AB SLC 5/05 Open Connections Problem

Problem:
I have a AB SLC 5/05 (L551 Series C FRN 11) which is erroring on MSG blocks (Error D1), which is Already at max number of open connections.

Details:
1. This PLC polls 14 remote PLC's, each with a Read & write MSG block. (14 connections or 28 connections depending on how you count them - see below)
2. Two HMI PC's with AB driver (4 connections)
3. Occasional RSlinx connection for going online for programming. With this connection open, two of the remotes MSG give the D1 error. With this connection closed, only one remote gives the error. (2 connections)

My Question:
I called AB tech support and they referred me to knowledge base 17469. According to this document, L551 w/ FRN 11 has 32 available connections. In the SLC instruction set manual it states a read and a write to the same address uses the same connection. By this count I should not be running out of connections (14 + 6 = 20). There is a note in this knowledge base that states "Note: If 5/05 'A' sends a MSG to 5/05 'B', 'A' uses one outgoing connection and 'B' uses one incoming connection. All subsequent MSGs sent by 'A' to 'B' use this same connection, as long as the "Inactivity Timer" in 'A' has not expired." If Im not conserving connections in this manner, then that explains my connection problem (14x2 + 6 = 34)

What is meant by Inactivity timer? The AB guy didnt know what it meant. Do I have to issue the Read and Write simultaneously? I have them spaced out by a few seconds in my polling loop right now.

I really hope someone has run into this problem before, because I dont want to upgrade to a L552 if at all possible. All this dumb PLC does is poll remotes, that would be a waste of money if you ask me.

Any input is greatly appreciated.

Inactivity Timeout is configured in the Channel Configuration of the SLC program. Under the Chan.1 - System tab, you will see that setting and it is entered in minutes. The default value is 30 minutes.

I would recommend using sequencing to make sure that the DN bit (bit 13 of the control block address) for the read is on before triggering the write to the same address and vice versa.

Yes, my MSG only go out if the previous ERR or DN bits are set.

Thanks for pointing out where Inactivity Timeout is. Now you've got me thinking. I wonder if I set the Inactivity Timeout to 1 minute, and slow my polling down to say 15 seconds between each MSG instruction - I should recover connections every 4 MSG blocks. This would be a slow polling loop but it would work fine for my application.

Is there anyway to close an open connection once a MSG goes out?

This whole connection thing seems way more complicated that it needs to be (although its a great way to sell L552 and L553's)!

SCADA_Dude said:

Yes, my MSG only go out if the previous ERR or DN bits are set.

Thanks for pointing out where Inactivity Timeout is. Now you've got me thinking. I wonder if I set the Inactivity Timeout to 1 minute, and slow my polling down to say 15 seconds between each MSG instruction - I should recover connections every 4 MSG blocks. This would be a slow polling loop but it would work fine for my application.

Is there anyway to close an open connection once a MSG goes out?

Click to expand...

Nope. SLC's just have to time out.

SCADA_Dude said:

This whole connection thing seems way more complicated that it needs to be (although its a great way to sell L552 and L553's)!

Click to expand...

Sometimes I am convinced that is their idea!