A long RS-422 HMI run needs termination resistors; the 6 ft panel unit ran without them because its reflections settled before they mattered. The value comes from the cable, never from the run length: set each resistor to the cable's characteristic impedance, typically 100 ohms for multipair instrumentation cable, within 20 percent of that figure. Fit one resistor across each pair at the receiving end.
Moving an HMI from a 6 ft cabinet run to an 80 ft wall run changes the serial wiring question. On the short runs you could leave the RS-422 pairs bare and the link never complained. On the long run, fit termination. The value is set by the cable: read the characteristic impedance off the cable datasheet, buy resistors of that value, typically 100 ohms for multipair instrumentation cable, and put one across each pair at the receiving end. This page gives the value, the standard behind it, where each resistor lands on a four wire link, and the checks that prove the fix.
The value comes from the cable, not from the run length
A termination resistor gives the fast signal edge arriving at the end of a pair somewhere to go. Its value has to look like the cable to the signal, so it equals the cable's characteristic impedance, the figure printed in the datasheet of the reel you actually installed. Typical listed values sit close together: multipair shielded instrumentation cable is commonly listed at 100 ohms and single twisted pair at about 120 ohms. Application notes for TIA/EIA-422-B drivers keep the resistor within 20 percent of that figure, which is 80 to 120 ohms on a 100 ohm cable, and a driver maker's comparison of RS-422 with RS-485 puts the usual range at 100 to 130 ohms.
The controller manual sets the frame. Its wiring instruction is to place termination resistors at both ends of the RS-422 network wiring, matched to the cable's impedance rating, bounded between 100 and 500 ohms. That range covers every cable a customer might pull; the datasheet figure for your reel is the target inside it.
Distance decides whether you fit them, never what value they take. TIA/EIA-422-B application notes recommend termination for the longer cable lengths, and the wall run here is a long one by panel standards. The bare 6 ft units worked because reflections on a pair that short settle before the receiver samples; nothing in that success carries to the longer run. Terminate it.
What an unterminated long pair does
Each symptom below tells you which layer to work on first:
- Framing and CRC errors that come and go, and multiply as you raise the baud rate. The bit time at 9600 baud is about 104 microseconds; at 115200 baud it is about 8.7 microseconds, a twelfth of the settle time for the same ringing, so errors hidden at 9600 show up at the faster rate. This is the classic reflection signature.
- A link that ran clean on the bench at 6 ft and misbehaves once the HMI is mounted far from the controller. Length is the variable that changed, so treat the physical layer first.
- Errors only in values sent from the HMI to the controller. Each direction rides its own pair, so one-sided symptoms point at the missing resistor on that direction's pair.
- Whole-link dropouts in both directions at once. Check the wiring, the port and the addressing before touching a resistor; Modbus RTU master troubleshooting walks the serial-side causes a resistor cannot reach.
- A stable link with values that are consistently wrong. Termination cannot cause or cure that; it is a scaling or tag mapping problem, covered in HMI shows the value ten times too small: fix the scaling.
Where the resistors go on a four wire link
Four wire RS-422 is full duplex: one pair carries the controller's transmit data to the HMI, a second pair carries the HMI's transmit data back. Each pair gets one resistor, across its two conductors, at the farthest receiving end of that pair: at the HMI for the controller's transmit pair, at the controller for the HMI's transmit pair. That puts one termination at each physical end of the link, which is what the manual's both ends instruction describes.
Multidrop wiring is the different case: when two or more drivers share one pair, each end of that pair gets a termination. Point to point RS-422 is not that case.
Three placement rules:
- Fit the resistor across the pair, conductor to conductor, never from a conductor to the shield or to ground.
- Check the HMI connector documentation first. Some connectors ship with a termination resistor fitted behind a jumper; an enabled one counts as the termination for that end, and adding a second beside it halves the resistance and doubles the driver load. The example below runs that arithmetic.
- Land the cable shield the way your plant standard specifies; the tradeoffs between grounding the shield at one end or both ends are set out in Instrumentation cable shield grounding: one end vs both ends.
Isolate power to the HMI and the controller before opening any connector, following the site's lockout procedure.
Example: sizing the resistors for the long run of 100 ohm multipair cable
Inputs, with where each comes from:
- Run length: 80 ft, the job at hand, an HMI mounted that far from the controller. This is the only place the length matters, and it matters only for the decision to terminate, not for the value.
- Cable: multipair shielded instrumentation cable with a datasheet characteristic impedance of 100 ohms, the common listing for this cable type. Confirm the figure on the reel actually installed.
- Acceptance window: within 20 percent of the cable impedance, per TIA/EIA-422-B application notes.
- Controller manual range: 100 to 500 ohms, the controller's wiring instruction for RS-422 network wiring.
- Driver swing: 3 V differential, an illustrative figure for the load check.
The rule in words: set each resistor equal to the cable impedance, one per pair at the receiving end, and confirm the value sits inside both the 20 percent window and the manual range. An enabled internal resistor counts as the termination for its end, with nothing added beside it.
The arithmetic:
- Resistor value: RT = Zo = 100 ohms.
- Window: 100 x 0.8 = 80 ohms; 100 x 1.2 = 120 ohms.
- Intersect with the manual range, 100 to 500 ohms: the passing band is 100 to 120 ohms. Buy 100 ohms.
- Duplicate check: if the HMI connector's internal 100 ohm resistor is enabled and a second 100 ohm resistor is added at that end, the parallel value is (100 x 100) / (100 + 100) = 50 ohms, below the 80 ohm floor and half the intended load. Remove the added resistor.
- Driver load check: 3 V / 100 ohms = 30 mA per terminated pair, so two pairs add 60 mA. RS-422 drivers are built to feed a terminated line, so this load is by design.
Result: 100 ohm resistors, one across each pair, fitted at the receiving ends.
Decision: on this cable any resistor between 100 and 120 ohms satisfies both limits, and 100 ohms is the value to buy. If the reel's datasheet instead lists 120 ohms, the window moves to 96 to 144 ohms and 120 ohm resistors are the pick. Both common cables give a clean answer inside every range.
Install, then prove it
- Pull the datasheet for the installed cable reel and note the characteristic impedance. Check the HMI connector documentation for a fitted, jumpered termination resistor.
- Isolate power per the site lockout procedure, then fit one resistor per pair at its receiving end, across the pair's two conductors.
- Record on the panel drawing which ends got resistors and whether an internal one is enabled.
- Re-energize, restore the link, and run the machine through a full cycle while watching for dropped or garbled updates at the production baud rate.
- Compare each displayed value with the controller's live value. A stable link with matching values is the proof the fix landed.
- If the link stays unstable, work the serial side in order: pair polarity and continuity, port and protocol configuration, then the value arithmetic again. A long programming cable is a different electrical problem with its own limits, covered in Extend an RS-232 HMI cable by 10 feet safely.
Recommendation: terminate the long run. Fit one resistor per pair at its receiving end, equal to the cable's listed characteristic impedance, kept within 20 percent of the datasheet figure and inside the controller manual's 100 to 500 ohm range. Leave the bare 6 ft cabinet units as they are, but write the resistor value on the panel drawing so the next time the HMI moves, the termination moves with it.
Questions
The cable datasheet lists 120 ohms instead of 100. What changes?
The resistor value moves with the cable. Fit 120 ohm resistors, one across each pair at its receiving end. The 20 percent window around 120 ohms runs from 96 to 144 ohms, and 120 ohms also sits inside the 100 to 500 ohm range the controller manual allows, so a standard 120 ohm resistor clears both checks. A 100 ohm resistor left over from the last job is 17 percent low on this cable, inside the window but off the datasheet value, so buy the listed 120 ohm part.
What power rating does a termination resistor need?
Work it from the drive swing. At an illustrative 3 V differential swing across 100 ohms, current is 3 divided by 100, which is 30 mA, and heat is current squared times resistance: 0.03 x 0.03 x 100, which is 90 mW. A quarter-watt film resistor runs at about a third of its rating there. A driver swinging 5 V doubles the current to 50 mA and the heat to 0.25 W, right at a quarter-watt part's limit, so step up to half a watt.
Errors multiply when the baud rate goes up. What does that show?
It points at the physical layer, at the cable and its termination. Halve the baud rate as a test: if the errors disappear at the slower speed, settle time was the problem and the termination fix is the cure, along with a check of pair polarity and the shield landing. If the errors persist at half speed, the wiring and data side come first: continuity and polarity of each pair, then addressing and tag mapping.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.