Communication RS485: RS-485 Termination Resistors

Why Use Termination Resistors?

RS-485 is excellent for long-distance communication because it uses differential signaling, which is less susceptible to noise. However, signal reflections can occur at the ends of the transmission line, potentially corrupting data. Termination resistors prevent this by matching the cable's impedance and absorbing the signal energy.

Placement

• Ideal Setup: Terminate at BOTH ends of the RS-485 chain. This means placing one resistor at the beginning (usually the master device or logger) and one at the very end (the last inverter in the chain)
• Our Inverters: Focus on terminating the end of the chain for the inverters. Add a 120-ohm resistor to the available RS-485 port on the last inverter (marked in yellow). You'll need to remove the front cover to access this port. Either disconnect existing cables or simply add the resistor ends to the unused + and - terminals.
  
  Choosing the Right Resistor

• Most RS-485 systems use 120-ohm resistors to match the characteristic impedance of the standard cable.

Important Considerations

• Cable Length & Data Rate: Longer cables and higher data rates increase the need for termination.
• Number of Devices: More devices can mean more reflections.
• Built-in Termination: Some devices have built-in termination, so check the documentation. Our inverters do not; check the master device (logger, plant controller, DAS, or SCADA system).  Newer firmware boards typically have built-in termination.

Key Takeaway

Terminating both ends of the RS-485 chain is standard MODBUS protocol and essential for reliable communication, especially in our inverter setups. Following these guidelines will minimize data errors and ensure smooth operation. 

Note: if the master device is a Solis Datalogger plugged in midway, and the daisy chain splits two ways from this 'master' device, both ends of the chain will need the resistors installed.