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Communication Between Two Controllers

IP.com Disclosure Number: IPCOM000088153D
Original Publication Date: 1977-Apr-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 15K

Publishing Venue

IBM

Related People

Anwyl, ED: AUTHOR [+6]

Abstract

In a terminal system, it is not unusual for a display terminal to be linked to a controller over a coaxial cable. Normally such a display terminal would have dedicated buffers for communicating with the controller and normally would be completely dependent upon the controller for its operation. In some circumstances, it may be desirable to replace the display terminal with a further intelligent controller acting as a slave. However, because the intelligent controller is performing tasks not dependent upon the communication link with the master controller and because communication is over a coaxial cable, communication failures can result unless special provision is made in the communication protocols.

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Communication Between Two Controllers

In a terminal system, it is not unusual for a display terminal to be linked to a controller over a coaxial cable. Normally such a display terminal would have dedicated buffers for communicating with the controller and normally would be completely dependent upon the controller for its operation. In some circumstances, it may be desirable to replace the display terminal with a further intelligent controller acting as a slave. However, because the intelligent controller is performing tasks not dependent upon the communication link with the master controller and because communication is over a coaxial cable, communication failures can result unless special provision is made in the communication protocols.

In the following handshaking routine, the assumption is always made that unless specifically instructed, the slave controller will always be setting itself up to repeat the communication it has just performed. Thus when the slave controller is in the transmit mode, it will prepare to retransmit the message until it has received a signal that the transmitted message has been received. When in the receive mode, the slave controller will prepare itself to receive a repeat message until the master controller itself acknowledges receipt of the acknowledgement signal from the slave controller.

The handshaking routine is as follows:- Master Slave
.1. Idle poll
. 2. Status (able to receive)
.3. Interrogate poll
. 4. Status (ready to receive)
.5. Write command and data
.6. Terminate poll
. 7. Status (data received or
. not - slave preparing to
. receive in event of
. failure)
.8. Acknowledge poll (only
. now can slave be sure
. data has been received
. at master)
. 9. Status (informing master
. that acknowledge poll
. received)
. 10. Idle Poll
. 11. Status (data to send)
. 12. Read command
. 13. Sends data (and sets up to
. retry)
. 14. Acknowledge poll (only
. now can slave be sure
. data has been received
. at master)
. 15. Status (informing master

1

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. that acknowledge poll
. received)
16. Idle poll

17. Status (unavailable)
18. Idle poll.

Various error possibilities during transmission of data from the master to the slave will now be discussed. If there is an error in the idle poll, the slave will ignore it; the master will retransmit the idle poll when there is no response from the slave. If there is an error in the interrogate poll, it too is ignored by the slave and in the absence of a response from the slave (or if the status response has bad parity) the master will retry. If the write command is received with bad parity, the slave will ignore it and the following data. When the terminate command is received, the slave will indicate data not received and the master will retry from the interrogate poll. If the slave receives a valid write command and then detects bad parity on the data, the same action is taken as with errors in the write command. If there is no response to the terminate...