Slowdown reading tags by RSLinx OPC Server


Thread Starter


I need info about how to optimize RSLinx comunications.

I'm involved in an SCADA application using 2xHMI/USDATA-FactoryLink connected to 5xPLC/ROCKWELL-ControlLogix by RSLinx OPC Server

When running the application the reading of tags from PLC slowdown (response time 10-20 seconds)

Someone can help me?
1. The OPC structure of the ControlLogix using native Tags adds *ALOT* of overhead to the Logix units in terms of memeory & speed - so it is best to use only one RSLINX server(or other data device) to grab the tags & then serve out to clients
2. Set HMI to get *ALL* tags - all the time this is more efficient using Logix (see AB Knowledge base)
3.This may be your main bottle neck - the Default time slice of the Logix Processors is set at 10% for peripheral tasks (read HMI) - bump this time slice up to as high as your program can handle & your graphics will improve - (this can only be done in program mode)
4. Hopefully you are using Ethernet not ControlNet to connect to HMIs - if you are trying to use CNet you will need a larger NUT to handle non periodic tasks. CNet is bad for HMI use - use Ethernet.
Here are several Rockwell articles on RSLinx

1.Getting Native tags from a 5550 Article number P1475
Several items to look but the Communication Timeout may need to be set to 6 or higher

2.Hot Fix Article Number A2019
Fixed an issue with request packets coming back as a response.
Customers experiencing problems with data not updating (aka "frozen data"), some instances
of accessing CPU should apply this update.

3.Number of Item Limitation for a topic Article Number Q974

4.Control Logix 5550 Com Optimization Article Number A1673
and Control Logix 5550 Com Optimization Article Number A1838

Also watch out for a timeslice in the PLC

Eric Zimmerman

On the read and write statments change from OPC Device to OPC cache. I found OPC device read and write does a complete all tag read and write to cache also. This had slowed down a vb program I had done greatly. This improved my read from 6 seconds to 0.1 seconds.

Eric Zimmerman