Wireless communication between PLC and PC


Thread Starter


I am under industrial training for B.Tech. (Electronics). I am trying to connect Siemen's CPU 928B with Step 5 Dos program on PC using Communication port. The communication is buggy.

if anyone can help with few suggestion/ experiences, it would be welcome.
"Buggy" - good old fashioned student logic. ;->

The S5 communication is "funny", maybe even "crazy", but I won't say "buggy". The AS-511 data comms can be slightly exaggerated as the Master asks "Are you there Slave A?" and Slave A says "Yes", then Master has to ask "Can I ask you about Y", and the Slave says "Ok", so then the Master says "Please tell me about Y", so the Slave says "Ok", but then the Master has to ask "Do you have an Answer yet?" and the Slave maybe says "No", so then the Master has to ask again "Do you have an Answer yet?" and now the slave says "Here's the Answer", But the Master needs to ask one more time "Anything more?" and the Slave says "No". Keep in mind this was design "long ago" when systems had less multi-tasking ability - plus the CPU port was designed
solely for a programmer device, not real-time data comms. Users are formally supposed to use the CP3964 interface for real-time data comms.

Contrast this with something like Modbus - the Master asks "Slave A tell me about Y", and the slave says "Here's the info". Done. Modbus has it's own funny features, but it's much less chatty.

So with all the chatty back-n-forth data to the S5, it is very sensitive to added timing delays. I've worked with AS-511 encapsulated with TCP on
Ethernet and as long as I keep the added latency per packet down to below 12msec it works. At around 30+msec it fails as the PLC aborts the data comm at some unexpected delay.

Your problem is likely that your wireless is packetizing the comms in such a way to disturb the timing. You'll probably have to play around with the settings in the modem to get a good match to the needs of S5. You'll probably have to sacrifice efficiency & send lots of small packets instead of trying to fill-out each packet fully. You may even find that your radio modems cannot be used with an S5.


Lynn August Linse, Senior Software Engineer
15353 Barranca Parkway, Lantronix Inc, Irvine CA 92618
[email protected] www.lantronix.com
Tel: (949)300-6337 Fax: (949)453-7152