT command, has means for sending an I'"'"'M NEXT command to its FROM station'"'"'s FROM station so as to identify itself to that station.

74. An access machine for initializing a first group of stations (token list) forming a perfect intersection or a subset of a second group of stations interfaced with a communication medium, the first group of stations to share periodic access to the communication medium, comprising:

(A) each station having means for powering up and for entering a state (NEW ONE) which upon expiration of a timeout (NEW ONE TIMEOUT

     66) causes entering of another state (BUS IS DEAD) for initial generation of a list (token list) of all stations which want to have periodic access to the medium (token ownership);

(B) each station which has entered the BUS IS DEAD state having means for waiting an amount of time proportional to a unique characteristic of the station (BUS REBUILD TIMEOUT

     68), which, for the first station to timeout, enters a search state (PASSING INITIALIZATION TOKEN) for determining the next station to enter the search state, and if such a station is found, for itself to enter the token list state (IN LIST) as the first station; and

(C) each station that enters the PASSING INITIALIZATION TOKEN state in turn having means for entering the token list after determining the next station to enter the PASSING INITIALIZATION TOKEN state unless the next station to enter the PASSING INITIALIZATION state is the first station to enter that state, at which point the initial token list is completed;

whereby an initialization of stations to have periodic access to the medium is obtained.

75. An access machine as defined in claim 74 wherein each station to enter the token list has means for knowing the station (FROM station) from which it is to receive the token as well as to which station (TO station) it is to pass the token.

76. An access machine as defined in claim 75 wherein each station has an address and wherein the unique characteristic of each station for determining its bus rebuild timeout period is the address of the station.

77. An access machine for allowing one of a plurality of stations interfaced to a communication medium to be able to control the transmission of information on the medium, comprising:

(A) means for generating a list of all stations wanting periodic access to the medium (TOKEN LIST);

(B) each station in the token list having means for knowing from what station (its FROM station) it is to receive access transfer information to allow it to obtain access to the medium, as well as to what station (its TO station) it is to next pass access transfer information after it has received access to the medium; and

(C) each station in the list having means for listening to the medium to ascertain when its FROM station has access to the medium so that when this condition occurs and if it (the station to next receive access) determines that its FROM station does not transmit on the medium within a predetermined length of time, further having means for initiating the obtaining its own access to the medium.

78. An access machine as defined in claim 77 wherein each station in the list after having access to the medium in turn having means for passing access information to its TO station, and further wherein each station has means for responding to the receipt of access transfer information, so that if the TO station does not appropriately respond to the transfer of access transfer information, the station which sent the access transfer information has means for determining the TO station'"'"'s TO station and for passing access transfer information to the TO station'"'"'s TO station.

79. An access machine as defined in claim 78 wherein each station which has access to the medium further has means for patching out its original TO station from the TOKEN LIST if that station is perceived as not appropriately responding to the transfer of access information.

80. An access machine as defined in claim 79 wherein the station which determines that its FROM station is not operating properly has means for determining its new FROM station when this new FROM station patches out its TO station by determining its TO station'"'"'s TO station.