Mircologix 1400 ModBus RTU Link Layer Error

[pandemonium]

Hi,

I'm having an issue with a mircologix not transmitting out.

The current setup is a mircologix 1400 connected to a Guardian 100 Radio through a serial to rs232 cable. I have replaced the radio , mircologix and cable multiple times and the issue persists.

It was working before and the program hasnt changed as far as i know.

I always get a link layer error code of 5 which according to the book is a crc error. I have been unable to resolve the issue, Our scada system is using a moxa software to request over rf.

Any Ideas?

 

[drbitboy]

Hi,

I'm having an issue with a mircologix not transmitting out.

The MicroLogix as Modbus Slave should not respond if it detects a CRC error in the Modbus request, which I assume will generate a timeout error on the scada side that made the request.

The current setup is a mircologix 1400 connected to a Guardian 100 Radio through a serial to rs232 cable. I have replaced the radio , mircologix and cable multiple times and the issue persists.

It was working before and the program hasnt changed as far as i know.

I always get a link layer error code of 5 which according to the book is a crc error. I have been unable to resolve the issue, Our scada system is using a moxa software to request over rf.

Any Ideas?

The only approach I can think of would be to "snoop" the protocol on the serial lines, and see the actual bits and bytes of the request; this approach is more complicated for RS-232 than RS-485, but it can be done.

At a minimum you might be able to connect a PC with a serial port directly to the scada system (Modbus Master) and read the Modbus request bytes, and from those bytes do a manual CRC check. You could also connect the PC in place of the MicroLogix and confirm whether the request bytes are the same on both ends of the communication hardware.

Assuming either the original or the replacement communication hardware are good, that leaves only the serial ports on the scada system (send) and the MicroLogix (receive). Reading serial data directly from the scada as suggested above, or writing a test program* for the MicroLogix that reads and displays data from the serial port, should identify where the error is occurring.

* this assumes the exisisting MicroLogix program can be replaced and whatever process it is controlling can be put into a safe state.

This approach is very much down in the weeds, so it is not for the faint of heart, but if nothing else you should find where the CRC error is occurring. Details of the Modbus protocol can be found here. Perhaps the scada system is sending bad data over good communication hardware (cables and radio)?