Bradley Smoker PLC-5 16Communicating with Remote I/O, Block-transfer module, Transfer read

6-16Communicating with Remote I/O

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.

The figure below shows the remote I/O block-transfer sequence.

Ladder Logic

]	[	( )
]	[
		BT
		1

2

Scanner within PLC processor

Block-	Discrete	4	9
Transfer
	I/O
Requests
		7	12

3

8

Remote I/O

510

Adapter		BT
	6	11
Adapter		BT

1Processor executes a block-transfer instruction.

2Processor sends the block-transfer request to its I/O scanner.

3Scanner places module control byte (MCB) into the discrete output image table.

4Scanner sends MCB as part of the discrete I/O update to the adapter.

5The adapter module sends the block-transfer request to the block-transfer module.

6The block-transfer module returns a module status byte (MSB) to the adapter.

7MSB returned to the scanner in addition to the discrete I/O by the adapter.

8The scanner forms a block-transfer packet.

Adapter BT

9The 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).

10	The adapter passes the block-transfer packet to
	the block-transfer module.
11	The block-transfer module sends status to the
	adapter (will also send data if it is a block-
	transfer read).
12	The adapter passes status to the I/O scanner;
	if the request is a block-transfer read, adapter

sends data.

Publication 1785-UM012D-EN-P - July 2005