Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 116)

Transferring Discrete and BlockĆTransfer Data
Chapter 8
8-21
In a distributed control system where your process is controlled by several
independent programmable controllers, make sure that your program
considers the status of the PLC processors and the integrity of the
communication link by using the status bits that the supervisory and
adapter mode processor provide for each other.
For example, consider how your process should respond if:
there is an incremental degradation of the systems control due to the loss
of one of the programmable controllers
the supervisory processor is in program mode and someone manually
activates a valve normally controlled by the supervisory processor
the adapter-mode processor faults
The adapter-mode processor can monitor the status of the supervisory
processor by examining the status bits in the first word of the the data
being transferred from the supervisory processor.
The supervisory processor can monitor the status of the adapter-mode
processor by examining the status bits in the first word of the data being
transferred from the adapter-mode processor. The supervisory processor
can also monitor the rack fault bits for the rack the adapter is emulating to
determine the integrity of the remote I/O communications between the
supervisor and the adapter-mode processors. For more information on rack
fault bits, see the faults chapter in the programming software
documentation.
General Considerations for BlockĆTransferring I/O Data
The following are general programming considerations when you are
block-transferring I/O data.
When performing block transfers (processor-resident local, extended-
local, or remote I/O) in any PLC-5 processor clear the output image
table corresponding to the block-transfer module rack location before
changing to RUN mode. If you do not clear the output image table, then
you encounter block-transfer errors because unsolicited block transfers
are being sent to the block-transfer module (i.e. if a block-transfer
module is installed in rack 2, group 4, clear output word O:024 to 0. Do
not use the word for storing data).
If you use remote block-transfer instructions and have the timeout bit
(.TO) set to 1, then the processor disables the 4 sec timer and requests
additional block transfers anywhere from 0-1 sec before setting the error
(.ER) bit.
Programming
Considerations