Qantum - TCP/IP Modbus comms problems

I have a Quantum CPU534-14 Processor, a NOE-771-01 Ethernet Module, and I am using Concept 2.6 SR2a configuration software.

I wish to read data from a modbus device via a 174CEV300 TCP/IP?Modbus Gateway. I am have configured the gateway as per the manual and all appears ok. But I cannot get the 534 Processor to talk to the modbus device.

It would seem like I have to set up the PLC program to point to port

502 on the Gateway? but I cannot find where to do this.The plc has a very simple test configuration using the TCP_IP_ADDR and READ_REG functions.

I would welcome all comments, and hints. Also any useful websites would be much appreciated.

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Well, the first thing to do would be to verify that the gateway/device, and the PLC are working separately, before trying to put them together. The best way to do this would be to use a Modbus Master simulation program on a PC such as the shareware one I wrote - Modbus Test Pro, available at

formatting link
, to test and verify that the gateway/device is responding properly to Modbus commands.

Then, once you have verified the gateway/device is working, you can test the PLC by connecting a PC or ethernet protocol analyzer running Modbus Protocol data analyzer software to the PLC and viewing the request messages the PLC is sending out over the bus, to make sure they are properly formatted Modbus commands. With this hooked up, you can play with the PLC settings until it starts sending out proper request messages, at which point the gateway/device should start responding to it. I believe the popular Linux application ethereal has a Modbus-TCP analysis plug-in available for it.

I haven't used your particular PLC, but one suggestion is that often in TCP/IP applications, the port number is assumed, or is entered along with the IP address in a format similar to like or with a space instead of a colon.

Tim Kirk

Alan wrote:


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