In modern manufacturing environments, many machines and control devices must communicate with each other.

A typical manufacturing system may include:

• PLCs
• Industrial robots
• PCs
• HMIs
• Servo drives
• Sensors
• Vision systems
• Motion controllers
• Remote I/O
• Industrial network switches

These devices exchange data continuously through communication networks.

When communication problems occur, engineers often focus first on software, PLC programs, or communication settings.

However, many communication problems can originate from basic hardware issues such as cable connections, termination, grounding, shielding, and impedance matching.

Understanding these basic electrical principles can significantly reduce troubleshooting time.

1. Why Cable Connection Is Important

Industrial communication signals travel through physical cables.

The communication cable is not simply a piece of wire.

At higher communication speeds or longer cable distances, the cable behaves as a transmission line.

Therefore, the cable has electrical characteristics such as:

• Characteristic impedance
• Capacitance
• Inductance
• Resistance
• Signal attenuation

If the cable is improperly connected or terminated, the signal can become distorted.

This may result in:

• Communication errors
• CRC errors
• Lost packets
• Intermittent communication
• Device disconnection
• PLC communication faults
• Robot communication alarms
• Random system stops

2. What Is Characteristic Impedance?

The characteristic impedance, usually represented by $Z_0$, is an important electrical characteristic of a communication cable.

For a transmission line, it can be approximately represented by:$$Z_0 \approx \sqrt{\frac{L}{C}}$$

where:

• $Z_0$ = characteristic impedance in ohms
• $L$ = inductance per unit length
• $C$ = capacitance per unit length

For example, many industrial communication cables are designed around standard impedance values.

Typical examples include:

Communication System	Typical Cable Impedance
RS-485	~120 Ω
CAN bus	~120 Ω
Ethernet twisted pair	~100 Ω
RS-422	~100–120 Ω depending on implementation

The exact value should always be verified against the equipment and cable manufacturer’s specifications.

3. What Is Impedance Matching?

When a signal travels through a long cable and reaches the end of the transmission line, part of the signal can be reflected if the load impedance does not match the cable impedance.

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

where:

• $Z_L$ = load impedance
• $Z_0$ = cable characteristic impedance
• $\Gamma$ = reflection coefficient

Example 1: Perfect Matching

Suppose:$$Z_0 = 120\Omega$$

and:$$Z_L = 120\Omega$$

Then:$$\Gamma = \frac{120-120}{120+120} =0$$

Therefore:

No idealized reflection occurs at the termination.

4. What Happens When the Termination Is Wrong?

Suppose the cable has:$$Z_0=120\Omega$$

but the termination resistor is:$$Z_L=60\Omega$$

Then:$$\Gamma = \frac{60-120}{60+120}$$$$\Gamma=-0.333$$

This means a reflected signal with approximately 33% of the incident amplitude and opposite polarity occurs in the idealized transmission-line model.

If the termination is open:$$Z_L \rightarrow \infty$$

then:$$\Gamma \approx +1$$

A large reflection can occur.

This is one reason improper termination can create communication problems.

5. RS-485 Example: Why Is 120 Ω Used?

RS-485 is widely used in industrial automation.

A typical RS-485 bus uses a twisted-pair cable with approximately:$$Z_0 = 120\Omega$$

A common configuration places a 120 Ω termination resistor at each physical end of the bus.

Consider:

PLC                         Robot
 |                            |
 |---- RS-485 Bus ------------|
120Ω                         120Ω
Termination                  Termination

The two 120 Ω termination resistors are electrically in parallel when viewed across the bus:$$R_{total} = \frac{120 \times 120}{120+120}$$

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

This gives us an extremely useful field troubleshooting method.

6. Measuring an RS-485 Network With a Multimeter

Suppose the system is powered OFF.

Measure the resistance between the two differential communication lines, for example A and B, according to the manufacturer’s naming convention.

If the bus has two 120 Ω termination resistors:$$120\Omega || 120\Omega = 60\Omega$$

Therefore, you may measure approximately:

60 Ω

across the bus.

Possible measurements

Measured Resistance	Possible Condition
~60 Ω	Two 120 Ω terminators connected
~120 Ω	One 120 Ω terminator connected
Very high / open	Termination missing or open circuit
Much lower than 60 Ω	Extra termination or another low-resistance path
Near 0 Ω	Short circuit or wiring problem

Important: This is a troubleshooting guideline, not a universal pass/fail rule. The actual network topology and equipment design must be checked first.

7. A Very Common Manufacturing Problem

Imagine a manufacturing line containing:

PLC
 |
 | RS-485
 |
Robot
 |
 | RS-485
 |
Vision Controller
 |
 | RS-485
 |
Remote I/O

An engineer may find that communication works when the machine is stopped but becomes unreliable when the robot starts moving or another machine turns on.

At first, this may look like a software problem.

However, possible hardware causes include:

• Loose communication connector
• Incorrect cable
• Broken twisted pair
• Poor shield connection
• Missing termination resistor
• Excessive termination
• Long cable
• Incorrect topology
• Ground-potential difference
• Electrical noise from motors
• VFD noise
• Poor cable routing

8. Daisy Chain vs Star Connection

For many bus-based systems, topology is important.

A typical RS-485 bus should generally look like:

PLC
 |
 +-----------------------------+
                               |
                            Robot
                               |
                               |
                         Vision System
                               |
                               |
                           Remote I/O

The main cable forms a bus.

A star configuration can create unwanted reflections:

                 Robot
                   |
                   |
PLC ---------------+--------------- I/O
                   |
                   |
                Controller

The exact allowable topology depends on the communication standard and equipment, but uncontrolled star connections and long stubs can cause signal-integrity problems on transmission-line networks.

9. Cable Length and Signal Reflection

As communication speed increases and cable length increases, transmission-line effects become more important.

A signal does not travel instantaneously.

For example, suppose a signal travels along a cable at approximately:$$2 \times 10^8 \text{ m/s}$$

For a 100 m cable:$$t=\frac{100}{2\times10^8}$$$$t=0.5\ \mu s$$

So the signal takes approximately 0.5 μs to travel 100 m in this example.

If the signal is reflected at the end of the cable, the reflected signal must travel back.

Therefore, the round-trip propagation time is approximately:$$1.0\ \mu s$$

At sufficiently fast signal edges, this delay can become important even if the nominal communication data rate does not appear extremely high.

9. Cable Length and Signal Reflection

As communication speed increases and cable length increases, transmission-line effects become more important.

A signal does not travel instantaneously.

For example, suppose a signal travels along a cable at approximately:$$2 \times 10^8 \text{ m/s}$$

For a 100 m cable:$$t=\frac{100}{2\times10^8}$$$$t=0.5\ \mu s$$

So the signal takes approximately 0.5 μs to travel 100 m in this example.

If the signal is reflected at the end of the cable, the reflected signal must travel back.

Therefore, the round-trip propagation time is approximately:$$1.0\ \mu s$$

At sufficiently fast signal edges, this delay can become important even if the nominal communication data rate does not appear extremely high.

11. Practical Troubleshooting Procedure

When an industrial communication system has intermittent communication problems, I recommend checking the hardware systematically.

Step 1 — Check the Communication Configuration

Verify:

• Baud rate
• Parity
• Stop bits
• Device address
• Communication protocol
• Network configuration

But do not assume the problem is software.

tep 2 — Check the Cable

Inspect:

• Cable type
• Cable length
• Connector condition
• Broken wires
• Loose terminals
• Twisted-pair configuration
• Shield connection

Physically move the connector and cable while monitoring communication if appropriate and safe.

An intermittent connector can create a difficult-to-reproduce communication fault.

---

Step 3 — Check Termination

For a typical 120 Ω differential bus:

       120Ω                         120Ω
PLC ───/\/\────── Communication ───/\/\──── Device

With power OFF and the correct topology:$$R_{measured}\approx60\Omega$$

may indicate two 120 Ω termination resistors are present across the bus.

Step 4 — Check for Extra Termination

Suppose three 120 Ω resistors are accidentally installed.

Then:$$R_{total} = \frac{120}{3}$$$$R_{total}=40\Omega$$

This can load the bus more heavily than intended.

Therefore, measuring the resistance can help identify an incorrectly terminated network.

12. Check the Signal With an Oscilloscope

A multimeter is useful for checking resistance and DC voltage.

However, an oscilloscope is much more useful when investigating actual signal integrity.

Look for:

• Overshoot
• Undershoot
• Ringing
• Slow rise/fall time
• Noise
• Differential voltage problems
• Reflections
• Ground-related disturbances

A simplified example:

Ideal:

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


Problematic:

      /\/\____/\/\____
─────/                 \──────────
       Ringing

Ringing can be an indication of signal-integrity problems, although the root cause must be determined from the actual circuit and measurement setup.

13. Check Ground and Shielding

Industrial environments contain many potential sources of electrical noise:

• Motors
• Servo drives
• Variable-frequency drives
• Contactors
• Solenoids
• Welders
• Power supplies
• High-current cables

Communication cables should be routed appropriately and separated from strong noise sources according to the applicable installation guidelines.

Shielding and grounding must also follow the communication equipment manufacturer’s recommendations.

A shield connected incorrectly can sometimes create additional problems rather than solving them.

14. A Real Manufacturing Troubleshooting Approach

In an actual manufacturing environment, I would approach an intermittent communication problem in this order:

Communication Failure
        ↓
Check Alarm / Error Code
        ↓
Check Cable & Connector
        ↓
Check Wiring Polarity
        ↓
Check Termination
        ↓
Measure Resistance
        ↓
Check Ground / Shield
        ↓
Check Power Supply
        ↓
Check Signal With Oscilloscope
        ↓
Check Network Topology
        ↓
Check Software / Protocol

This approach helps prevent engineers from immediately replacing PLCs, robots, or communication modules when the actual problem may be a simple wiring or signal-integrity issue.

Conclusion

Industrial communication reliability depends on much more than software and communication protocols.

The physical layer is equally important.

Cable selection, cable connection, characteristic impedance, termination resistance, topology, grounding, shielding, and signal integrity can all affect communication reliability.

For a typical 120 Ω differential bus, two 120 Ω termination resistors result in:$$120\Omega || 120\Omega = 60\Omega$$

This simple calculation can become a very useful field troubleshooting tool.

However, engineers should remember that impedance matching is not simply the same thing as measuring resistance with a multimeter.

A multimeter can help identify wiring and termination problems, while an oscilloscope can reveal the actual behavior of the communication signal.

Before replacing a PLC, robot controller, or communication module, check the cable, termination, grounding, and signal integrity first.