Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 113)

Transferring Discrete and BlockĆTransfer Data
Chapter 8
8-18
Block Transfers of Remote I/O Data
Block transfers of I/O data to remote I/O follow these procedures.
block-transfer requests are queued for each addressed remote I/O rack
Each active buffer transfers one data block per remote I/O scan.
The processor momentarily interrupts program scan when the active
buffer performs a block-transfer request to access the block-transfer
data file.
If program scans are two or three times longer than remote I/O scans, the
processor can run two or three remote block transfers per program scan
and interrupt the program scan two or three times.
Important: If you split remote rack numbers between scanner channels,
block transfers to lower priority scanner channels do not work. Discrete
transfers function properly. Scanner channels have priority according to
the following order: 1A, 1B, 2A, then 2B. If you configure channels 1B
and 2A as remote scanners and split rack #2 between them, for example,
block transfers will work to 1B (the higher priority channel) but will not
work to the second half of rack #2 (2A, the lower priority channel).
Block Transfers in Fault Routines (STIs)
If the processor runs a fault routine that contains block-transfer
instructions, the processor performs these block transfers immediately
upon completing any block transfers currently in the active buffer, ahead of
block-transfer requests waiting in the queue.
The block transfers in a fault routine should only be between the processor
and processor-resident local I/O.
ATTENTION: The program scan stops when the PLC-5/15 or
-5/25 processor runs a fault routine with a block-transfer
instruction to a remote chassis. The delay for a block transfer
could be unacceptable for your application.