In the DeviceNet industrial communication system, we often encounter such problems: the network is sometimes good and sometimes bad, the signal is occasionally lost, and even the device is frequently disconnected. Many engineers attribute the problem to interference, cable quality, or node failure, but ignore a key and low-key component - the terminating resistor. Don't underestimate this small resistor, it is the "invisible hero" who ensures the stable communication of the DeviceNet system.
1. Why must the DeviceNet system use terminal resistors?
DeviceNet uses differential signal transmission technology based on the CAN bus. In order to ensure signal integrity and prevent reflection and interference, a terminal resistor (typical value is 121Ω) must be installed at both ends of the bus.
If the termination resistor is missing, it will cause:
- Signal reflection: the data packet is reflected back to the trunk line at the end of the bus, causing interference;
- Communication abnormality: node recognition failure, data conflict or increased bit error rate;
- System instability: disconnection or "freeze" occurs when the network load is slightly higher;
- Difficult debugging: faults occur irregularly, and it is difficult to locate the source of the problem.
2. Correct installation position of terminal resistors
According to ODVA's DeviceNet network specification, terminal resistors should be installed at both ends of the network trunk line, namely:
- One resistor is installed at the end of the master station (such as a PLC controller)
- The other resistor is installed at the end of the branch where the farthest slave device is located
3. Recommended terminal resistor models
Common terminal resistor modules on the market, such as Turck's RSM 57-TR2, have been widely used in DeviceNet systems, with advantages such as plug-and-play, standardized design, and IP67 protection level. Its typical configuration is:
- Resistance value: 121Ω ±1%
- Interface type: 7/8"-16UNF 5-pin male connector
- Installation method: plug-type module, adapted to DeviceNet trunk line
- Features: solder-free, waterproof and dustproof, fast on-site deployment
However, such products usually have problems such as long delivery time, high price, and insufficient customization flexibility.
Premier Cable provides DeviceNet terminal resistor alternatives with equivalent electrical performance and mechanical compatibility, and supports:
- Customized line length and connector type (such as M12 A-coded or 7/8")
- One-piece waterproof injection molding structure
- Industrial grade IP67 protection
- Large-volume fast delivery and small-batch customized production
4. Common Installation Errors and Troubleshooting Recommendations
| Installation Error | Description | Symptoms | Troubleshooting Recommendations |
|---|---|---|---|
| Only one termination resistor installed | Only one 120Ω resistor used instead of two | Unstable communication, intermittent data loss | Ensure a 120Ω resistor is installed at both the master (PLC) and the farthest I/O module |
| Termination resistor placed in the middle of the bus | Resistor not located at network endpoints | Devices fail to respond or show communication faults | Move resistors to correct positions: one at each end of the trunk line |
| More than two termination resistors installed | Over-termination lowers the total network impedance | Bus cannot start, abnormal voltage readings | Remove excess resistors, leaving only two - one at each network end |
| Incorrect resistor value (not 120Ω) | Using 100Ω, 150Ω, or other values | Signal reflection, bus instability | Replace with proper 120Ω resistors designed for DeviceNet use |
| Loose or poor contact in connector | Improper tightening or worn-out connectors | Intermittent signal, voltage drop, device disconnection | Use high-quality connectors (e.g., Premier Cable M12 or 7/8" DeviceNet cables); check all terminations |
| No termination resistors installed | System lacks any impedance matching | Network not operational | Install one 120Ω resistor at each end of the DeviceNet trunk line |
| Resistors installed on drop lines (not trunk) | Incorrect placement on branch lines or device stubs | Weak signal, random communication errors | Resistors must only be installed on the two ends of the trunk cable |
5. Summary: Small accessories, big role
In the DeviceNet system, the cable is the skeleton, the connector is the joint, and the terminal resistor is the "nerve center" that maintains the balance of communication. Don't let a small resistor that is ignored drag down the stability of the entire automation system.
Premier Cable provides DeviceNet special cables (M12 and 7/8" interfaces) that meet ODVA standards, as well as a full set of connector products that adapt to the installation requirements of terminal resistors. We support special scene requirements such as customized length, customized branching, oil resistance and corrosion resistance. Please contact us for technical information or samples.
