Universal Robots Forum

MODBUS communication issue

I have a problem with my MODBUS communication with PC application.
I have 8 output register and 3 Input register for PC to Robot Comunication.
All it works fine but some time when in script code I call “modbus_set_output_register” or “modbus_get_signal_status” the robot stop the program with this error:
“MODBUS Signal Signal_Name in not connected to slave”.
If I check on installation panel in MODSUB window this signal is connected (green light on led).
This issue is happen only with PC interfacing : I also have a gripper interfaced with MODBUS and I never have this error.
The frequency refresh of this register is 2Hz so very low…
The issue is less frequent if I read less requently the register.

There is a way to check before read this register in order to avoid the STOP of the program?


Hi Michele, there is no way to check if signal is connected.
Modbus signal must be connected all the time while program is running.
Can it be that network issues are causing packet loss, and connection drop?
Actually making signal request rate higher can help, since controller will assume that connection was lost if there is no reply from device after 2 seconds.

If you’re running your server on PC it would help a lot to record traffic on port 502 with wireshark.

You can also try to enable “sequential mode” in advanced settings on modbus client tab. That enforces client to wait for response before sending next request.