If the two phrases never change, light two backlit cut-out legends, one PLC output each. If the wording can change, buy a serial dot-matrix display sized by character height for the viewing distance and match its documented baud rate and command format before ordering.
A sign that shows one of two phrases from a PLC needs no touchscreen, no buttons and usually no programmable display at all. It needs one decision first: will the wording ever change? If both phrases are fixed, cut them into a black face plate, put an LED lamp circuit behind each, and switch each legend with its own PLC output. If the wording must change, buy a serial dot-matrix message display sized to the sign envelope, about 12 in wide by 3 to 4 in tall for a bench or cell, and drive it from the PLC serial port at the display's documented baud rate and command format. This page sizes the letters for the viewing distance, checks the output ratings against the load, and lists what to confirm before you order either one.
First decide whether the wording can change
Write the message list down before shopping. Two phrases that are frozen, such as a pass mark and a rework call, point to two illuminated legends. A list that starts at two and is expected to grow, or wording a supervisor edits, points to a text display.
Read the requirement literally. "Display a message" and "display these two messages" are different jobs. The legend route needs no serial settings, no command format and no display firmware; the sign is wiring and a plate. The text route buys flexibility and buys port setup with it. Everything else on this page follows from that one line in the requirement.
Fixed wording: two backlit legends
The hardware is a black face plate with one cut-out window per message, a lamp circuit behind each window, and one discrete PLC output per lamp circuit. Machine-side discrete I/O runs at 24 V DC under IEC 61131-2, so pick lamp circuits and wiring for that voltage and the rest is ordinary cabinet wiring.
Size the lamps to the load. A typical 22 mm panel LED indicator draws about 20 mA at that voltage. A 12 in wide window lit evenly takes more; an LED strip or edge-lit assembly around 0.2 A per legend is a workable planning figure, and the lamp datasheet gives the real number.
Interlock the two outputs in the logic so both cannot be on at once, and decide the state for "neither condition valid": both windows dark is the honest answer. Wire the supply from a fused branch and isolate the panel before wiring.
Changing wording: a serial character display
Character displays are sold by character height, commonly 0.5 in, 1 in and 2 in, and by characters per line. Pick the character height from the viewing distance. The FHWA planning figure gives 30 ft of viewing distance per inch of capital height, and 20 ft per inch is the safer choice for fast reading, so 1 in capitals read at a glance from 20 ft.
Check the port at both ends. RS-232 needs three conductors, transmit, receive and signal ground, crossed so one side's transmit lands on the other's receive; its classic planning limit is 50 ft. RS-485 is a differential multidrop standard with a familiar reach of 1200 m at low baud rates, and several displays can share one twisted pair. Hobby-grade 5 V serial modules speak TTL levels and will be destroyed by RS-232 voltage swings; they need a converter and their own 5 V supply, which a cabinet supply at 24 V does not provide.
Set the PLC port to the display's documented values; character displays commonly default to 9600 baud. The discipline is the same as connecting an HMI to a PLC port with fixed serial settings. The command format, meaning the start byte, the address and how a line of text is framed, comes from the display's own manual. Use its documented example exactly and never borrow a command string from a different model.
Match the output rating to the load
Add the load current and compare it with the output rating per point and per common terminal. A transistor output switches DC loads and carries a few tenths of an amp per point on small controllers. A relay output takes AC or DC, but its rating is quoted for resistive load, and PLC makers' own support notes derate inductive loads well below the printed figure; a 3 A resistive-rated relay contact is the classic worked example, and inductive loads sit far under it. LED lamps and display electronics are resistive or electronic loads; solenoid and contactor coils are the inductive ones that eat the margin.
If a lamp assembly draws more than the output is rated to carry, a discrete FET between the supply and the load does the actual switching, a standard protection arrangement covered in semiconductor makers' application notes on protecting PLC digital outputs. Check the assembly's current before wiring it to an output sized for sensing loads.
Example: a 12 by 4 inch bench sign
A QC bench needs a sign read by the tester from 12 ft. The two phrases are "Unit OK" and "Rework required", and the wording is frozen. The envelope is 12 in wide by 4 in tall.
Letter height: the fast-reading figure is 20 ft per inch of capital height, so 12 ft ÷ 20 ft/in = 0.6 in capitals.
Width: plan an average character advance of 0.6 times the capital height for a dot-matrix font. "Rework required" is 15 characters, so the line is 15 × 0.36 in = 5.4 in wide at 0.6 in capitals.
Room to grow: at 1.0 in capitals the same line is 15 × 0.6 in = 9.0 in, still inside the 12 in plate, and the reach rises to 1.0 in × 20 ft/in = 20 ft at a glance, or 30 ft at the planning figure. Take 1.0 in if the sign may also be read from the aisle.
Electrical: two legends at 0.2 A each put 0.4 A on the fused branch, and each output carries 0.2 A, inside a typical 0.5 A transistor rating with margin.
Decision: two backlit legends, 1.0 in capitals, on two interlocked transistor outputs. If the supervisor wants editable wording, the same envelope becomes a serial display with a 1 in character height on RS-232 at 9600 baud, because the bench run is a few feet.
The options compared
| Option | Hardware | PLC side | Choose it when |
|---|---|---|---|
| Two backlit cut-out legends | Black plate, one LED circuit per legend on the 24 V machine supply | One discrete output per message, interlocked | The wording is frozen, one or two messages |
| Serial dot-matrix message display | Display with 0.5 to 2 in character heights, its own low-voltage supply, an enclosure | A serial port set to the documented baud and command format | The wording can change or the list will grow |
| Small HMI panel | Touchscreen terminal and its development software | Its driver and tag database | Operators also enter data; a sign with no buttons is the wrong shape |
When to pick something else
If the machine already carries an HMI panel, the simplest path is no new hardware: one PLC bit can flip its text between two states. If the goal drifts toward live machine states on a bigger screen, showing 16 PLC input and output bits on one screen is its own project. Shift notices, quality targets and visitor information that ignore machine state are offline signage, and shop-floor signage displays that play files from a memory card cover that job.
Distance changes the serial choice: beyond the 50 ft RS-232 planning figure, move to RS-485 and its 1200 m reach. Bright surroundings argue for the taller character height, because letter height is the lever that buys reading distance.
What to ask for before you order
- The envelope in inches and the reading distance in feet, so the character height follows from 20 to 30 ft per inch.
- Character height and characters per line, or the lamp circuit current per legend in amps.
- Supply voltage for each part, in volts DC, from each part's datasheet or manual.
- Port type at both ends, RS-232, RS-485 or TTL, and the cable run in feet.
- The documented command format and default baud from the display manual.
- Load current summed against the output rating per point, and the fuse size for the sign branch.
Recommendation: For two phrases that will never change, build two backlit legends and switch each with its own interlocked transistor output, with both dark as the invalid state. For wording that can change, order a serial dot-matrix display whose character height is chosen from the viewing distance, confirm its documented command format and default baud against the PLC port you have, and check its supply voltage and current draw against the output or supply feeding it before the order goes in.
Questions
How tall do the characters need to be on a sign read from 30 ft?
Work from 20 ft of viewing distance per inch of capital height when the sign is read at a glance, and 30 ft per inch for slower reading. A sign read from 30 ft therefore needs 1.5 in capitals at the glance figure, or 1 in at the planning figure. If the display wraps a message onto two lines, space the lines 1.7 times the capital height apart.
Could I pre-load both messages as slides and trigger them from the PLC?
No. A signage player is designed to play the files loaded on its memory card or USB stick, with no subscription, and nothing in its documented function switches slides from a machine signal or takes live PLC text. Driving a sign from PLC states needs discrete outputs on lamp circuits or a serial text link, which is the hardware this page picks. Use the player for notices that do not depend on machine state.
What does the sign show when the PLC stops or the power drops?
Legend lamps go dark as soon as their outputs drop, which is the honest indication on a stopped machine. A character display keeps the last text it received until power is removed or new text arrives, so a stale message can outlive the condition that set it. If a stale message would mislead someone, add a heartbeat check or a clear-text command to the PLC logic and test it with the display powered from the panel.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.