Function of Termination Resistors in MODBUS 485-Compatible Stepper Drives

What is the purpose of a termination resistor?

Electrically, when an RS-485 signal encounters an impedance mismatch at the end of a cable, signal reflection occurs. The reflected signal superimposes itself onto the original signal, causing waveform distortion (ringing). This leads to data transmission errors, which manifest as sporadic non-responsiveness, positional anomalies, or communication interruptions in the stepper drive.

A termination resistor is an "energy absorber" connected between the signal lines (A+ and B-). Its core function is impedance matching; it absorbs energy at the signal's endpoint, thereby eliminating signal reflection. This ensures a clean waveform and accurate data. In scenarios requiring high-speed communication, eliminating reflection also allows the system to operate stably at higher baud rates.

Most commercially available MODBUS 485-compatible stepper drives feature a built-in termination resistor, which can be enabled using a DIP switch (e.g., SW8). Taking the DM420PR and DM556PR as examples, it is enabled at the position marked in red in the diagram.

Point-to-Point Connection: Termination Resistor Not Required

• Scenario: One RS-485 port on the controller is connected to only one stepper drive, and the connecting cable is very short (e.g., less than 1 meter).
• Reason: In this dedicated, very short-distance connection, the signal transmission delay is minimal, and the impact of reflection is negligible. Adding a termination resistor in this case unnecessarily increases the burden on the drive.

Bus Topology Connection: Termination Resistors are Mandatory at Both Ends

• Scenario: One RS-485 port on the controller connects to multiple stepper drives in a daisy-chain fashion via a single trunk cable, forming a true bus network.
• Reason: The signal needs to travel a long distance along the line, and each drive node introduces a slight impedance change. Ultimately, at the physically farthest end, significant signal reflection occurs, which travels back along the bus, interfering with communication across the entire network.

Updated on: 30/10/2025