I come across wiring issue related to modbus. I have PRO380-Mod energy meeter which is capable of measuring 3 phases and other various aspects of energy consumption for reasonable price. Communication interface is based on modbus, however manufacturer did not implement “full modbus specification”:
When connecting the meter through a serial converter (RS485) for testing, please be aware that because of not implementing the complete Modbus infrastructure, there will be a need to put an additional resistor (120 ohms/0.25 watts) across the terminals (22 & 23) on the meter side.
I tried to place 100 ohms resistor, however I see that RX diode on serial adapter does not blink. It does blink few times when I plug USB cable into it, but when I attempt to start modbus scanner it doesn’t show any activity.
From this place I would like to ask you what are your experiences with devices such above which requires extra resistors to communicate. I bought a dedicated cable for modbus communication which does have 120 ohm resistance (I’ve heard that cat5 cable requires different resistors). Will whole thing work with 120 ohm resistor? Have you ever tried to use potentiometers to adjust modbus lanes?