Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers User Manual

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.

] [

( )

] [

Block-
Transfer
Requests

Discrete
I/O

Adapter

Remote I/O

Scanner within PLC processor

Ladder 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.