Deterministic timed bus access method

• US 5,434,861 A
• Filed: 02/04/1994
• Issued: 07/18/1995
• Est. Priority Date: 02/02/1989
• Status: Expired due to Fees

• Current Assignee: University of Strathclyde
• Original Assignee: David Pritty, Derek Masson
• Inventors: Pritty, David, Masson, Derek
• Primary Examiner(s): Olms, Douglas W.
• Assistant Examiner(s): Nguyen, Chau T.
• Application Number:
  US08/192,206
• Time in Patent Office: 529 Days
• Field of Search: 370/85.1-85.3, 370/85.6, 370/85.8, 370/94.1, 370/95.3, 370/95.2, 370/13, 370/100.1, 340/825.07, 340/825.08, 340/825.5
• US Class Current: 370/449
• CPC Class Codes: H04L 12/403 with centralised control, e.g. polling
• CPC Class Codes: H04L 12/417 with deterministic access, e.g. token passing

Abstract

An improved timed deterministic bus access method and system for a bidirectional linear bus, a unidirectional linear bus, a star and a tree type of bus structure. The method employs a master timing reference signal and discrete values of time delay at each node together with a cycle start signal to provide orderly deterministic access by a number of nodes to a shared bus structure. The master timing reference signal is generated by a polling node which can also transmit messages. The time delays in the nodes can correspond to their physical order from the polling node when this is located at one end. Alternatively values of timing delays can be chosen to make the physical location of the polling node of the other nodes unimportant. The method provides for prioritised access to the bus structure using a reservation hold.

Claims

1. A method of providing deterministic access to a bus structure having a plurality of nodes coupled thereto, in a data communications network, each node being capable of transmitting and receiving data packets to and from said bus structure respectively, and each node having a physical node address, which is defined as the address for said data packets, said method providing round robin scheduling of all nodes waiting to transmit data packets, and said method comprising the steps of,

a) disposing a polling node, capable of generating a polling signal, defined as a timing reference signal, at a suitable point on the bus structure;

b) generating said timing reference signal which is independent of any data packets from said polling node to invite transmission of the data packets onto the bus structure after a period of silence;

c) providing each node coupled to the bus structure with a unique predetermined time delay, defined as the node time delay, said node time delay starting from receipt of said timing reference signal and said time delay being fixed at network configuration time, which is prior to the start of normal transmission of data packets on the network, each said node time delay at a respective node being independent of said physical node address;

d) detecting said timing reference signal at each said node as said timing reference signal travels along the bus structure, said timing reference signal being used by each of the nodes as the only timing reference signal to control its transmission activity onto the bus structure;

e) including within each of said nodes two internal flags to control the operation of the node in respect of transmission of data packets, said flags being defined respectively as a Next Slot flag and a Transmit flag, the operation of each said nodes being such that transmission of a data packet onto the bus structure from an active node is permitted if the bus is silent at the expiration of said node time delay, providing that neither said Next Slot flag nor said Transmit flag is set, the method including the further steps of;

i) setting said Next Shot flag, until the next timing reference signal is received by said node, if either said node does not have a data packet ready to transmit at the time it receives said timing reference signal or, if said node is unable to transmit a data packet when its said node time delay expires, as the bus is busy at that time,

ii) setting said Transmit flag when said node starts to transmit a data packet;

f) arranging that the said polling node generates the next said timing reference signal either;

i) after said polling node has detected that the bus has gone silent after the end of a data packet or

ii) after said polling node has detected that a sufficient period of silence has elapsed for said polling node to know that no other nodes are going to transmit a data packet in response to the current timing reference signal;

g) arranging that each of said Transmit flags in each of said nodes is reset when a said sufficient period of silence has elapsed that no other nodes are going to transmit a data packet in response to the current timing reference signal, and

h) supporting communications for said data packets and said timing reference signals over a single shared bus structure such that data packets follow the timing reference signals in same shared data communications channel.

2. A method as claimed in claim 1 wherein the said polling node is disposed at one end of the bus.

3. A method as claimed in claim 2 wherein when said bus structure is a bidirectional linear bus, and said polling node is at one end of the bus the value of the node time delay at each one of said nodes increases according to the order of the physical location of the node on the bus structure reckoned from the polling node.

4. A method as claimed in claim 2 wherein when the polling node is located at one end of the bus structure, the said node time delay of each node is calculated using the formula n.t_d where n is an integer representative of the node position reckoned from the polling node and t_d is a small time delay which is sufficient to permit the active node nearest the polling node to transmit first.

5. A method as claimed in claim 1 wherein the method includes the step of equipping a number of nodes with polling facilities whereby on failure of the present polling node the operational node closest to the failed polling node automatically assumes the polling function.

6. A system for providing deterministic access to a bus structure having a plurality of nodes coupled thereto, in a data communications network, each node being capable of transmitting and receiving data packets to and from said bus structure respectively and each node having a physical node address which is defined as the address for said data packets, said system providing for round robin scheduling of all nodes waiting to transmit data packets and said system comprising:

a) polling means coupled to the bus structure at a suitable location for generating a polling signal which is defined as a timing reference signal, which is independent of said data packets on said bus structure, said timing reference signal being used by each of said nodes coupled to the bus structure, as the primary means to initiate transmission of said data packets onto the bus structure after a period of silence,

b) each of said nodes coupled to the bus structure having timing reference signal detecting means for detecting said reference signal as it passes said node,

c) each of said nodes having means for storing a unique predetermined time delay, the value of which is independent of said node'"'"'s physical node address, said unique predetermined time delay being defined as the node time delay, and said node time delay being fixed at network configuration time, prior to the start of normal transmission of data packets,

d) each of said nodes having bus activity monitoring means for monitoring the transmission activity of the bus structure for timing reference signals and data packets,

e) providing communications means over said bus structure for supporting communications for said data packets and said timing reference signals such that said data packets follow the timing reference signals in the same shared data communications channel,

f) each of said nodes having transmission control means provided by internal bits within each of said nodes which are used as two flags which control the transmission of data packets onto the bus, said flags being defined as a Next Slot flag and a Transmit flag, the operation of said node being such that said bus activity monitoring means causes a data packet to be transmitted from said node when said bus activity monitoring means detects that the bus is silent at the time said node time delay has expired add neither said Next slot flag nor said Transmit flag is set,

The system being arranged to Operate such that;

i) said Next Slot flag is set, until the next timing reference signal is received by said node, if either said node does not have a data packet ready to transmit at the time it receives said timing reference signal or when said node is unable to transmit a data packet when its said node time delay expires because the bus is busy at that time,

ii) the said Transmit flag is Set when said node starts to transmit a data packet,

iii) the next said timing reference signal is generated after said polling node has detected that one of two next timing reference signal conditions has arisen, firstly that the bus has gone silent after end of a data packet or, secondly after said polling node has detected that a sufficient period of silence has elapsed for said polling node to know that no other nodes are going to transmit a data packet in response to the previous timing reference signal,

iv) arranging that when next said timing reference signal is generated as a result of the said second timing reference signal condition each of said Transmit flags in each of said nodes is reset.

7. A system as claimed in claim 6 wherein when said bus structure is a bidirectional linear bus and the polling means is located at one end of the bus structure the value of the node time delay at each one of said nodes increases according to the order of the physical location of the node on the bus structure reckoned from the polling node--so that a node further from the polling node always has a node time delay greater than its immediate neighbor closer to the polling node whereby the active node closest to the polling node transmits first.

8. A system as claimed in claim 6 wherein the bus structure is selected from the group consisting of a bidirectional linear bus, a star (hub) and a tree.