Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 112)

Transferring Discrete and BlockĆTransfer Data
Chapter 8
8-17
The processor block transfers data to and from its processor-resident local
and remote I/O chassis when operating in scanner mode. The processor
performs block transfers asynchronously to the program scan. The
processor also interrupts the program scan asynchronously to momentarily
access BTW and BTR data files. The processor performs one remote
block transfer per addressed rack per remote I/O scan in classic PLC-5
systems.
Queued BlockĆTransfer Requests
If your ladder program requests more than one block transfer to or from the
same I/O chassis in the same program scan, the processor queues the
requests. The PLC-5/12, -5/15, and -5/25 processors can handle up to 17
requests per rack address.
After the processor queues the requests, a PLC-5/12, -5/15, or -5/25
processor runs the block transfers in the order they are requested. The only
exception is a block-transfer request in a fault routine.
The processor has an active buffer. The processor places a block transfer
in the active buffer when the processor takes the request off the queue.
The processor places a block-transfer request directly in the active buffer
only if the queue is empty.
When the processor is changed to program mode, the block-transfer
instructions still in the buffer are cancelled.
Block Transfers to ProcessorĆResident Local I/O
Block transfers to processor-resident local I/O follow these procedures.
Block-transfer requests are queued for the addressed processor-resident
local I/O rack.
The active buffer handles all block-transfer modules whose block-
transfer instructions were enabled in the program scan continuously via
the queue scan in the order the requests were queued.
The processor momentarily interrupts program scan when the active
buffer performs a block-transfer request to access the block-transfer
data file.
Block transfers of I/O data can finish and the done bit can be set
anytime during the program scan.
The processor runs all enabled block transfers of I/O data to processor-
resident local I/O continuously as each block-transfer request enters
the active buffer. The processor does not wait for the I/O scan to queue
the requests.
Programming Block
Transfer in Scanner Mode