An RS-485 device might have a small switch labeled TERM, EOL, 120Ω, or simply S1. That switch may enable a termination resistor rather than changing the device’s network address. Setting it incorrectly can cause intermittent communications, especially on long cables or at higher data rates.
The short answer is that a conventional linear RS-485 bus normally needs correctly matched termination at the two physical ends, not at every device. The exact switch position and whether termination is already built in depend on your hardware, so use the equipment manual before making changes.
Quick Answer: Turn Termination ON at the Bus Ends
Think of the RS-485 trunk as one main cable with devices attached along it. The devices at the two farthest physical ends are typical locations for termination when the installation needs it. Devices in the middle generally leave their selectable termination disabled.
A common value is 120 ohms because many twisted-pair cables used for RS-485 have a nominal differential characteristic impedance near 120 ohms. Termination is intended to limit signal reflections caused by a mismatch between the cable and its electrical load.
These are common design rules, not permission to blindly flip a switch. Some equipment contains a permanently installed termination resistor, some uses jumpers or software settings, and others are designed for short links that can work without added termination.
- End of a conventional linear RS-485 trunk: enable the correct termination if not already present.
- Intermediate node: leave selectable end-of-line termination off in the normal two-end arrangement.
- Unknown switch function or network topology: consult the manufacturer’s schematic before changing anything.
What an RS-485 Termination DIP Switch Actually Does
On supported products, a termination switch connects a resistor across the differential signaling pair. This is different from an address switch that represents a binary value. The term DIP describes the small physical switch package; it does not describe the electrical function behind it.
Some manufacturers label the relevant control with TERM or EOL. Others use numbered DIP switches and publish a table in their installation guide identifying each position’s purpose. An unlabeled SW8, for example, cannot safely be assumed to enable a resistor.
Always confirm which physical direction means ON. On a miniature switch, the moving lever may be on the opposite side of the board from where you expect, and an incorrectly oriented photo can make a good wiring diagram misleading.
Example: Three Devices on One RS-485 Bus
Suppose a Modbus RTU controller communicates with two field devices in a linear daisy-chain: Controller A, Device B, and Device C. If the controller sits at one end and Device C at the other, those two physical ends are the usual termination points. Device B is in the middle.
In a typical implementation with matching cable and two 120-ohm end terminations, Device B’s selectable termination would remain off. If Device A is already internally terminated, adding another separate resistor there can create an unintended extra load.
The position on a drawing is what matters. A controller might appear at the top of a schematic while actually being installed physically in the middle of the cable. Trace the cable run, not just the logical master/slave roles.
- Controller A, physical start: one appropriate terminator.
- Device B, in the middle: no additional end terminator.
- Device C, physical end: one appropriate terminator.
Why Too Many Termination Resistors Can Cause Problems
Two 120-ohm resistors connected across the same differential bus create an effective 60-ohm parallel resistive load, ignoring other equipment loading. That loading is anticipated in many RS-485 driver designs. Adding an extra 120-ohm termination lowers the equivalent resistance further.
Excessive loading may reduce differential voltage margin or prevent a marginal driver from meeting its operating conditions. The symptoms can look like bad wiring, an intermittent device, or a software configuration problem.
Texas Instruments discusses how RS-485 drivers, receiver loading, and line termination interact. If an installation becomes less reliable after changing a DIP switch, check whether that position actually adds a resistor, and whether more than two end terminators are active.
Why Missing Termination Can Also Cause Problems
Fast signal transitions travel down the cable and can reflect at an impedance discontinuity. Those reflections may interfere with later transitions, which is particularly troublesome when the propagation delay along the cable becomes significant compared with the signal’s timing.
Long trunk lines, higher data rates, and noisy industrial environments increase the importance of sound signal integrity. A short, low-speed link may work without explicit termination, but that does not justify removing the resistors from every field installation.
Texas Instruments’ RS-485 basics guidance explains both the benefits of proper termination and cases where it may not be essential. The correct decision depends on actual cable characteristics, signal timing, device design, and manufacturer instructions.
Termination, Biasing, and Addressing Are Three Separate Settings
Termination matches the transmission line impedance at the ends of the trunk. Fail-safe biasing helps establish a known receiver state when no driver actively drives the bus, depending on the transceiver design. Addressing identifies devices at a higher protocol layer such as Modbus.
A single equipment housing may have separate switch controls for all three features. Enabling bias resistors at every device is not the same as enabling termination at both ends. Some modern transceivers provide internal failsafe behavior while older devices may require different measures.
If your device has a bank of eight switches, do not assume all eight are address bits. Read the legend, find the manufacturer’s electrical interface diagram, and confirm the electrical meaning of every numbered position.
RS-485 Troubleshooting Checklist
First document the actual cable path and identify the two physical endpoints. Note whether the bus uses daisy-chain wiring, whether long stubs branch away from the trunk, and whether external adapters have built-in resistors. Many problems attributed to addresses are really wiring or physical-layer issues.
Next compare the baud rate, parity, stop bits, and protocol settings at every participating device. Confirm that each Modbus RTU subordinate has an appropriate unique serial address; see our Modbus DIP Switch Address Calculator for a general binary address reference.
If appropriate for the device, have a qualified technician inspect the powered-down bus resistance and physical connections. The result depends on connected components, so a single resistance reading is not proof of a healthy installation. Avoid probing unknown energized wiring.
- Check that only the intended physical endpoints are terminated.
- Identify any permanently installed or software-controlled termination.
- Verify consistent serial parameters and no duplicate application addresses.
- Use a suitable twisted pair with short stubs as specified for the installation.
- Keep termination and biasing roles distinct.
- Follow lockout, isolation, and site-specific electrical safety procedures.
What About BACnet MS/TP?
BACnet MS/TP also uses a shared serial data link in many building automation systems, so proper trunk design and end termination matter. However, the BACnet MAC address rules are separate from the resistor switch that terminates the RS-485 line.
If you need help converting an MS/TP MAC address to a seven-switch pattern, use our BACnet MS/TP DIP Switch Address Calculator. It includes narrowly defined model profiles and warns that a manufacturer may reserve some switch positions for other functions.
Different vendors may also specify shield grounding, reference conductors, allowed stub length, and commissioning practices. Treat the relevant controller’s current installation guide as the final authority.
RS-485 Is the Electrical Layer, Not a Programming Language
RS-485 defines important electrical characteristics used by many serial communication networks. Modbus RTU is one protocol that often uses RS-485 hardware; BACnet MS/TP uses RS-485 for its common physical connection as well. Their address schemes and diagnostic tools are not the same.
That distinction is useful when identifying DIP switches: a resistor switch belongs to electrical installation, while a Modbus or BACnet address bank serves protocol configuration. Changing the wrong control can create a second problem even when the original device address was correct.
For foundational context, read What Is RS-485 and Why Industrial Networks Still Use It and Modbus RTU vs Modbus TCP.
Frequently Asked Questions
Should RS-485 termination always be ON?
No. The usual linear bus has termination at the physical ends and no extra termination at intermediate nodes. Some short links or device-specific designs have different requirements.
Is a 120-ohm resistor always the right value?
120 ohms is a common nominal value, but the termination should suit the cable’s characteristic impedance and the manufacturer’s electrical specification.
Can DIP switch 8 control RS-485 termination?
On some devices it can, but on others it controls address, protocol, or a special mode. Never infer the function from the number alone.
Can the same switch set the Modbus address and line termination?
A manufacturer could implement a combined configuration scheme, but termination and addressing are separate functions. Use the device’s exact switch table to understand its design.
What happens if every RS-485 device has termination enabled?
The combined resistive load can become too low, potentially reducing signal margin and interfering with communication. Check the hardware’s internal and external terminators.
Sources and Further Reading
- Texas Instruments: RS-485 Basics, when and how to terminate
- Texas Instruments: RS-485 links without termination
- Texas Instruments: reference design for selectable bus termination
- Analog Devices: MAX485 termination and failsafe guidance
Editorial note: This independent educational guide is not manufacturer support. Confirm your exact equipment model and follow the original operating and electrical safety instructions.