Buy an industrial serial character display that accepts true RS-232 input, power it from the port's power pin when its draw fits, match baud, data bits, parity and stop bits, and drive it with the CLICK's Send instruction. A hobby serial LCD needs an RS-232-to-logic-level converter first. Check interface, wiring, power, settings and commands in that order.
You want a screen on a CLICK PLC that shows messages and values and offers an operator nothing to press. The part that fits is an industrial serial character display with a true RS-232 input: three wires from the port, power from the port's 5 VDC pin, and ASCII strings your ladder sends with the Send instruction. The cheaper hobby serial LCDs speak logic-level serial the port cannot drive directly, so they need a converter in the line before they survive the port. Work the checks in order before wiring anything: signal levels, wiring, power, serial settings, command set. This page gives the numbers for each, a 20 x 4 worked example with its timing at 9600 baud, and the cases where a different class of screen is the better buy.
Signal levels come first
RS-232 is an electrical standard as much as a protocol. A driver swings between roughly ±5 V and ±15 V, negative for a mark (the idle state) and positive for a space, and the receiving threshold in the standard is ±3 V. A logic-level serial line, the kind hobby boards use, idles near the positive rail and swings 0 to 5 V.
That difference sets your shopping list. An industrial character display with an RS-232 input stage wires straight to the port: its receive pin is built for those swings. A hobby serial character LCD puts the standard character-module controller behind a small backpack board, and the backpack's input is logic-level; character LCDs driven through serial backpack chips show how those boards work. The port's negative idle voltage lands on an input that never sees a negative level in its own world, so wire one of these boards only through an RS-232-to-logic-level converter, which translates the levels in both directions.
The decision at this step: a display whose data sheet prints RS-232 input levels connects directly. One that prints TTL or logic-level serial needs the converter, powered from the same rail as the display.
Wiring: three conductors and no assumptions
The cable is PLC transmit to display receive, display transmit to PLC receive if the display answers back, and signal common to signal common. Port power joins on the power and ground pins when you use it.
Which conductor carries which signal differs between makers even when the connector shells match. Several CLICK models bring the port out on a modular phone-style connector, while displays use anything from a DB9 to a pin header. Take the function of each pin from both manuals, write it on the cable drawing, and check the finished cable against that drawing before power goes on. A mating connector tells you nothing about the function of the pins inside it.
Power: what the 5 VDC pin may carry
The CLICK's RS-232 ports carry that power pin, and on the two-port models both supply it. The CLICK hardware manual documents the port powering one small monochrome graphic display: one display, that class of load.
Before using port power, read the display's current draw with the backlight on and compare it with the limit the manual states for the pin. Character displays with LED backlights draw more lit than the bare controller does. If the draw sits above the port budget, feed the display from a separate supply and tie that supply's ground to the port's signal common, so both ends share one voltage reference and the serial lines still decode.
Settings: one set, both ends
Baud rate, data bits, parity and stop bits must match at both ends. 9600 baud with 8 data bits, no parity and 1 stop bit is the common default on serial displays and a safe choice at panel-cable lengths. Displays store their settings in a setup menu, a jumper block, or a command you send once and save; the manual says which, so set it before the display is bolted in. The PLC port takes the same numbers in its configuration. Fixed serial settings on a PLC port walks through locking these down when the device on the other end offers no negotiation.
Commands the ladder can send
A character display takes printable ASCII for text and reserved command bytes for control: clear screen, cursor positioning, backlight level. The command reference in the display's manual lists the exact bytes. In the CLICK, string characters live in data registers one byte each and the Send instruction transmits them; command bytes are stored the same way and sent as data, so the ladder builds a whole update as one block. When one refresh needs several Send instructions, run them one at a time and trigger the next on the instruction's status bits (receiving, success, error), so the next send starts only after the previous one finishes.
Numeric values need fixed-width formatting. Print a count padded to a set number of characters on every update and a shrinking number replaces the old one completely. Cursor positioning commands let you rewrite only the lines that changed.
Your options compared
| Option | Wiring to the port | Power | Best for | Watch |
|---|---|---|---|---|
| Industrial 4 x 20 serial character display, RS-232 input | Three conductors, direct | Port power pin when the draw fits | Read-only messages and values at the machine | Command sets differ by maker; order the command reference |
| Hobby serial character LCD plus level converter | Through the converter | Separate supply; the backlight often pushes past the port budget | Lowest-cost fixed text | The converter adds a part and a wiring fault to chase |
| Touch panel HMI used read-only | Per model: serial or Ethernet | Its own supply | Graphics, trends, screens that reformat often | The touch surface stays live; lock it down or mount it out of reach |
| Wall notice board playing a show from a memory card | None | Its own supply | Fixed safety notices read across the plant | No live PLC values on it |
Driving a bare character module from PLC outputs saves the serial port and costs six I/O points plus hand-timed strobes in ladder; interfacing character LCDs: commands, wiring and code shows what that involves. For a CLICK, the serial route wins.
Worked example: station status on a 4 x 20 display
An end-of-line station needs a no-button screen showing a header, a state, a parts count and a cycle time. The display is 4 lines by 20 characters with RS-232 input, powered from the port pin, set to 9600 baud, 8 data bits, no parity, 1 stop bit, matching the PLC port.
Layout, counting characters per line:
- Line 1, fixed:
STATION 3 EOL CHECK, exactly 20 characters. - Line 2:
STATUS:is 8 characters, so the state text gets 20 − 8 = 12.RUN,JAM STA 2(9 characters) andCHANGEOVER(10 characters) all fit. - Line 3:
PARTS:is 7 characters, leaving 13 for the count. Pad the count to 5 characters every update, so 09999 replaces 10000 cleanly. - Line 4:
CT:plus12.4sfor the cycle time.
Timing at 9600 baud with 8 data bits and 1 stop bit: one character is 10 bits, 10 ÷ 9600 = 1.04 ms. Rewriting both live lines is 24 characters × 10 bits = 240 bits ÷ 9600 bit/s = 25 ms. Refreshing once per second spends 240 of that second's 9600 bits, under 3 % of the port's capacity. A full-screen rewrite, 80 characters, is 800 bits ÷ 9600 bit/s = 83 ms, still easy on a 1 s cycle. If updates ever feel slow, a faster RS-232 baud rate will not fix HMI animation; rewriting only the changed lines does.
Reading distance: a 10 mm capital is 0.39 in, so at the safer 20 ft of viewing distance per inch of letter height it reads to about 8 ft, and the FHWA planning figure of 30 ft per inch gives about 12 ft. That is machine-side reading distance, exactly where this display belongs.
The decision
Recommendation: order an industrial 4 x 20 character display whose data sheet states RS-232 input levels, power it from the port's power pin when its current draw fits the manual's stated limit, and drive it at 9600 baud, 8 data bits, no parity, 1 stop bit with the Send instruction and fixed-width numbers. If the only display you can get is a logic-level hobby board, put an RS-232-to-logic-level converter in the line and budget a separate supply for the backlight. Move up to a touch HMI only when the job needs graphics or constant reformatting, and then lock its touch surface down or mount it out of reach.
When a different screen class fits
Two situations push you off the character display. When operators must change setpoints or acknowledge alarms, a read-only screen is the wrong tool; an interactive touch panel is the right one, and the button-temptation problem returns with it, so configure its documented lockdown or mount it where only maintenance reaches it. When the message is a fixed notice read from across the plant, with no live values, shop-floor notice boards that play a show from a memory card do that job with no PLC link at all. The basics of matching a display to two fixed messages are in choosing a PLC-driven message display for two fixed texts.
What to ask for before you order
Get these six answers from the display data sheet before you buy:
- Electrical interface: RS-232 input levels stated, or logic-level stated.
- Current draw with the backlight on, in mA, at the port pin's voltage.
- Connector drawing showing which pin carries which signal.
- Baud, data bits, parity and stop bit range, and where the settings are stored.
- Command reference with the exact bytes for clear screen, cursor positioning and backlight.
- Operating temperature range, if the display mounts on the machine.
Order the mating connector for the PLC port at the same time, and keep a spare level converter on the shelf if you went the logic-level route.
Questions
Can the CLICK PLC's RS-232 port power the display directly?
Yes, within the documented limit. The CLICK hardware manual states the port provides 5 VDC, sized for one small monochrome graphic display, and on two-port models both RS-232 ports carry the pin. Compare the display's current draw, backlight on, with the limit the manual states. Above that limit, use a separate supply and tie its ground to the port's signal common.
Why does a hobby serial LCD need a converter with a PLC serial port?
Level mismatch. The port drives RS-232 swings, negative for the idle mark and positive for a space, from the plus and minus 3 V threshold up to about 15 V. Hobby boards decode signals between ground and their positive supply rail. The converter translates both directions; without it, the port's negative idle voltage sits on an input never built to see it.
How fast can a 4 x 20 display refresh at 9600 baud?
At 9600 baud with 8 data bits, no parity and 1 stop bit, each character is 10 bits, about 1 ms. A full 80-character rewrite takes 83 ms; the two live lines of a typical status screen, 24 characters, take 25 ms. Refreshing once per second uses under 3 % of the port's bit budget.
How do I keep stale digits off the screen when a value shrinks?
Print every value at a fixed width. On a 20-character line, a 7-character label leaves 13 for the value; pad the number to a set 5 characters with leading zeros or spaces on every update, so 09999 fully replaces 10000. A fixed-width send rewrites the whole field without clearing the line first.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.