Allen-Bradley PLC-5 - Page 88

Publication 1785-UM012D-EN-P - July 2005
6-16 Communicating with Remote I/O
The figure below shows the remote I/O block-transfer sequence.
IMPORTANT
If you split remote rack addresses between scanner
channels, block-transfers to lower priority scanner channels
do not function properly. Scanner channels have priority
according to the following order: 1A, 1B, 2A, then 2B.
For example: if you configure channels 1B and 2A as
remote scanners and split rack #2 between them,
block-transfers to channel1B (the higher priority channel)
will be completed, but block-transfers to the second half of
rack #2 (2A, the lower priority channel) will not be
completed.
Although splitting remote I/O racks across scanner
channels does not affect discrete transfers, I/O status bits
such as Fault and Present may not indicate the correct
status.
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Block-
Transfer
Requests
Discrete
I/O
Adapter
BT
BT
AdapterAdapter
BT
Remote I/O
Scanner within PLC processorLadder Logic
Processor executes a block-transfer instruction.
Processor sends the block-transfer request to
its I/O scanner.
Scanner places module control byte (MCB) into the
discrete output image table.
Scanner sends MCB as part of the discrete I/O
update to the adapter.
The adapter module sends the block-transfer request
to the block-transfer module.
The block-transfer module returns a module status
byte (MSB) to the adapter.
MSB returned to the scanner in addition to the
discrete I/O by the adapter.
The scanner forms a block-transfer packet.
The scanner sends the block-transfer packet to
the adapter for the block-transfer module
(the packet includes data if it is a block-transfer write).
The adapter passes the block-transfer packet to
the block-transfer module.
The block-transfer module sends status to the
adapter (will also send data if it is a block-
transfer read).
The adapter passes status to the I/O scanner;
if the request is a block-transfer read, adapter
sends data.
BT
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