Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 114)

Transferring Discrete and BlockĆTransfer Data
Chapter 8
8-19
BlockĆTransfer Sequence
Figure 8.11 shows the sequence the processor follows to run a block
transfer.
Figure 8.11
BlockĆTransfer Sequence
2
Q17 Buffer
for 17 BT
Requests
3a, 3b, 5
Active BT
area
4a, 4b I/O Chassis
1, 7
Ladder
Program
6 STI
Data
Files
Request
Priority Request
Data
Data
Request and Outgoing Data
Acknowledgement
and Incoming Data
1. Ladder logic enables the block transfer.
2. The processor places the blockĆtransfer request in the queue, or in the active buffer if the queue is empty. If the queue is full, the
request is ignored until the next scan.
3. If the block transfer is a:
BTW: The processor interrupts the program scan momentarily to transfer data from the BTW file to the active buffer. The active
buffer transfers the request and outgoing data to the processorĆresident local I/O module or to the remote I/O adapter.
BTR: The active buffer sends the blockĆtransfer request to the processorĆresident local I/O module or remote I/O adapter. In
the same local blockĆtransfer update or in the next remote I/O scan, the active buffer receives the blockĆtransfer
acknowledgement and incoming data.
Important : The processor interrupts the program scan momentarily to transfer incoming data to the BTR file one word at a
time; therefore, some ladder logic could execute in between word transfers to the BTR file. We recommend that you buffer your
BTR data with a fileĆtoĆfile move or a copy instruction using a BTR done bit to condition the rung if you need file integrity of
the data.
4. If the block transfer is to:
ProcessorĆResident Local I/O: The processor continuously runs blockĆtransfer requests for all processorĆresident local I/O
modules in the order the processor queues the requests.
Remote I/O: The processor runs one blockĆtransfer request for one blockĆtransfer module per rack address per remote
I/O scan.
5. The processor clears the active buffer and the active buffer accepts the next request after the buffer receives a confirmation of a
valid read or write.
6. When the processor enables a fault routine or STI, the processor runs any blockĆtransfer program in the fault routine or STI ahead
of any blockĆtransfer requests in the queue, as soon as the active buffer completes any block transfer currently in the active buffer.
The program scan is stopped until the STI or fault routine block transfer is complete.
7. The blockĆtransfer process runs asynchronously to the program scan, so data can change during program scan.