AB SLC 5/04 + OPC polling time issues over rs232/DF1

jwb

Member
Join Date
Jun 2003
Posts
1
Hello all,

I am on a project using an Allen-Bradley SLC 5/04 communicating to a Windows 2000 pc over RS232/DF1 and KepWare OPC. I am experiencing comm buffer overruns when serving tags, using polling rates of 50 - 10000 ms.

I only have 714 tags (mixed integers and floats, with about 15 string registers), so I don't think the volume of tags is the problem, especially since the overruns occur later if I use a faster polling rate.

Can anyone refresh my memory as to how the SLC 5/04 handles communications? Has anyone seen this problem?

Thanks!

jwb
 
Are those tags as one from there, one from here, one from there etc..
That means OPC must poll them one by one, it means a'lot of frame trafic.

714*4*10+15*8 = 28780 bits : 38400 bps means 1 scan/second, but there are not frame/handshaking trafic in 2 direction and not of PLC scanning times etc...
 
The SLC responds to incoming message requests during the housekeeping time at the end of each program scan, or when an SVC instruction is encountered in ladder logic.

There's also a status bit you can set or clear to determine whether the SLC will process just one incoming message request or all incoming message requests per program scan.

from the online help in RSLogix 500:

Comms Servicing Sel

When set (1), only one channel 0 communication request/command will be serviced per END, TND, REF, or SVC instruction. When clear, all serviceable incoming or outgoing communication requests/commands will be serviced per END, TND, REF, or SVC instruction.

When clear, your communication throughput will increase. Your scan time will also increase if several communication commands/requests are received in the same scan. (Status bit S:33/5 with SLC 5/03, 5/04 and 5/05, and Status bit S:2/15 with SLC 5/02.)
 
In my experience, 714 tags is a lot to scan over a 19.2 kb/s or 38.4 kb/s RS232 link, if your scan rate is less than 2 seconds or so.

The burden for making this efficient is on the client software.

RSLinx software tries to optimize the requests that are sent to it by reading contiguous blocks of data instead of making several individual requests. If your OPC client requests N7:10, N7:11, and N7:36 from RSLinx, RSLinx will read N7:10-N7:36 as a block then parse out the required data to the client.

I don't know the version of Kepware driver you're using does this.

Where do you get the "buffer overrun" error... is this a Kepware error? Does Kepware Help extend it's explanation of what it means by buffer overrun ?

Very often an HMI project that has slow communication can be tremendously optimized by changing the ladder logic to place all the data the HMI is reading into contiguous registers in a minimum number of data files.
 

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