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On one of our station gas compression, we got a big problem.

The DCS ( APACS MOORE ) is connected to Speedtronic Mark 5 via MODBUS.

Along the circuit, we find from Port 2 HMI to RS232/485 Converter then 4 wires to another Converter LDM422 DATAFORTH then to ACM 40 of DCS.

I checked all the elements noted above and that is ok.

I think that the problem come from really from the modules of APACS ACM.

Who can help me to resolve the no communication HMI and DCS.
DCS MASTER HMI SLAVE.
Thanks more and pardon for my english

Hello Damene;
First checking the cabling is a good start, but not the only one.

Modbus requires that m,aster ansd slave use exactly the same data parameters: data speed, number of bits per character, parity bit number od Stop bits (ex: 9600, 8,E,1). Please check the requirements of the master so you can set the slave to the same values.

The master must have a configured "job" for each block of data it needs to read or write in the slave; is this done? For example, you may want to read multiple registers (Function 3) from Slave #6, starting at (slave's) internal address 120, with a length of 22 registers, mapped to master's internal address WX4000.

The best way to test a Modbus application in the field is to have a Modbus master software on your PC (especially if there is a "real" ComX port available). Download Modtest or Modscan from the Internet, try connecting directly to the master and the slave using a null-modem RS-232 cable; if you are able to see data directly, do the same type of check using the RS-232 to RS-485 converters installed in the field.

Hope this helps,
Daniel Chartier