g_dickinson Posted July 2, 2009 Share Posted July 2, 2009 I have a PLC that is acting as as ModBus Slave device. I'm using Modbus ASCII as the communications protocol and I am puzzled by the "Alert" messages that I'm receiving: 07/01/09 00:41:48.329 P-ModbusRTU 0011: CRC Fail 07/01/09 01:25:48.334 P-ModbusRTU 0011: CRC Fail 07/01/09 06:57:49.003 P-ModbusRTU 0010: Timeout 07/01/09 06:58:19.003 P-ModbusRTU 0010: Timeout I'm puzzled by the "P-ModbusRTU" as I am not using this protocol, but rather Modbus ASCII for all channels. I'm puzzled by the "CRC Fail" as Modbus ASCII does not use CRCs. Are the "0011" and "0010" values helpful in figuring out what these communications errors actually mean? Link to comment Share on other sites More sharing options...
AzeoTech Posted July 2, 2009 Share Posted July 2, 2009 Looks like there are a couple typos in the protocol. They have been fixed. The code base for RTU and ASCII are almost identical and someone forgot to make the text change in the error message. But that doesn't address your problem. Yes, ASCII does not use CRC, but it does use LRC, and that's what the message should say. Basically it means that the packet LRC didn't match what was returned. Timeout simply means the device didn't respond within the timeout period, or responded with a completely invalid packet. To help debug it, I recommend using the comm monitor. If you need help, please paste a portion of the comm monitor here so we can see your traffic. Link to comment Share on other sites More sharing options...
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