| 摘要 |
A control system for controlling an industrial process includes a plurality of remotely located process control units (remotes) each coupled to an associated input/output device(s) and adapted to communicate with one another through a dual channel communications link. The system permits each of the remotes to assume supervisory control of the communications link on a master-for-the-moment basis with the remotes communicating with each other over the communications link by simultaneously sending redundant blocks of information over each of the dual channels of the communications link. The receiving remote tests the validity of the received messages by first selecting one of the two channels and providing an acknowledgement (ACK) if the received information on the first-selected channel is valid. If the information on the first-selected channel is invalid, the receiving remote then selects and tests the validity of the information on the other, alternate channel. If the information received on the other, alternate channel is valid, an acknowledgement is sent to the transmitting remote, and, if the information received on both channels is invalid, a non-acknowledgement (NAK) is sent to the transmitting remote to request retransmission of the invalidly received information. Other features include performing the information validity check on alternate channels of the communications link with each successive transmission of information, providing redundant remotes for detecting the failure of one of the controlling remotes and for assuming control responsibility thereover; observing the status of the controlled devices and recording changes in that status along with the pertinent time-of-day information; and detecting an open or degraded portion in either of the dual channels of the communications link and inserting a line terminating impedance on either side of the open or degraded portion of the channel. |
