Hello,

I have a newly installed Valmet Maxum II GC analyzer installed that I'm attempting to communicate via Modbus to a Honeywell C300 DCS system.

I received the Modbus map, but cannot get good values to register. I cannot tell if this is a data format issue or configuration issue in the DCS/GC.

I have the analyzer assigned to an IP address on the same subnet as the DCS. Although i cannot ping the IP (it times out) I can see it on the network via cmd "arp -a".

Thoughts?

 

Hello and welcome to the forum.

I'm going to assume that "...but cannot get good values to register" means you are reading values, just not the values you expect. If this is the case, the issue is not a connection issue (since you must have successfully established a connection in order to read any values).

The most common reason is an off-by-one register addressing issue. Some vendors start their Modbus register numbering from 0 (0-based), while others start from 1 (1-based). Therefore, try adding or subtracting 1 from the register number you're trying to access.

Data formatting issues typically only apply to 32-bit registers (which are composed of 2 consecutive, standard 16-bit registers). Some devices use most-significant register first (lower register address), while others use least-significant register first. In addition to register order, devices might encode a 32-bit register as either a 32-bit integer or a 32-bit floating point value (using IEEE 754 single precision floating point encoding).

Other suggestions would be to try reading only a single register that has a known value. You could also try using a Modbus client simulator on a computer to confirm communication to the analyzer, such as ModScan, Simply Modbus, or Modbus Poll.

If you're still having issues after trying the above suggestions, please attach the Modbus map and indicate which register(s) you're attempting to read.