I am trying to Poll K095 controller with my PLC and when I monitor the protocol I see a wierd 00 before the echo at the beginnig of the message. It is causing a bad message. Any input.

Example: my message is for example 01 04 00...
my expected reply should be 01 04 data..
but I get 00 01 04 and the PLC thinks this is a bad message.

I tried terminating with cap and Res in series and biasing the lines with 1 K resistor... No help.

PLEASE HELP ANYONE

 

> I am trying to Poll K095 controller with my PLC and when I monitor the protocol I see a weird 00 before the
> echo at the beginning of the message. It is causing a bad message. Any input.

> Example: my message is for example 01 04 00...
> my expected reply should be 01 04 data..
> but I get 00 01 04 and the PLC thinks this is a bad message.

> I tried terminating with cap and Res in series and biasing the lines with 1 K resistor... No help.

I have seen this problem before when tying an RTU to a MDS4710B radio. I determined that it was due to a very short spike on the RS232 interface appearing when the radio is keyed (DCD goes high, prior to data transmission/rts delay). This spike was only apparent when communicating with certain devices with particularly fast/sensitive UARTS. The spike would be interpreted as a start bit, and thus a null character was being inserted at the beginning of the message. Then depending on the hosts ability to parse messages it may or may not cause the message to be seen as invalid. I have a product called the SS10 SCADASwitch which has a feature to remove the offending '00' byte.

hope this helps...
-jER

<b>Moderator's note:</b> yes I know this is an old post. Since no one ever replied, I thought having a reply would be a good thing in case someone found this on Google search.