Ten printing stations and four PLCs whose serial resources stop at two RS-232 ports each: point-to-point wiring reaches eight printers, and a fifth PLC bought only to feed labels is the expensive direction. A bus is the small one. RS-485 was built for this shape of problem — one differential twisted pair, many stations, one talker at a time — and industrial thermal label printers are sold with an RS-485 serial interface as a factory or bolt-on option, each station answering to its own number. This guide wires one PLC serial port through an RS-232-to-RS-485 converter to ten or more printers: the port arithmetic, the addressing scheme, termination and bias, the wire-time numbers, the PLC send routine and the checks that prove the bus before a shift depends on it.
The port math that forces the decision
Four PLCs with two RS-232 ports each give 4 × 2 = 8 point-to-point links. You need ten printers, so you are two ports short with nothing spare, and every future station widens the gap. Expansion serial cards or a fifth PLC close it at the worst cost per drop.
One RS-232-to-RS-485 converter on a single port changes the arithmetic. The RS-485 physical layer is specified for 32 unit loads with classic transceivers, so ten printers plus the converter is 11 of 32 — 21 node slots in reserve. The other three PLCs keep their ports for real I/O and hand their print requests to the bus-owning PLC across the network they already share.
Below eight dedicated links, plain point-to-point RS-232 stays the simpler build. At ten printers on four PLCs, one multidrop bus from one port is the answer, with capacity for the next decade of stations.
What you need
- One isolated RS-232-to-RS-485 converter with automatic send-data control and DIP switches for the 120 Ω termination and the bias pair. The same conversion wired point-to-point is covered in wiring an RS-232 device to RS-485 with a converter; here it carries a bus instead of one peer.
- Label printers ordered with the RS-485 serial option. On industrial thermal families the option board carries the station addressing, and its programming manual is part of the order.
- One shielded twisted pair, 120 Ω characteristic impedance, rated to about 1.2 km at these data rates — any plant on one run.
- Legs and drops from cables — the accessories finder filters by type and length — and an enclosed 12, 24 or 48 V rail from power supplies if the cabinet needs a new one.
Between the PLC port and the converter, a lead with a connector on one end and open cores on the other keeps the cabinet tidy. The LS-CBL-0224 is a DB9F-to-open cable, 7 m long:
- Type
- Frame grabber I/O cables
- Length
- 6-8 m
Down each drop, an 8-core lead gives the A/B pair, the reference conductor and spares. The LS-CBL-0300 is an 8-core M8-to-open power and I/O cable, 7 m, built for drag-chain use where a printer rides a moving frame — one lead per drop:
- Type
- Power and I/O cables
- Length
- 6-8 m
If the line must also verify what it prints, put the reader on the plant network rather than on the printer bus. The fixed code readers include the LS-CRD-0052, a 1.2 MP model with a 12 mm focal length and an Ethernet interface.
- Resolution
- Up to 3 MP
- Shutter
- Global
- Interface
- Ethernet
Wire the bus in eight steps
- Set the converter before power-up: 120 Ω termination ON — it is one end of the bus — bias ON, data settings at 19200, 8 data bits, no parity, 1 stop bit.
- Wire the RS-232 leg with transmit crossed to receive and signal ground joined, and use the converter's automatic direction control rather than driving it from a handshake line.
- Run the pair as a daisy chain — converter, station 1, station 2 through station 10 — landing on terminal blocks at each printer. No stars, no tees.
- Ground the shield at the converter end only and carry one reference conductor common to every station. Between cabinets, let the converter's isolation do that work instead of bonding two grounds.
- Enable the second 120 Ω terminator at station 10, the far end, by menu or switch as its manual shows. Stations 1 through 9 stay unterminated.
- In every printer menu set RS-485 mode, the same 8N1 data settings, and a unique station number: 1 through 10, with 11 and 12 left free for drops you will add.
- Route the pair away from motor and VFD cabling, crossing at right angles where they must meet, and feed the converter from the same protected 24 V branch as the PLC I/O so a supply fault cannot reach the serial line.
- Draw the bus map — station number, location, terminal block — and put a copy inside the door. Ten anonymous drops are un-debuggable in year three.
Give every printer a name on the bus
RS-485 has no arbitration: the converter's port is the only master, and a printer speaks only inside a reply the PLC asked for. Each job is prefixed with the destination station's number, only that printer acts, and the other nine ignore the frame. The prefix itself is the printer dialect — a leading address character on some interfaces, a structured directive on others — and the option board's programming manual states it exactly.
The frame discipline is worth borrowing from a protocol that has carried multidrop for decades: station address, payload, then a quiet gap. Connecting an RS-232 device to an RS-485 multidrop network works through those frame rules as Modbus RTU applies them on an RS-485 pair.
Two habits keep ten slaves orderly: disable unsolicited transmissions, with status sent on request only, and hold the turnaround gap between jobs.
Worked example: wire time for ten printers at 19200 baud
The inputs: ten printers, the count from the line-up that started this; one bus at this setting with 8N1 framing, as set in the steps above; and a 200-character print job — format header plus variable fields. The 200 characters is the assumed input, so measure one of your own format strings and substitute it.
In words, the rule: 8N1 puts 10 bits on the wire per character — 1 start, 8 data, 1 stop — so a job occupies the bus for characters × 10 ÷ baud. Between jobs, hold 3.5 character times of silence, the frame gap Modbus RTU defines; above 19200 that standard fixes the gap at 1.75 ms flat.
The arithmetic, carried with its units:
- bits = 200 characters × 10 bits/character = 2000 bits
- job_time = 2000 bits ÷ 19200 bit/s = 0.104 s = 104 ms
- gap = 3.5 × 10 bits ÷ 19200 bit/s = 1.8 ms
- round = 10 × (104 ms + 1.8 ms) = 1060 ms ≈ 1.1 s
| Baud | One 200-character job | Ten-station round |
|---|---|---|
| 9600 | 212 ms | 2.1 s |
| 19200 | 106 ms | 1.1 s |
| 38400 | 54 ms | 0.54 s |
| 115200 | 19 ms | 0.19 s |
A label takes seconds to print, so the bus drains the queue long before the printers do. The decision: a full dispatch round costs 1.1 s of wire time — comfortable for any cycle that hands out labels once per pass. Double the job to 400 characters and the round reaches 2.1 s; that is the trigger to set the whole bus — printers, converter, PLC port — to 38400 and buy the round back down to 0.54 s.
The PLC send routine: one talker, disciplined gaps
The routine is a small sequencer with a state per station:
- Queue. One string buffer and a pending flag per station number. Writers anywhere in the program — or the other three PLCs over the plant network — fill a buffer and set its flag.
- Send. On a timer, when the bus is idle and a flag is set, frame that job with the station's address and transmit.
- Wait. Hold the gap, 2 ms of silence, then advance. If the printer's dialect acknowledges jobs, wait for the reply and count it in the budget.
- Advance. Next station, round-robin, so no printer starves behind a busy neighbour.
Building the label string and firing the port is the same one-shot send you would use point-to-point; sending ASCII label data from a PLC serial port to a barcode printer covers the string side. For the transmit instruction on a serial module, a barcode-printer serial-port programming walkthrough runs one platform end to end. Status replies deserve the same framing care as scanner data — the PLC receive-buffer framing guide shows the nulls and the races — and the reply wait is latency you budget, exactly as the scan-to-flag latency breakdown does for readers.
Prove it before a shift depends on it
- Power down and measure across A and B at the converter: two 120 Ω terminators in parallel read about 60 Ω. Near zero means a stray terminator somewhere; open means a broken pair or a missing far end.
- Power up idle: the bias should hold more than +200 mV between A and B, the receiver threshold it exists to clear. Noise centred on zero means the bias switches are off.
- Send to station 3 only. One printer prints and nine stay silent; walk through all ten.
- Garbage at one station means a menu mismatch — baud, parity or a station number used twice. The garbage-characters serial link fixes walk the same faults from the receive side.
- Queue all ten and time the round: close to the 1.1 s the example computed. Minutes mean retries and collisions, not wire time.
When a multidrop is the wrong answer
- Past about fifteen stations — 1.6 s per round — or the moment printers spread across buildings, buy Ethernet-interface printers: one switched link each, no shared wire, requests from anywhere. It is the same trade sending vision data to a PLC over Ethernet instead of serial makes for cameras.
- Printers with no RS-485 option need a converter each; past two or three of those, the per-device hardware outweighs the bus and an Ethernet model wins.
- If ten labels must land inside the same fraction of a second, the bus serialises them by design. Split onto two ports, five stations each, and the round halves to 0.54 s — or go Ethernet.
- Migrating an existing bank of legacy serial printers rather than building new has its own cut-over pattern; replacing legacy serial printers through RS-232 multidrop configuration covers it.
Commissioning checklist
- Wiring: daisy chain with no tees; shield grounded at the converter end only; 120 Ω at exactly two ends; one reference conductor; isolation between cabinets.
- Settings: 8N1 identical in the PLC port, the converter and all ten printer menus; station numbers 1-10 unique with 11 and 12 reserved; unsolicited transmit disabled everywhere.
- Acceptance: about 60 Ω across the pair powered down; idle line above +200 mV; a single-station send silent on the other nine; a ten-station round timed near 1.1 s.
The configurator matches a camera, lens, light, light controller and cables to each other, step by step, and lists only parts that fit what you already picked.
If the station count, distances or cycle time sit outside the numbers above, bring them to our applications desk — sizing the bus is a short conversation.
Questions
How many printers can one RS-485 bus carry?
Classic transceivers load the bus as one unit each and the physical layer is specified for 32 of them; receivers built as 1/8 unit load raise the ceiling to 256 stations on one pair. Your ten printers sit far below both, so the real limit is wire time: each 200-character job occupies the pair for about 106 ms at the bus rate, and a ten-station round costs 1.1 s. Grow the bus until a round crosses your cycle budget, then split it or raise the baud.
Should every printer have a 120 Ω terminator?
No — terminate only the two physical ends. Termination absorbs reflections on a long line; it does nothing useful at a mid-bus node, and ten 120 Ω resistors in parallel present 12 Ω to the drivers, far below the load an RS-485 driver is built for. Enable the converter's termination at one end and the last printer's at the other, leave stations in between unterminated, and keep the bias resistors in one place only, on the converter.
What baud rate should a printer bus run?
Set 19200 with 8 data bits, no parity and 1 stop bit as the default. A ten-station round of 200-character jobs then costs 1.1 s of wire time, which label printing never notices against its own seconds per label. Move the whole bus to 38400 when jobs pass 400 characters or the cycle needs every label dispatched inside half a second; that halves the round to 0.54 s. Change every printer menu, the converter and the PLC port together, because one mismatched node garbles the bus.
Can printers share the RS-485 bus with other serial devices?
Yes, when every device obeys master-slave discipline. Printers that stay silent until the PLC frames their address coexist with Modbus RTU slaves on the same pair, because both wait to be called. Devices that transmit unsolicited — a scanner streaming ASCII the moment it reads a code — collide with print jobs and belong on their own port or behind a converter that buffers by address. Keep one master per bus; two masters polling one pair will interleave frames.
When should I buy Ethernet printers instead?
Past about fifteen stations — 1.6 s per round at the bus rate — or across buildings and floors: Ethernet gives each printer a switched link at full rate with no bus arbitration, at the cost of a switch port and a cable drop per station. Below that, on one line at up to about a kilometre, the multidrop pair does the work with none of that infrastructure and uses a port you already own. Mixed plants run both: RS-485 inside a cell, Ethernet between cells.