Wiring one HMI panel to three PLCs on an RS422 multidrop

What you need

• Cable: one run of shielded twisted-pair cable with at least two twisted pairs, 24 AWG, at 100 Ω or 120 Ω characteristic impedance. Published lab work behind manufacturer application notes on RS-422 and RS-485 network configurations commonly used 100 ft of 100 Ω, 24 AWG twisted pair, so that cable is a sensible pick for a shop-floor chain; your port manuals name the parts each maker recommends.
• Two termination resistors, one per physical end, each matched to the cable: 100 Ω on 100 Ω cable, 120 Ω on 120 Ω cable.
• A terminal-block style connector or breakout at each node. Every intermediate PLC lands two cables on the same terminals, the pair arriving from upstream and the pair continuing downstream, and screw terminals make that possible without soldering a one-off plug.
• An RS232-to-RS422 converter for any PLC whose port is RS232 only.
• The pin table for every port. Differential pair naming on RS422 ports is not standardized across makers, so read each port's own table before landing anything.
• Power off. Kill power at the panel and at every PLC before you land a conductor, and leave it off until the ring-out check passes.

Wire the chain, step by step

1. Check each port's mode. Confirm the serial port supports RS422, and where a port offers selectable RS232 and RS422 modes, set the RS422 mode by that port manual's procedure. Where a port is RS232 only, plan the converter at that node.
2. Land the panel end. Identify TX+, TX-, RX+, RX- and the signal ground pin on the panel's serial connector from its manual, and label the terminals before wiring. If the panel hangs on a wall with little clearance behind it, a right-angle serial adapter keeps the plug inside the enclosure's depth.
3. Chain the PLCs. Run cable from the panel to PLC 1, from PLC 1 on to PLC 2, and from PLC 2 on to PLC 3. A multidrop is a chain, not a star: the line runs straight through every junction. Cross the pairs so the panel's transmit pair lands on every PLC's receive terminals, and every PLC's transmit pair lands on the panel's receive terminals. Keep each stub of cable inside an enclosure as short as the routing allows.
4. Bond the shield at one point. Run the shield through the whole chain unbroken and connect it to earth ground at the panel end only. If a port manual requires a signal ground reference, run signal ground on its own conductor inside the cable. The shield never carries signal current.
5. Terminate the two physical ends only. One resistor across RX+ and RX- at the panel, which ends the line the PLCs drive, and one across RX+ and RX- at PLC 3, which ends the line the panel drives. The middle PLCs take no resistor. A terminator in the middle loads the line and steals drive margin from the two ends that need it.

Give each PLC a station address

1. Connect to each PLC one at a time with its programming software and set the serial port's station number: 1, 2 and 3 across the three PLCs. Three PLCs left on the same factory station number answer the panel together, and two drivers driving one shared pair at the same time corrupt each other's output.
2. Match the remaining serial settings on all four nodes: the protocol the panel's driver expects, plus the same baud rate, parity and stop bits everywhere. A mismatch here gives a silent line, not a noisy one.
3. In the panel project, configure one serial port for the PLC driver and write tags that carry the station number in the address. One port entry, three station addresses, and the PLC 1, PLC 2 and PLC 3 tags stay separate.
4. Apply each PLC's new port settings the way its manual requires, power-cycle where the manual says to, and label each enclosure with its station number while you are at the cabinet.

Example: three PLCs at 15 ft spacing

Inputs: the panel is on a wall column, PLC 1 sits 15 ft away in the main enclosure, PLC 2 15 ft beyond it, PLC 3 15 ft beyond that (the chain layout above). The cable is shielded twisted pair, 100 Ω characteristic impedance, 24 AWG, the same cable the application-note lab work ran over 100 ft. The two termination resistors are 100 Ω each. Station addresses 1, 2 and 3 are illustrative.

Rule in words: an RS422 multidrop is one electrical line per direction. Each line terminates at its far physical end with a resistance equal to the cable's impedance. TIA/EIA-422 defines the line for one driver and up to 10 receivers.

Arithmetic: total run is 15 + 15 + 15 = 45 ft of cable. The panel's transmit line carries one driver and three PLC receivers, 3 of the 10 the standard allows, seven spare. Termination is two resistors in all, one 100 Ω part at each physical end. Drive check: an RS-422 driver must deliver at least 2 V across a 100 Ω terminated load, so each end resistor dissipates 2² / 100 = 0.04 W, far inside a common 0.25 W part. Distance check: 45 ft is under half the 100 ft that lab work ran, and the standard's rate-distance curve allows about 4000 ft at 100 kbit/s and about 40 ft at 10 Mbit/s; polling three PLCs runs at ordinary serial baud rates, the slow, long-distance end of that curve.

Result: a 45 ft chain with three receivers on one driver, two 100 Ω end terminations and the shield bonded at the panel end is a fully specified RS422 network with margin everywhere.

Decision: at 45 ft and three nodes, wire it exactly as above. The only extra hardware is a converter at any RS232-only port, and no network card enters the build.

Check that it works

1. Ring it out dead. With power still off, continuity-check every conductor end to end. Each conductor must reach the terminal its label names, and nothing may short to the shield.
2. Bring up one PLC at a time. Power the panel and PLC 1 only, confirm the panel reads PLC 1's registers, then add PLC 2, then PLC 3. Address and setting errors surface with one node online, where they are easy to name.
3. Force three different values into the same register. Write 1 into that register in PLC 1, 2 into PLC 2 and 3 into PLC 3. The panel's three on-screen fields must read 1, 2 and 3. Identical values across all three fields mean a duplicated station address in a PLC or in a tag.
4. Watch a minute of live polling with all three nodes running: fields refresh in order and none holds a stale number. If values arrive but drop out under load, the polling side of that fault is covered in fix dropped PLC-triggered messages on a serial HMI panel.

If the line stays dead

A line that passes the ring-out and still shows nothing points at polarity first: a differential pair fails quietly when one conductor is swapped, and swapping the pair at one end costs one minute. The com-light side of a dead serial link has its own checklist in HMI to PLC serial timeout with a dead com light.

When to take the Ethernet route

Ethernet wins in two cases: a programming PC must sit on the same network as the panel and the PLCs and reach them all at once, or a serial port has no RS422 path even through a converter. It needs an interface at every node plus a switch. For three PLCs shown side by side on one panel, the serial multidrop is the simpler build: two resistors and one reel of twisted pair against an interface at every node.

Recommendation: Build the four-wire RS422 daisy chain for a one-panel, three-PLC job: one shielded twisted pair per direction, terminations only at the head and tail and matched to the cable impedance, the shield bonded to earth at the panel end, and a unique station address on every PLC. Take the Ethernet route only when a programming PC must join the panel and the PLCs on the same network, because that decision needs an interface at every node.

Questions