All-in-One PLC + HMI vs Separate PLC and HMI: Which to Choose
Every machine needs a controller and an operator interface. The traditional approach is a separate PLC in the control cabinet and a separate HMI panel on the door, linked by a communication cable. An all-in-one controller, such as the Unitronics range, puts the PLC, the HMI and often the I/O into one unit. Both approaches work; the right choice depends on machine size, panel layout and how the system will grow.
What an all-in-one controller is
An all-in-one controller (Unitronics calls it an OPLC) mounts in a panel cut-out like an HMI. The display and keypad or touchscreen face the operator, and the PLC and I/O sit behind the panel. The range runs from text-display units to large touchscreens:
- The Unitronics Jazz is a full PLC with a two-line text display, a 16-key keypad and up to 40 built-in I/Os, priced to replace smart relays on small machines.
- The V350 adds a 3.5 inch colour touchscreen with onboard I/O and expands up to 512 I/Os, in a 92 x 92 mm cut-out.
- The V700 has a 7 inch colour touchscreen and expands up to 1000 I/Os, with Snap I/O modules that plug directly onto the back to create a self-contained unit.
- The UniStream Built-in combines PLC, HMI and I/O in one unit with a 5, 7 or 10.1 inch touch panel and expands up to 2048 I/O points.
Advantages of all-in-one
Less wiring and fewer parts
There is no communication cable between PLC and HMI to route, terminate and troubleshoot, and no separate power supply feed for the panel. With onboard or snap-on I/O, field wiring goes straight to the unit. Fewer parts also means fewer item codes to stock as spares.
Panel space
The controller lives in the door cut-out, so the cabinet does not need DIN rail space for a CPU and I/O cards. On compact machines this can allow a smaller enclosure, or remove the cabinet altogether in favour of a small operator box.
One software project
Ladder logic, HMI screens, alarms and communication settings are programmed in the same software and downloaded as one project. Tags are shared directly, so there is no tag mapping between two programs and no version mismatch between PLC and HMI after a change. Many Unitronics controllers include built-in alarm screens and multi-language display support.
Cost
For small and medium machines, one unit usually costs less than a comparable PLC plus a separate HMI, before counting the saved wiring and panel space.
I/O expansion
A common concern with all-in-one units is outgrowing the onboard I/O. Most Unitronics controllers expand well beyond what is built in:
- Expansion I/O modules mount on a DIN rail through a local or remote adapter, with digital, analog, high-speed, temperature and weighing options.
- Vision and M91 controllers can connect remote I/O over CANbus through the UniCAN remote adapter.
- UniStream supports Ethernet-based remote I/O adapters, up to 8 per controller, which lets you place I/O in separate panels along a long line.
So I/O count alone is rarely the reason to move to a separate PLC and HMI.
When separate PLC and HMI is better
- Several operator stations. A line with operator panels at both ends, or a central control room, needs more than one display connected to one controller.
- Operator position far from the cabinet. If the panel must sit on a pendant arm or at a distance from the control cabinet, a separate HMI with one network cable is simpler than routing all I/O to the operator position.
- Harsh operator location. If the operator position sees heavy washdown, heat or vibration, keeping the controller in a protected cabinet can be the safer layout.
- Specified PLC platform. Some customers, especially exporters and large OEM buyers, specify a PLC brand. A separate HMI then connects to that PLC.
- Independent replacement. On machines with long lifetimes, a separate HMI can be replaced or upgraded without touching the control program.
Middle ground
Unitronics also covers layouts between the two extremes. The UniStream Modular lets you choose a 7, 10.4 or 15.6 inch HMI panel, plug a CPU onto its back and snap on I/O and communication modules, so the system is sized to the machine while staying in one software project. The UniStream PLC has no local display: it runs the HMI application inside the PLC as a virtual HMI, viewable through VNC on a phone, PC or remote display. That suits unattended pumping stations and utility skids where nobody stands at the machine, and supports OPC UA and MQTT for data exchange with SCADA and IT systems.
Quick comparison
| All-in-one PLC + HMI | Separate PLC and HMI | |
|---|---|---|
| Wiring | No PLC to HMI link; I/O direct or via adapter | Communication cable and separate HMI power |
| Panel space | Controller in the door cut-out | CPU and I/O on DIN rail plus HMI cut-out |
| Software | One project for logic and screens | Two programs, tag mapping between them |
| Operator stations | One per controller | Several HMIs can share one PLC |
| Best fit | Small and medium machines, OEM series builds, skids | Long lines, multiple panels, specified PLC brands |
How to decide
- Count the operator stations. One station favours all-in-one.
- Count the I/O, including future additions, and check the expansion limit of the chosen model.
- Check where the operator stands relative to the control cabinet.
- Check whether the end customer specifies a PLC brand.
- List the communication needed: Modbus, CANopen, EtherNet/IP, OPC UA, MQTT, SMS or e-mail alerts.
- Choose the display: text and keypad for simple machines, touchscreen for recipes, trends and alarm history.
PLC and HMI from Rajdeep
Rajdeep Automation supplies Unitronics controllers across India, from Jazz and Vision units for compact machines to UniStream for larger lines and connected plants. Browse the full range under PLC and HMI.
Send us your I/O count, display size, communication needs and machine layout through the enquiry form, and an application engineer will recommend a controller with the datasheet and price.