Sequence PLC-5 Block Transfers

I have several "smart" PLC-5 modules (Analog ins and outs, etc) in a RIO remote rack that I need to communicate with via block transfers. From what I've read online, I keep getting a lot of errors because I have to sequence them so that only one block transfer is running over the RIO bus at a given time. However...I can't seem to figure out a workable way to keep them running in a loop but only have one transfer happening at a time. I've tried different combinations of DN/EN bits, a counter to keep track of which one to run, but I still keep getting errors and hangups.

Is there an easy way to do this that I'm totally missing?

Is it possible to increase the baud rate on your RIO link? If so, I'd try it. I've had to do sequencing on start-up for the BTWs to configure a heavy load of analog I/O on remote chassis before, but I've never had to sequence reads.

If you have to do sequencing, I wouldn't do it with a counter. I'd use a series of latch bits that trigger the next BT after the previous BT is Done, but also monitors for faults.

The best way is to sequence them so they execute only one logical rack at a time. it is best to have them controled with their DN and EN and ER bits.

Control by the previous BT using the EN and DN bits as a block transfer can take more than 1 scan to complete depending on multiple factors.

I do think you should have a timer /counter control also to fore firing of the next BT if the current one gets stuck / locked up,etc. It should not happen but it does from time to time even on the best systems this should only force the next BT on a ER bit or a excessive amount of time.

I use couters to count the ER bits from BT's also as they help when you have a problem with something.

If you have access to the Rockwell Knowledge base there are good examples in the tech notes.

Also try to use BTW as less as possible. Try to use 2 reads where possible.

First of all what kind of PLC-5 are you using. The classic is very different from the NP. The NP-5 (5/20, 5/40, 5/60, 5/80) have Block transfer queues which should give the RIO enough time to process the block transfers independently of the PLC scan.

Generally speaking you shouldn't see errors unless you're using continuous mode.

It's usually enough to condition the Block Transfer on it's own EN bit (use an XIO). If you're communicating with a module like the IFE which only needs configuration data on power-up condition the BTW with the start-up bit from the BTR data file. If you need to constantly read and write from/to a module (like the DB) you should condition both the BTR and the BTW with each other's EN bits (using XIO, again).

Hope this helps.

Good luck,

(8{)} .)

(Yosi)