One RS-485 bus carries one HMI panel and four PLCs when the panel is the only bus master and every PLC answers at its own station address. Run the bus as a daisy chain, terminate the two cable ends at 120 ohm, keep each drop under 1 ft, and fit a signal isolator at each node. The planning limit is 4,000 ft of cable at 100 kbps.
Four PLCs and one operator panel belong on one serial cable when that cable is an RS-485 bus. The panel is the bus master: it sends every request. Each PLC answers as a slave at its own station address, so address 1 through address 4 select the machines one at a time. Wire the bus as a daisy chain, terminate it at the two cable ends, and fit a signal isolator at each node that sits on a separate machine or power feed. The planning limit is 4,000 ft of cable end to end at 100 kbps. This page gives the wiring order, the address plan, the bring-up test and the arithmetic for the poll cycle.
One master, four addressed slaves
An RS-485 bus in its usual form is half duplex on one twisted pair: two wires, labelled A and B, shared by every node, with one node transmitting at a time. The panel asks; the PLC whose station address matches the request replies; the other three stay quiet. That addressing is what a bundle of point-to-point cables lacks: fork one RS-232 port with a splitter and you get four cables but no way to select a machine.
Keep the panel as the only master. Two masters polling the same slaves transmit over each other and both lose; why two Modbus RTU masters collide on one RS-485 bus walks through that failure. A second operator panel is a second master, so it stays off this bus.
Recommendation: when one panel must serve several PLCs over serial, build one RS-485 daisy chain with the panel as the only master, one unique station address per PLC, a 120 ohm termination resistor at each of the two cable ends, and a signal isolator at every node on its own machine or power feed.
What you need
- Twisted shielded pair with a characteristic impedance near 120 ohm, cut for the route plus a service loop.
- Two 120 ohm resistors, one for each cable end.
- One signal isolator per node: five total, one at the panel and one at each PLC.
- A screw-terminal adapter for the panel's serial connector, if you prefer terminals to soldering a short cable.
- The serial settings sheet for the PLCs: protocol name, baud rate, parity, stop bits, station address range.
- A small DC feed for each isolator's field side; a DIN-rail supply in the cabinet keeps that wiring tidy.
- One scratch register in each PLC, loaded with a value you know, for the bring-up test.
Wire the bus in this order
- Set one serial format for all five nodes before any wiring: 19,200 bps, 8 data bits, even parity, 1 stop bit, the Modbus RTU default.
- Give each PLC a station address: 1, 2, 3 and 4. Write the number on each cabinet door.
- Run one continuous cable from the panel past PLC 1, PLC 2, PLC 3 and PLC 4 in that order. Break the cable at each node so every drop stays under 1 ft; long stubs reflect signal edges back down the pair.
- Carry the shield through every node and bond it to ground at the panel end only.
- Insert one isolator at each node. Isolation is the safety item: machines on separate power feeds sit at different ground potentials, and a difference outside the -7 to +12 V common-mode window the standard defines for the bus pins will damage a port. The isolator breaks the copper path so that difference stops at its barrier.
- Fit the 120 ohm resistors across A and B at the panel end and at PLC 4's end. Two resistors total, never mid-bus.
- Point the panel's protocol driver at the PLCs' protocol and enter addresses 1 through 4 in its poll list. Match the driver against the PLC protocol first; check the panel driver list before tuning serial settings is the short version of that check.
The 4-wire variant
Some PLC serial ports expose separate transmit and receive pairs. Those ports form a full-duplex, 4-wire bus: the panel's transmit pair fans out to every PLC's receive pair, and every PLC's transmit pair returns to the panel's receive pair. Each connection crosses the pairs, TX+ to RX+ and TX- to RX-, per the wiring charts in the vendor application notes for RS-485 master-to-multiple-slave layouts. Terminate the panel end of the return pair and the far end of the transmit pair. For the node-by-node layout of this arrangement, read one HMI panel and three PLCs on an RS-422 multidrop.
Bring the network up one PLC at a time
- Connect the panel and PLC 1 only. Poll address 1 and read the scratch register. The value you loaded should appear.
- Add PLC 2 and poll addresses 1 and 2. If address 2 fails while address 1 still reads, check PLC 2's station address first, then its drop wiring, then its format.
- Repeat for PLC 3 and PLC 4, one at a time. The newest address is always the suspect.
- If every address fails while the cable tests clean, compare the format at both ends and the driver against the PLC protocol. A port that answers a scan yet fails every read points at the driver; when a panel reaches the PLC but every read fails covers that case.
- Leave the panel polling all four addresses for a full shift with the machines running. An error count that climbs only when large contactors kick in points at a missing isolator or a loose shield bond.
Example: four PLCs spread down a 1,000 ft line
Say the panel hangs at the head of an assembly line and the four PLCs stand 250 ft apart. The trunk is 4 x 250 ft = 1,000 ft. The standard RS-485 design guides plan 4,000 ft of cable at 100 kbps, so the run fits with 3,000 ft to spare even before dropping to 19.2 kbps, where the reach is longer. Termination is the two 120 ohm resistors at the ends.
Now the poll arithmetic. Each cycle the panel reads 10 holding registers from each PLC. One request is 8 bytes; one reply is 25 bytes: 1 address + 1 function + 1 byte count + 20 data bytes + 2 CRC. Per PLC that is 8 + 25 = 33 bytes.
At 19,200 bps with even parity the frame carries 11 bits per byte (start, 8 data, parity, stop), so the bus moves 19,200 / 11 = 1,745 bytes/s. One poll takes 33 / 1,745 = 18.9 ms, call it 19 ms. Add a 10 ms turnaround for the PLC to compose its reply: 29 ms per slave. Four slaves: 4 x 29 ms = 116 ms per full cycle. Polling each address every 250 ms keeps the bus busy 116 / 250 = 46 percent of the time, and every value on the panel is at most a quarter second old.
Those are illustrative inputs; your register counts and the panel's turnaround time set the real cycle. For scale, published RS-485 quick references top the interface at 10 Mbps, a rate reached only on short runs; a plant floor of this length lives comfortably at 19.2 kbps.
How to check it before you close the panel
- Write 1234 into PLC 3's scratch register and read all four addresses. Only address 3 should return 1234; if address 1 shows it too, two nodes share an address.
- Toggle one bit at each PLC in turn and watch it change on the panel within one poll period, 250 ms in the example.
- Power the panel down and back up and confirm it resumes polling all four addresses with no operator action.
- Re-read the panel's error counter after a shift with the machines running. A steady zero closes the job.
- Record the addresses, the serial format, the cable route, the isolator locations and the termination positions in the maintenance log.
When a different design fits
One PLC and one panel is a point-to-point job, and an RS-232 port does it with a straight cable; connecting a Modbus RTU HMI to a legacy RS-232 port shows that wiring. A run longer than 4,000 ft takes an RS-485 repeater at mid-span: a 6,000 ft route becomes two 3,000 ft segments, each inside the design guide's limit at 100 kbps, and each segment gets its own two end terminations. A floor-facing status board is a different screen with its own content feed, which is what our shop-floor screens are built for, while the serial bus stays dedicated to the panel's polling.
Questions
How many PLCs can one RS-485 segment carry?
The standard budgets 32 unit loads per segment. Early transceiver chips each drew one full load, so the old rule was 32 devices. Most current transceivers present one eighth of a load, which raises the ceiling on paper well past that. Plan on the 32-load budget and the design stays valid with either kind of chip. Past it, fit a repeater and start a new segment.
Can a PLC with only an RS-232 port join the bus?
Not as wired. RS-232 is a two-device point-to-point link, so the port has no address of its own. A small converter at that PLC turns its RS-232 side into an RS-485 node; the converter joins the bus and the PLC keeps a station address like the rest. The converter needs a small DC supply on the plant side.
Where exactly do the two 120 ohm resistors go?
Across A and B at the two ends of the cable, nowhere else. One sits at the panel end, the other at the last PLC. A resistor added at a mid-bus node loads the bus at that point and shrinks signal swing for every device on the segment. If a machine moves later, the resistors stay with the cable ends.
Is 19,200 bps the right speed for this network?
It is a sound default for a four-slave line. At 19,200 bps with even parity, the example cycle lands near 116 ms and the format holds up in plant electrical noise. At 38,400 bps the same four polls take about 78 ms, because the fixed 10 ms turnaround per slave caps the gain. Speeds approaching 10 Mbps need short runs and rarely suit a plant floor.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.