By Eileen Harvey
In automation, reliable data transmission is just as important as power delivery. Every controller, sensor and actuator in a factory communicates through a network of bus cables. These cables are designed not just to carry signals, but to preserve their integrity over long distances and in electrically noisy environments. They are engineered not merely to transmit signals but to maintain their integrity across extensive distances and withstand the challenges posed by electrically noisy environments, which are commonplace in industrial settings. The ability to transmit data accurately and consistently is crucial for the seamless operation of automated processes, influencing everything from production efficiency to equipment longevity.
A pivotal aspect of ensuring this reliability lies in impedance control. When impedance is not managed correctly, it can lead to a host of issues, including signal degradation, reflections and ultimately network downtime. Such disruptions can halt production and result in costly repairs or delays. Therefore, understanding the role of impedance in bus cables is essential for industries seeking to optimize their communication performance.
What Is Impedance in Bus Cables?
Impedance is the total opposition a cable presents to alternating current (AC) at a specific frequency. Unlike pure resistance, impedance takes into account both resistance and reactance, which includes inductance and capacitance. It is measured in ohms (Ω), and most industrial bus cables are designed with characteristic impedances of 100Ω, 120Ω or 150Ω depending on the requirements of the communication protocol. To achieve this, bus cables are engineered with controlled impedance design, meaning that conductor spacing, dielectric materials and shielding are carefully managed to ensure constant impedance along the cable’s length.
Examples of Standard Impedances
Different bus systems require different impedance values. PROFIBUS cables are typically designed with 150Ω impedance. CAN-Bus and DeviceNet generally use 120Ω cables. Fieldbus AS-i does not have a fixed impedance standard, but shielding and precise cable geometry still play an important role in performance.
Why Impedance Matters in Bus Cables
When impedance is not properly matched, the signal cannot fully transfer into the receiving device. Instead, a portion of the signal reflects back along the cable. This causes multiple issues. Signal reflection produces echoes that distort the original transmission. These distortions lead to data errors and bit misinterpretations, which disrupt industrial communication protocols. If the degradation becomes severe, devices may drop off the network altogether, resulting in communication failures. Reflections also increase electromagnetic emissions, creating excessive EMI that can interfere with nearby equipment.
Signal Integrity in Bus Cables
Signal integrity refers to a cable’s ability to carry signals without distortion, delay or loss. This is especially critical in industrial environments where motors, welding machines and variable-frequency drives generate strong electrical noise. Several factors affect signal integrity. Impedance matching prevents reflections and helps maintain clean waveforms. Shielding reduces electromagnetic interference. Twisted pair construction cancels out external noise through balanced transmission. Finally, proper termination with resistors, such as 120Ω for CAN-Bus and 150Ω for PROFIBUS, ensures stability at both ends of the network.
Anatomy of a High-Integrity Bus Cable
To protect signals and maintain impedance, high-quality bus cables include several critical layers. The conductors, usually solid or stranded copper, are designed for consistent resistance. Dielectric insulation keeps spacing consistent and controls capacitance. Shielding, which can be foil or braided, blocks electromagnetic interference. An outer jacket provides protection against oil, UV exposure, chemicals and mechanical stress, ensuring long-term durability.
Practical Considerations in Industrial Applications
Cable Selection
Choosing the correct cable begins with matching the impedance requirements of the network. For example, PROFIBUS commonly uses a 150Ω shielded 22 AWG cable. CAN-Bus often requires a 120Ω double-shielded 24 AWG cable. AS-i Fieldbus systems use unshielded 16 AWG yellow cable.
Cable Length and Routing
Cable runs should not exceed the limits specified by each protocol. For instance, CAN-Bus is limited to about 40 meters at 1 Mbps. Sharp bends should be avoided since they can alter the cable’s impedance. Signal cables should also be routed away from high-voltage power cables to minimize interference.
Proper Termination
Termination resistors must be installed at both ends of the bus to prevent reflections. For example, PROFIBUS networks require 150Ω resistors across the cable pairs to ensure clean communication.
Testing and Monitoring
Diagnostic tools are essential for maintaining network reliability. A Time-Domain Reflectometer (TDR) can be used to verify cable impedance and identify mismatches or faults along the length of the cable. Routine monitoring helps catch potential problems before they lead to failures.
Facts About Bus Cable Impedance
- A mismatch as small as 10Ω can reduce signal strength by 20% to 30%.
- Shielding doesn’t directly affect impedance but protects signal integrity by blocking external noise.
- Impedance is designed into the cable geometry during manufacturing. Cutting corners with “generic cables” can cause failures.
- Proper bus cable design allows communication speeds up to 12 Mbps in PROFIBUS and beyond 1 Mbps in CAN-Bus.
FAQs (Frequently Asked Questions)
Q1: What happens if I use a bus cable with the wrong impedance?
A: Mismatched impedance causes reflections, leading to communication errors and device dropouts.
Q2: Can I use Ethernet cables instead of industrial bus cables?
A: No. Industrial bus protocols like PROFIBUS or CAN-Bus require precise impedance and shielding that standard Ethernet cables don’t provide.
Q3: How do I know if my bus cable has signal integrity issues?
A: Symptoms include device dropouts, communication delays and increased error messages. Testing with a TDR or oscilloscope can confirm.
Q4: Why do some cables have double shielding?
A: Double shielding (foil + braid) enhances protection against EMI in harsh environments, maintaining signal clarity.
Q5: How often should bus cables be tested in an industrial setting?
A: At least once annually, or more frequently in high-vibration or EMI-heavy environments.
In industrial networks, impedance and signal integrity go hand in hand. A carefully designed bus cable ensures reliable communication, minimizes downtime and supports high-speed automation. Using the right cable type, whether PROFIBUS, CAN-Bus or AS-I, protects your data signals from degradation.
For long-term reliability, industries should rely on tested, high-quality bus cables like those from TKD Kabel, engineered with controlled impedance, robust shielding and durable jacket materials.
About L-com
Explore the full range of TKD Kabel through L-com. We’ll fill your orders fast, with same-day shipping on all qualified, in stock, online orders received Monday through Friday before 5 p.m. EST. For TKD-related inquiries, connect with our U.S-based TKD expert Eileen Harvey at eharvey@infiniteelectronics.com.
