Dual master implied token communication system

• US 5,122,794 A
• Filed: 10/29/1990
• Issued: 06/16/1992
• Est. Priority Date: 08/11/1987
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Warrior, Jogesh
• Primary Examiner(s): Yusko, Donald J.
• Assistant Examiner(s): ZIMMERMAN, BRIAN A
• Application Number:
  US07/604,168
• Time in Patent Office: 596 Days
• Field of Search: 340/825.20, 340/825.05, 370/85.1, 370/85.2, 370/85.3, 370/85.15
• US Class Current: 370/453
• CPC Class Codes: G08C 19/02 in which the signal transmitted is magnitude of current or voltage G08C19/36, G08C19/38 take precedence
• 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

A pair of master units share a common communication link to communicate with one or more slave units. Each slave unit transmits a message only in response to a message from one of the master units. When an initial transmission request is received by a master unit, it monitors the communication link and only initiates transmission of a message when the link has been quiet for a link quiet time period. After completing a transmission of a message, the master unit waits a link grant time period before transmitting again to permit the other master unit an opportunity to transmit a message if it desires. Synchronization of the two master units, therefore, is achieved quickly and with minimum overhead.

Claims

1. For use in a two-master communication system in which first and second master units are capable of communication with one or more slave units, and in which the master units operate so that when both master units are active they alternate in sending messages over a shared communication medium to the one or more slave units;

the first master unit comprising;

interface means for connection to the communication medium to transmit messages from the first master unit, for receiving messages from the second master unit and the slave unit and for sensing the presence of messages on the shared communication medium;

input means for providing input signals;

output means for providing an output based on output signals;

means for causing the interface means to send a message on the shared communication medium, setting a link grant timer to a link grant time period and setting a link quiet timer to a link quiet time period;

means for sending messages according to a first protocol by causing the interface means to send an initial message on the shared communication medium if no messages from the second master unit are detected during the link quite time period;

means for sending messages according to a second protocol by causing the interface means to send a subsequent message on the shared communication medium if the link grant time period times out and no messages from the second master unit are detected on the shared communication medium wherein the subsequent message is different from the initial message; and

controller means connected to the interface means, the input means, the output means, the means for sending messages according to a first protocol and the means for sending messages according to a second protocol, for controlling operation of the interface means, the means for sending messages according to the first protocol and the means for sending messages according to the second protocol whereby the controller means activates the means for sending messages according to a first protocol if the interface means has detected presence of a message from the second master unit on the communication medium and activates the means for sending messages according to a second protocol if the interface means has not detected message activity from the second master unit, the second protocol providing a shorter time period between successive transmissions by the interface means than the first protocol.

2. The two-master communications system of claim 1 including means for detecting a transmitter time-out of a slave unit if a message is sent to a slave unit and no response is received from the slave unit before expiration of a time-out period.