PLC-5 BT Queue


Thread Starter

Bill Code

One of my customers has stated the following as part of a technical review of an Allen-Bradley PLC-5/40E system design:

"Smart card (block transfer required) in remote rack. This requires Block Transfer queue checks in program."

I have placed cards requiring block transfers in remote racks before and AFAIK this has not created a problem. I have seen the block transfer queue status bits in the S file but I have not implemented any use of them in any applications to date. Nor have any of my colleagues.

Has anyone had a problem with block transfers to remote racks? Could anyone point me to a document that describes what is being asked for?

Bill Code
MPM Engineering Ltd.
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Phone: 1-604-534-6605 Fax: 1-604-534-6693
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Bob Desrochers

Hi Bill,

AB RIO will only move a certain amount of BT data between rack I/O updates. It is possible to ask for BT data (too much and/or too often) for the RIO to keep up. I don't recall the formula but AB does have one for determining BT capacity. I never read it but was informed of it in detail by AB tech support. I have had situations where too many BT's filled the queue and the data being moved was late arriving. By the time I got in trouble I had in excess of 12 BT's executing as often as possible at 64 words each on a 230 kbps RIO with 15 nodes. I prefer not to manage BT's by looking at the queue. The "queue full" bit will only tell you when it is too late for time sensitive data. I have had good results with timed events triggering BT's at a frequency sufficient to meet data needs yet never set the queue full bit.

Bob Desrochers
[email protected]