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I am an undergraduate researcher trying to connect my PC to a Eurotherm 3508 controller (see Eurotherm 3508 manual here: https://www.eurotherm.com/download/3504-3508-engineering-manual-ha027988-iss-19/). My PC is connected via a usb to a USB-RS485 converter (see https://www.waveshare.com/wiki/USB_TO_RS485#Resources).

I then connected the ground, A+ and B- ends of the USB-RS485 converter to the HD, HE, and HF ports on my Eurotherm controller (see figure).

enter image description here

I then used Modbus Poll to try and send requests from my PC and receive responses from the controller. When I send messages, I see a red light blink on the USB-RS485 converter, so I think my messages are reaching the USB-RS-485 device. However, I do not receive any response from the Modbus. Instead, I always receive a timeout error on Modbus Poll.

To troubleshoot, I have checked the baud rate, parity, and address that my Eurotherm is set to, and I've made sure that my Modbus Poll is configured to those parameters. I am wondering if it's possible my Eurotherm is in "listen only" mode. If so, is there any way I can test that or change it do a different mode that would allow it to respond? Any other advice?

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  • \$\begingroup\$ This is about product usage, not about electrical engineering. Can you get support from manufacturer? Are you sure your product was bought with 2-wire Modbus option? Are you sure you don't have A and B wires swapped (try it)? Are you sure the product is not expecting a 4-wire Modbus? Frankly, there is not much to debug - timeout happens because PC receives nothing, but it might be because the other product fails to send on same wires, or it does not even see valid comms from PC. And RS-232 tag is irrelevant, there is no RS-232 involved. \$\endgroup\$ Commented May 2 at 16:49
  • \$\begingroup\$ This device seems to talk many different protocols, have you checked the correct comms adapter is installed, in which slot, if it is configured to use this adapter in this slot, and if the device is set to Modbus (Slave not Master)...? \$\endgroup\$ Commented May 2 at 16:54
  • \$\begingroup\$ Have you tried swapping the A and B wires? There seems to be various interpretations of what is + and - regarding RS485. One way works, the other doesn’t. \$\endgroup\$ Commented May 3 at 8:30
  • \$\begingroup\$ @bobflux What do you mean by a comms adapter? Are you referring to the USB-RS485 converter? And yes, I ensured the device is set to MODBUS, not master. \$\endgroup\$ Commented May 4 at 15:34
  • \$\begingroup\$ The manual looks like the modbus port is on a plug-in module or adapter that you have to put in a slot \$\endgroup\$ Commented May 4 at 17:47

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