RS-485 is widely used in industrial automation systems to connect PLCs, robots, PCs, remote I/O, sensors, and controllers.

Although RS-485 is designed for reliable differential communication, incorrect termination, biasing, wiring, or grounding can cause intermittent communication problems.

This article explains how to calculate the termination and bias resistors in a practical RS-485 network.

1. Basic RS-485 Network

A typical RS-485 network can be represented as:

             120Ω                         120Ω
       ┌────/\/\/────┐             ┌────/\/\/────┐
       │             │             │             │
     PLC            Robot        Vision        Remote
                                  Controller      I/O
       │             │             │             │
       └─────────────┴─────────────┴─────────────┘
                    RS-485 BUS

The communication cable commonly has a characteristic impedance of approximately:$$Z_0 = 120\Omega$$

Therefore, a 120 Ω termination resistor is commonly placed at each physical end of the bus.

2. Why Do We Need Termination?

When a fast-changing signal travels through a transmission line, a mismatch between the cable impedance and the termination can cause signal reflections.

The reflection coefficient is:$$\Gamma = \frac{Z_L-Z_0}{Z_L+Z_0}$$

For a 120 Ω cable terminated with 120 Ω:$$\Gamma = \frac{120-120}{120+120} =0$$

Therefore, the idealized reflection coefficient is zero.

This is the basic reason for impedance matching.

3. Why Does the Multimeter Show Approximately 60 Ω?

Suppose there are two 120 Ω termination resistors:

       120Ω                    120Ω
A ─────/\/\/──── BUS ─────────/\/\/──── B

From the perspective of the bus:$$R_{total}=120\Omega || 120\Omega$$

Using the parallel-resistance equation:$$R_{total}= \frac{120\times120}{120+120}$$

Therefore:$$\boxed{R_{total}=60\Omega}$$

So, with the system powered OFF, measuring between the differential lines may produce approximately:

60 Ω

when two 120 Ω termination resistors are connected across the bus.

4. What If We Measure 120 Ω?

If the measured resistance is approximately:$$120\Omega$$

it may indicate that only one termination resistor is present.

For example:

PLC                         Robot
 │                           │
120Ω                       No 120Ω
 │                           │
 └──────── RS-485 BUS ───────┘

However, the actual result depends on the network equipment and topology.

Always check the equipment documentation before adding or removing termination.

5. What If We Measure 40 Ω?

Suppose three 120 Ω termination resistors are accidentally installed.

The equivalent resistance is:$$R_{total}= \frac{120}{3}$$$$\boxed{R_{total}=40\Omega}$$

This creates a heavier electrical load on the RS-485 driver.

For example:

120Ω
  │
  ├──── Bus
  │
120Ω
  │
  ├──── Bus
  │
120Ω
  │
  └──── Bus

Therefore, measuring approximately 40 Ω can be a useful indication that too many termination resistors may be connected.

6. What Are Bias Resistors?

Termination resistors and bias resistors have different purposes.

Termination resistor

The termination resistor primarily helps reduce signal reflections.

Bias resistor

Bias resistors establish a defined differential state on the bus when no transmitter is actively driving it.

A simplified circuit looks like this:

             +5 V
              │
             Rpull-up
              │
              A
              │
          RS-485 Bus
              │
              B
              │
           Rpull-down
              │
             GND

The exact resistor values depend on the transceiver, supply voltage, network loading, and design requirements.

7. Practical Bias Resistor Calculation

Let’s use a simplified example.

Assume:

• Supply voltage = 5 V
• Pull-up resistor = 680 Ω
• Pull-down resistor = 680 Ω
• Two termination resistors = 120 Ω each

The two termination resistors are:$$120\Omega || 120\Omega=60\Omega$$

The bias resistors form a network around the bus.

A simplified DC model is:

             +5V
              │
            680Ω
              │
              A
              │
             60Ω
              │
              B
              │
            680Ω
              │
             GND

The total series resistance is:$$R_{total}=680+60+680$$$$R_{total}=1420\Omega$$

Therefore, the approximate bias current is:$$I=\frac{5V}{1420\Omega}$$$$I\approx3.52mA$$

The voltage across the 60 Ω equivalent termination is approximately:$$V=IR$$$$V=3.52mA\times60\Omega$$$$V\approx0.211V$$

So this simplified example produces approximately:

The important point is that the actual RS-485 idle differential voltage must be checked against the specific transceiver and network design requirements. This simplified calculation is for understanding the resistor network, not a universal design recommendation.

8. Why Bias Voltage Matters

Without an appropriate biasing strategy, the differential bus can become electrically undefined when no driver is active.

For example:

Driver OFF

A ─────────────── ?
                   \
                    Undefined
                   /
B ─────────────── ?

Noise or interference can potentially cause the receiver to interpret the bus incorrectly.

A properly designed bias network establishes a preferred idle state.

This can help prevent:

• False data detection
• Random communication errors
• Receiver instability
• Intermittent communication faults

9. Termination and Bias Are Different

This distinction is very important when troubleshooting.

Component	Main Purpose
Termination resistor	Reduce signal reflections
Bias resistor	Establish a defined idle state
Pull-up resistor	Bias one differential line toward the positive supply
Pull-down resistor	Bias the other differential line toward ground

An engineer should not assume that adding more resistors will automatically improve communication.

Too much termination or excessive bias loading can actually make the electrical conditions worse.

10. A Real Manufacturing Troubleshooting Example

Imagine a manufacturing line with:

PLC
 ↓
Robot
 ↓
Vision Controller
 ↓
Remote I/O

The system normally operates correctly.

However, the communication occasionally fails when:

• The robot starts moving
• A motor starts
• A vision system starts processing
• Another machine turns ON
• The production line starts after a long shutdown

Instead of immediately replacing the PLC or communication module, check the physical layer first.

Troubleshooting sequence

1. Turn OFF power according to the applicable safety procedure.

2. Check the cable connections.

3. Verify A/B or differential-pair wiring according to the manufacturer’s labeling.

4. Measure resistance across the communication pair.

5. Check termination resistors.

6. Check bias resistors.

7. Inspect cable routing and shielding.

8. Check grounding and potential differences.

9. Use an oscilloscope to inspect the differential signal.

10. Compare the waveform during normal operation and during the failure condition.

11. Don’t Forget Electrical Noise

Manufacturing environments are electrically noisy.

Communication cables may run near:

• Servo motors
• VFDs
• Contactors
• Solenoids
• Welding equipment
• High-current power cables
• Switching power supplies

A communication cable routed directly alongside a high-power motor cable can be exposed to significant electromagnetic interference.

A better approach is generally to maintain appropriate separation and follow the cable-routing and grounding recommendations for the specific communication system.

12. Oscilloscope Troubleshooting

Resistance measurements can identify some wiring and termination problems.

However, an oscilloscope can show what is actually happening to the communication waveform.

Look for:

• Ringing
• Overshoot
• Undershoot
• Excessive noise
• Slow edges
• Differential amplitude problems
• Reflections
• Voltage disturbances synchronized with motor operation

For example:

Normal Signal

      ┌─────────────┐
──────┘             └──────────────


Possible Signal Integrity Problem

      /\/\───────────/\/\
─────/   \__________/   \──────────
        Ringing

The waveform alone does not prove the exact root cause, but it can help identify where further investigation is needed.

13. The Most Important Engineering Principle

When troubleshooting industrial communication, don’t look at the system only from the software side.

Think about the entire signal path:

PLC
 ↓
Transceiver
 ↓
Connector
 ↓
Cable
 ↓
Termination
 ↓
Connector
 ↓
Transceiver
 ↓
Robot / Controller

Every part of this path can affect communication reliability.

A communication problem may originate from something as simple as:

one loose terminal, one incorrect termination resistor, or one damaged cable.

Conclusion

Reliable industrial communication requires both correct software configuration and good physical-layer design.

Understanding the relationship between:

• Cable impedance
• Termination resistance
• Bias resistors
• Differential voltage
• Cable topology
• Grounding
• Shielding
• Electrical noise

can make troubleshooting much faster.

For a typical 120 Ω RS-485 bus with two 120 Ω termination resistors:

120Ω∣∣120Ω=60Ω

This simple calculation is one of the most useful starting points when troubleshooting an RS-485 network.

Before replacing an expensive PLC, robot controller, or communication module, check the physical communication layer first.

Cable → Connector → Termination → Bias → Ground → Signal → Software

That troubleshooting sequence can save significant time in a manufacturing environment.