A computer monitor is only a panel; it needs a PC running HMI software with a driver for your controller. Size the screen by capital height, 30 ft of viewing distance per inch for legibility, half that for fast reading, so a glance-read line 20 ft away needs 1 in capitals. One or two big lines suit a message display; detailed screens suit the PC route.
Yes, a computer monitor can serve as a shop floor HMI display, and dropping the touchscreen is a sound choice for a board that only shows values. A monitor is a panel with a video input, so it needs a PC behind it running HMI software with a driver for your controller. Once that link works, the screen decision is arithmetic: a status line read at a glance from across the cell needs about an inch of capital height for every 20 ft of distance, and a 21.5-inch 1080p monitor delivers that on one large line. This page gives the sizing rule, a worked example, and the choice between a monitor, a message display and a static signage panel.
A monitor needs a PC and a driver behind it
A monitor ends at its video input; nothing inside it speaks a controller protocol. The chain you build is controller, then a PC running HMI software with a driver for that controller, then the monitor. PC-based HMI packages publish driver lists, and the mainstream ones cover most major controller families, so the first check is your exact model and protocol against that list. The license also has to cover runtime display on the PC you pick. A compact HMI panel works at a machine because panel and driver ship together; on the PC side the driver match is your job, and checking the PLC serial protocol before you replace a legacy HMI walks through it.
Size the text before the screen
Operators read capital letter height, not diagonal inches. The planning figures: 30 ft of viewing distance per 1 in of capital height for legibility (the FHWA planning figure), 20 ft per 1 in when the value has to be read at a glance, and line spacing of 1.7 × the capital height. A compact panel that fails at that range has on-screen capitals under 0.67 in, because 0.67 in × 30 ft/in is exactly where legibility ends.
On a 21.5-inch 16:9 monitor at 1920 × 1080, the panel is 10.54 in tall, so the pixel grid works out to 102.5 px per inch. Multiply the capital height you need by 102.5 to get the font size in pixels.
| Viewing distance | Legibility minimum | Fast reading | Font size on a 21.5-inch 1080p panel |
|---|---|---|---|
| 15 ft | 0.50 in | 0.75 in | 77 px |
| 20 ft | 0.67 in | 1.00 in | 102 px |
| 25 ft | 0.83 in | 1.25 in | 128 px |
| 30 ft | 1.00 in | 1.50 in | 154 px |
For the general method behind this table, see how screen size and viewing distance work together.
Example: one status line read from 24 ft
Scenario: a packaging cell where the farthest operator position is 24 ft from the board. The screen must show a state line (running or stopped, plus piece count) and an alarm line, display only, no touch.
- Fast-reading capital height: 24 ft ÷ 20 ft/in = 1.2 in. Alarm line at the legibility minimum: 24 ft ÷ 30 ft/in = 0.8 in.
- Convert to pixels on the 21.5-inch 1080p panel: 1.2 in × 102.5 px/in = 123 px, so set the status line at 125 px. Alarm line: 0.8 in × 102.5 px/in = 82 px.
- Check the fit: the status line plus its 1.7 × spacing is 212 px and the alarm line adds 82 px, so the pair occupies 294 px of the 1080 available, leaving room for small detail text aimed at people standing at the machine.
Result: a 21.5-inch 1080p monitor with a 125 px status line reads at a glance at that distance. A full screen of fast-reading text there is about five lines (four line pitches of 209 px plus one 123 px line is 959 px of 1080), so the real limit is content density: past five lines the layout is too fine for the cell, and the fix is fewer lines or a bigger panel, never smaller text.
The options side by side
| Option | What drives it | Reads well | Choose it when |
|---|---|---|---|
| PC monitor with PC-based HMI software | The software's driver for your controller | Several lines, trends, alarm detail, multiple screens | Operators need more than one or two big values |
| Large message display (andon board) | Its own serial or Ethernet input; check the input list against the controller | One to a few very large lines with beacon color | One status line must read far across the hall with no PC to maintain |
| Compact HMI panel kept at the machine | Its built-in driver | A full operator interface at arm's length | Operators stand at the machine and only supervisors read from afar |
| Offline signage panel playing image files from a memory card | Nothing; there is no controller link | Static notices, shift targets, hand-updated status | The content changes rarely and can be published as images |
That last row is where our displays fit: an Android signage panel plays images and video from a memory card with the software included and no subscription, which suits a shift board, but it has no controller driver and cannot show live values. Our display range covers it; for the choice between a status board and a working operator interface, read factory status display or operator HMI.
When a PC monitor is the wrong choice
- One counter or one temperature must read across the hall: a large display wired to the controller is the classic answer, and how to display a PLC counter value where operators can see it compares the routes.
- One line and an alarm are the whole job: a message display needs no PC license, no patches and no restarts, and our guide to andon boards covers where a small screen fits and where it does not.
- The operators who need the data stand at the machine: keep the compact panel and spend nothing.
The monitor route earns its keep when the content is genuinely rich: trends, batch detail, several screens a supervisor walks through. The price is a PC to license, configure and restart cleanly.
What to check before you order
- Driver and license: your controller model and protocol on the software's driver list, and a runtime license for the PC.
- Data path: a serial link is dependable to about 50 ft (the RS-232 standard's usual 15 m planning figure); beyond that, put the PC on Ethernet. For a long run, see connecting a remote HMI screen 1,500 feet from the PLC.
- Video path: copper HDMI at 1080p is usually quoted as dependable to about 15 m (50 ft); a longer run needs an active extender.
- Heat and mount: most panels this size use a 100 × 100 mm VESA mount, and many office-grade panels are rated only to about 35 °C ambient, so check both on the datasheet before the panel goes into a hot cell.
- Startup and stale data: set the PC to boot straight into the HMI runtime and give the screen a visible connection status, so a dropped link is obvious and a frozen value is never mistaken for a live one.
One safety note: treat the screen as read-only, and lock down the PC's keyboard, USB ports and remote access so no one on the floor can write to the controller through it.
The decision
Recommendation: for a display-only screen read from far across the floor, put a PC with a verified driver for your controller behind a 21.5-inch 1080p monitor, set the main status line to 125 px capitals for the 24 ft example above, and scale it with distance at the fast-reading rate in the table. Take a purpose-built message display when one or two big lines well past the table's last row is the whole job and a PC on the line is more maintenance than you want. Keep the compact HMI panel for anyone standing at the machine.
Questions
Can a computer monitor connect straight to a PLC?
No. A monitor stops at its video input and has nothing that speaks a controller protocol. A PC, often an industrial panel PC mounted right behind the screen, runs HMI software with a driver for the controller and feeds the monitor its picture. The driver list, not the monitor, decides whether the link works.
How do I keep the screen showing after a power cut?
Set the PC to boot without a login prompt, launch the HMI runtime on startup, and enable restart after power loss in the PC's firmware settings. Then test it: switch the power off for 30 seconds and time how long the screen takes to show live values again. A screen that needs a keyboard and a login stays dark until someone walks over.
What if the HMI software has no driver for my controller?
The PC route stops there. Pick software whose driver list includes your controller, or keep the HMI panel as the data source and let the monitor show only text the PC generates on its own. Reconfiguring controller logic or network settings to suit a screen belongs in an approved change, not in a display purchase.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.