When a legacy machine needs an operator interface but the panel already has a working Omron PLC, the natural upgrade path is a matching programmable terminal rather than a whole new control architecture. The Omron NT31-ST123-EV3 is a 5.7-inch monochrome STN touch-screen HMI from Omron's NT31 series, designed to sit on a panel door and talk to Omron PLCs over Host Link or NT Link. It is a mature, widely installed panel; Omron now lists it as discontinued and recommends the NS5 family as the successor. This guide covers the verified display, power, communication, and touch-panel specifications, how it connects to an Omron PLC, and what maintenance teams should expect when supporting installed NT31 terminals. SHLY-PLC supplies Omron HMI parts as an independent industrial-automation distributor; every unit is inspected and tested before dispatch.
What Is the NT31-ST123-EV3?
The NT31-ST123-EV3 is part of Omron's NT31/NT31C family of programmable terminals (PTs). The "-ST123" model is the monochrome version: a 5.7-inch STN LCD showing black and white (two colors), while the NT31C-ST143 is the color variant. The "-EV3" suffix denotes the English-language, EV3 firmware generation. The panel is programmed with Omron's NT-series screen-development software and displays process values, lamps, touch switches, trend graphs, and messages that are read from and written to a connected Omron PLC.
It is not a PLC itself: it has no I/O of its own beyond the touch screen. It acts as an operator interface—displaying PLC data and forwarding button presses to the host controller over a serial link.
Key Specifications
| Parameter | Specification |
|---|---|
| Model | NT31-ST123-EV3 (monochrome, English version) |
| Display device | Monochrome STN LCD |
| Display size | 5.7 inch (118.2 x 89.4 mm effective area) |
| Resolution | 320 dots horizontal x 240 dots vertical |
| Display colors | Black / white (2 colors) |
| Backlight service life | 50,000 hours minimum (brightness adjustable in 3 levels) |
| Touch panel | Pressure-sensitive; up to 192 touch switches (16 x 12 matrix) |
| Touch operating force | 1 N maximum; service life 1,000,000 operations minimum |
| Rated supply voltage | 24 V DC |
| Allowable supply range | 20.4 to 26.4 V DC (24 V DC -15 % / +10 %) |
| Power consumption | 15 W maximum |
| Operating temperature | 0 to 50 °C |
| Storage temperature | -20 to 60 °C |
| Operating humidity | 35 % to 85 % RH, no condensation |
| Serial port A | EIA RS-232C, D-sub 9-pin female; +5 V (250 mA max.) on pin 6 |
| Serial port B | EIA RS-232C (RS-422A/485 selectable by memory switch), D-sub 25-pin female |
| Parallel interface | Centronics-compatible, 20-pin half-pitch connector |
| Expansion interface | Dedicated expansion connector |
| Host protocols | Host Link, NT Link (1:1 and 1:N), Memory Link (RS-232C) |
| Successor (Omron-recommended) | Omron NS5 series |
Display and Touch Operation
The 5.7-inch monochrome screen is resolvable at 320 by 240 dots and is well suited to status screens, push-button panels, and trend displays rather than high-resolution graphics. The touch panel is pressure-sensitive and supports up to 192 touch switches arranged on a 16 by 12 matrix; individual touch-switch graphics can be set to standard, shadow, 3D, rectangle, circle, polygon, or sector shapes. The screen supports fixed character strings, numeric displays (up to 256 per screen), bar graphs, analog meters, trend graphs (one frame per screen, up to 50 graphs), lamps, and image-library displays. Contrast is adjustable over 100 levels from the front touch panel, and backlight brightness has three settings.
Because the panel uses a pressure-sensitive membrane, each touch needs a firm press of around 1 N. Omron rates the touch surface for at least one million operations, which is long life for light-to-medium machine interface use but still means a high-frequency push-button location is the first thing to wear out on a heavily used panel.
Connectivity and Host Communication
The NT31-ST123-EV3 is built around two serial ports:
- Port A is an RS-232C D-sub 9-pin female connector and supplies +5 V at up to 250 mA on pin 6, which can power small accessories such as a programming adapter.
- Port B is a D-sub 25-pin female that runs RS-232C by default and can be switched to RS-422A/485 via a memory-switch setting, allowing longer cable runs and multi-drop NT Link networks.
For host communication the panel supports Omron Host Link, NT Link in 1:1 and 1:N configurations, and a Memory Link mode over RS-232C. That means it pairs naturally with Omron C-series, CV, and CVM1 PLCs and with later Omron controllers that speak the same link protocols. The Centronics-compatible parallel port and the dedicated expansion connector are used for programming peripherals and optional units rather than for field I/O. Screens are developed on a PC with the NT-series support tool and downloaded over a serial connection.
Power, Mounting, and Maintenance
The terminal runs from a regulated 24 V DC supply in the range 20.4 to 26.4 V DC and draws up to 15 W. It is panel-mounted through a rectangular cutout and sealed to the panel with the supplied gasket; operating conditions are 0 to 50 °C and 35 to 85 % RH without condensation. Green POWER and RUN LEDs show power and operating status, and the RUN LED turns orange when the back-up battery voltage is low, which is the usual reminder to replace the PLC-backup battery that retains screen and system memory. Field wiring consists of the 24 V DC supply and the serial cable to the host PLC; no other field connections are required.
Common maintenance tasks on installed NT31 panels are: replacing the backlight when it dims below the 50,000-hour service point, replacing the backup battery on the orange-RUN warning, and reseating the serial connector when communication to the PLC is intermittent.
Typical Applications
- Packaging, textile, and machine-tool panels running Omron PLCs
- Conveyor and material-handling cells needing local push-buttons and status lamps
- Small process skids and stand-alone machines with operator recipes
- Retrofit of existing Omron-equipped lines where the HMI has failed but the PLC is healthy
- Test rigs and equipment panels that need a serial-linked operator interface
For these duties the NT31 remains a cost-effective way to keep an installed Omron control system operable. Related HMIs and Omron controllers can be browsed in our Omron range and the broader HMI / touch-screen collection.
Lifecycle and Spare Parts
Omron lists the NT31/NT31C family as discontinued and recommends the NS5 series as the direct replacement family. The NS5 offers a wider screen, brighter color displays, longer backlight life, and modern communications, but it uses a different screen-development environment and form factor, so a like-for-like NT31 swap on an existing panel usually involves rebuilding the screen project. For plants that cannot justify a redesign, tested surplus and pre-owned NT31-ST123-EV3 panels are the practical spare-parts route. SHLY-PLC stocks discontinued HMIs in our Discontinued Products range; if you have a failed NT31, send us the full model number and we will confirm a tested replacement and discuss the NS5 upgrade path if you want one.
Frequently Asked Questions
What type of display does the NT31-ST123-EV3 have?
It has a 5.7-inch monochrome STN LCD with a resolution of 320 x 240 dots, showing black and white (two colors). The color variant in the same family is the NT31C-ST143.
What power supply does the NT31-ST123-EV3 need?
It runs from 24 V DC, allowed over a range of 20.4 to 26.4 V DC, and draws up to 15 W. A regulated 24 V DC panel supply is required.
How does the NT31-ST123-EV3 connect to a PLC?
It uses two serial ports: Port A is RS-232C (D-sub 9-pin) and Port B is RS-232C or RS-422A/485 (D-sub 25-pin), selectable by memory switch. It communicates with Omron PLCs over Host Link, NT Link (1:1 or 1:N), or Memory Link.
Is the Omron NT31-ST123-EV3 still available?
Omron has discontinued the NT31 family and recommends the NS5 series as the successor. Tested surplus and pre-owned NT31-ST123-EV3 panels remain available through independent distributors such as SHLY-PLC for maintaining installed systems.