Stay on RS-232 while the HMI sits within 50 ft of the PLC at the standard rate, the reach the interface standard assumes with typical cable. Longer runs, higher baud rates or a route near drives call for RS-422 over shielded twisted pair, which carries HMI-speed links to about 1200 m.
Keep an HMI to PLC link on RS-232 only while the panel sits within 15 m (50 ft) of the PLC at 9600 baud. That is the reach the interface standard assumes, and higher baud rates or heavier cable shrink it fast. Past that, or wherever the route runs beside drive output or contactor wiring, switch to RS-422 over shielded twisted pair. TIA-422 carries HMI serial speeds to about 1200 m, so one cable plan covers a panel next to the enclosure and one at the far end of the line.
What decides the interface
Three things set the choice: how far the run goes, how fast it clocks, and what else is in the tray.
RS-232 is unbalanced. Every signal rides one conductor against a shared ground wire, so a voltage difference between the panel's ground and the PLC's ground lands straight on the signal. Distance hurts twice: cable capacitance slows each edge until the receiver reads it late, and a long unbalanced line picks up whatever the room radiates.
RS-422 is balanced. Each signal travels as the voltage between the two wires of a twisted pair. Noise that arrives on both wires cancels at the receiver. That is why TIA-422, the balanced counterpart to TIA-232, is rated to roughly a kilometer while RS-232 stops near the 50 ft mark.
The RS-232 limit and the arithmetic behind it
TIA-232 charges the cable as a capacitance, and the driver is specified for a total load of 2500 pF. The familiar reach figure is that budget divided by typical cable: 50 ft at 50 pF per foot gives 50 x 50 = 2500 pF, and 50 ft is about 15 m.
The budget is the working number, so reach in feet = 2500 pF / capacitance per foot. Cable rated 25 pF/ft doubles the reach to 100 ft (30 m). Baud rate cuts it the other way. Reach figures published for real-time RS-232 data are 61 m (200 ft) at 4800 baud, 30 m (100 ft) at the standard rate, 15 m (50 ft) at 19200 and 7.6 m (25 ft) at 38400, all on low-capacitance cable in a quiet run.
Decision line: on a clean route at the standard rate, plan RS-232 up to the reach the budget allows, and stretch to 30 m only when you know the cable capacitance. Protocol changes nothing here; the DF1 version of the question is worked through in how far an HMI can be from the PLC on DF1 RS-232. A short stretch inside the budget is a different job, covered in extend an RS-232 HMI cable by 10 feet safely.
What RS-422 buys
TIA-422 sends each direction over its own twisted pair and holds full rated reach while the data rate stays at or below 100 kbps. HMI links sit far inside that: the 9.6 kbps of the standard rate is roughly a tenth of the ceiling, and 38.4 kbps is still under it. A link pushed to 115200 baud sits just above, so expect less than the maximum there.
Pull cable built for the job: shielded twisted pair, one pair per direction, characteristic impedance around 120 ohms. Termination belongs on long runs, and the value follows the cable and the port; RS-422 termination resistor value for a long HMI cable works that number out.
RS-422 is also the doorway to a multidrop, where one panel port feeds several receivers; wiring one HMI panel to three PLCs on an RS422 multidrop shows the arrangement.
Options at a glance
| Setup | Reach at the standard rate | Pick it when |
|---|---|---|
| RS-232 on a ready-made 3 m assembly | 3 m, as built | Panel hangs beside the PLC, no extension planned |
| RS-232 on typical cable, 50 pF/ft | 50 ft | Run inside the standard reach, tray clear of power |
| RS-232 on low-capacitance cable, 25 pF/ft | 100 ft (30 m) | Run lands in the 15 to 30 m band and the cable spec is known |
| RS-422 on shielded twisted pair | About 1200 m | Run past 30 m, rate above 38400 baud, or route passes drives |
| Serial to fiber converters | Converter rating, commonly 2 km or more | Run past the copper reach or building to building |
Example: 100 ft to the operator station, drive feeder in the tray
Scenario: the HMI is in the operator station at the end of a packaging line. The PLC panel sits 100 ft (about 30 m) away, and for the last 33 ft (10 m) the only clear tray path shares with a 480 V drive feeder. The link runs at the standard rate.
RS-232 check. Typical cable at 50 pF/ft loads the driver to 100 x 50 = 5000 pF, twice the 2500 pF budget, so the interface is out on capacitance alone. Getting back inside the budget takes 40 ft or less of that cable: 40 x 50 = 2000 pF. The layout puts the panel where it is, so shortening the run is off the table.
RS-422 check. The rate is 9.6 kbps, about a tenth of the 100 kbps ceiling that comes with the interface's rated reach, and 100 ft is about 2.5 percent of 1200 m. Length is trivial for the interface. The drive feeder is the real risk, and a twisted pair cancels voltage that lands on both wires equally.
Decision: pull shielded twisted pair, one pair per direction, land the shield the way the panel and PLC wiring diagrams show, and set both ends to the original rate on the RS-422 interface. Terminate the receiver end and retest reads and writes with the line in a safe state before production.
When to pick something else
Fiber, when distance or ground potential rules out copper. A serial to fiber converter pair carries the link well past the copper reach; ratings are commonly 2 km or more and are printed on the converter data sheet. Pick it for building to building runs, or anywhere the two ends sit on grounding systems a copper shield would tie together.
RS-485, when several devices share one run. It is the multi-driver member of the same electrical family, and the multidrop arrangement linked above is the pattern for one panel serving several PLCs.
What to ask for
Collect before you order cable or cut anything:
- the PLC port documentation, showing whether that port offers RS-422 and on which pins. A breakout cable exposes the signals a port already has, so the interface has to exist at the port first
- the panel's wiring diagram for its RS-422 connection, including where the shield lands
- the current settings on both ends, written down, in case the change has to be rolled back
- the termination the port documentation calls for, sized to the cable you buy
- a cable spec: shielded twisted pair, one pair per direction, data grade for the run length
Keep the old short assembly. It is the fastest way to confirm the panel and PLC still talk before you blame the new run.
Recommendation: on a clean route at the standard rate, keep RS-232 while the run fits the capacitance budget; allow the 15 to 30 m band only with cable you know is 25 pF/ft or better; past 30 m, above 38400 baud, or anywhere the route passes drives or contactors, pull shielded twisted pair and run RS-422, which carries the rest to full rated reach.
Questions
Can I fix a long RS-232 run by raising the baud rate?
Higher speed shortens the reach, it never extends it. The published reach for real-time RS-232 data is about 15 m (50 ft) at 19200 baud and 7.6 m (25 ft) at 38400, and the capacitance budget of 2500 pF stays the same. If the run is long, change the interface, not the clock rate: RS-422 at the original rate sits far inside its rated envelope.
What cable do I pull for an RS-422 HMI run?
Shielded twisted pair, one pair per direction, with a characteristic impedance around 120 ohms. Keep it out of the same tray section as motor and drive output wiring, and cross any unavoidable power run at right angles. Land the shield the way the wiring diagrams for your two devices show; the right end for the shield depends on how each port grounds it.
Can a passive adapter convert an existing RS-232 cable to RS-422?
Electrical conversion takes an active converter, powered from its own supply or from the port's handshake lines, placed at the RS-232 end. A passive box that only reroutes pins has no line driver behind it, so it survives only over short, quiet runs and defeats the purpose of the change. Convert once, at the RS-232 end, then run shielded twisted pair the whole way.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.