UK How remote I/O works
Remote I/O is input and output hardware that sits close to the sensors and actuators it serves, out on the machine or in a field cabinet, and reports back to the controller over a network or fieldbus instead of through individual wires. It is also called distributed I/O.
A remote I/O station has two parts. A network adapter, sometimes called a bus coupler or head station, talks to the controller. Behind it sit I/O modules, often slim slices on a DIN rail, that wire to the field devices: digital inputs for switches and proximity sensors, digital outputs for relays and valves, analogue inputs for 4–20 mA and 0–10 V transmitters, and modules for temperature, counters or serial devices.
The controller, usually a programmable logic controller but just as often an industrial PC or gateway, exchanges a block of I/O data with each station on every cycle. To the control program, a remote input looks the same as one wired into the controller’s own rack.
Local vs remote I/O
Local I/O plugs into the controller itself or its expansion rack, so every field wire runs back to one cabinet. Remote I/O moves the terminals out to the plant and replaces the bundle of field wiring with a single network cable. Most systems use both.
| Local I/O | Remote I/O | |
|---|---|---|
| Where it sits | In or next to the controller | Near the field devices, up to hundreds of metres away |
| Link to controller | Backplane or expansion bus | Fieldbus or industrial Ethernet |
| Field wiring | Every signal runs back to the main cabinet | Short runs to a local station, one network cable back |
| Response time | Fastest, no network in the path | Typically a few milliseconds per network cycle |
| Expansion | Limited by rack slots and cabinet space | Add stations on the network as the plant grows |
| Diagnostics | At the controller | Per channel and per station, reported over the network |
| Best suited to | Compact machines and single cabinets | Long lines, large sites and modular machines |
The saving is mostly in cable and labour: one Ethernet run replaces dozens of multicore cables, marshalling terminals and the cable tray to carry them. The cost is a network that has to be designed, segmented and monitored like any other part of the control system.
Remote I/O networks and protocols
The protocol is set by the controller, not the I/O, so remote I/O is sold in versions for each major network. The choice usually follows the PLC vendor or the plant standard.
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Modbus RTU and Modbus TCP. The simplest and most widely supported option. Modbus runs over RS-485 serial or Ethernet and is the usual choice when an industrial PC or gateway is the controller.
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EtherNet/IP and PROFINET. The two most common industrial Ethernet protocols for PLC-based remote I/O, each tied closely to one family of controllers and published by its own user organisation (ODVA and PI).
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EtherCAT. Used where cycle times well under a millisecond matter, such as motion control and high-speed test.
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CANopen and PROFIBUS. Older fieldbuses still found on many installed machines, mobile equipment and process plants.
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MQTT and cellular. Some remote I/O reports to the cloud or a SCADA server directly, which suits pumping stations and other remote sites where no controller is close by.
Remote I/O with an industrial PC
A PLC is not required. An industrial PC or IIoT gateway can act as the network master, polling remote I/O stations over Modbus TCP or another protocol and running the control or data-logging application in software. This is common in test rigs, building services, water and energy sites, and anywhere the I/O data also feeds a database, dashboard or analytics.
Our industrial IoT gateways and DIN rail PCs are a natural fit for this role: they mount in the same cabinet as the I/O, run from 24 V DC and offer serial and Ethernet ports for the field network. Where the I/O count is small and the cabinet is local, board-level digital I/O modules inside the computer can replace a separate station altogether.
On an Ethernet-based system, the switch matters as much as the I/O. Managed industrial Ethernet switches add VLANs to separate I/O traffic, quality of service to prioritise it, ring redundancy to survive a cable break and port diagnostics to find a fault quickly.
Quick answers on remote I/O
Remote I/O: common questions
Is remote I/O the same as distributed I/O?
Yes, in practice. Both describe I/O stations placed away from the controller and linked by a network. Some vendors use “distributed I/O” for stations on a fast fieldbus close to the machine and “remote I/O” for longer distances, but the idea is the same.
What is a remote I/O panel?
A small enclosure that houses a remote I/O station. It typically holds the network adapter, I/O modules, a 24 V power supply, terminals and sometimes a small switch, mounted near the equipment it serves.
Does remote I/O need a PLC?
No. Any device that speaks the network protocol can be the master, including industrial PCs, IIoT gateways and SCADA servers. Modbus TCP remote I/O is particularly easy to read from software.
Is remote I/O slower than local I/O?
Slightly, but rarely in a way that matters. An industrial Ethernet network usually updates every few milliseconds, which is fast enough for most process and machine control. Motion control and very fast counting may still need local or specialised I/O.
Can remote I/O be used in hazardous areas?
Yes, with the right ratings. Stations are available for Zone 2 and Class I, Division 2 installation, and intrinsically safe versions can connect to field devices in Zone 0 and Zone 1.
We supply industrial PCs, gateways and switches as part of a remote I/O system, with the protocol stack and network configured as standard before the system ships.