Shared access only works when ownership, diagnostics, and failure behaviour are clear
- A shared device can serve multiple people or multiple control systems, but the access model must be explicit.
- In automation, Siemens describes shared-device setups as one I/O device whose modules or submodules are split between different IO controllers.
- The biggest upside is less duplication of hardware, cabling, and panel space.
- The biggest downside is that faults, permissions, and responsibilities can become blurred if the design is rushed.
- For safety-critical equipment, sharing is only acceptable when the safety function stays independent.
What the term means in electrical systems
In ordinary use, a shared device is simply equipment that several people use at different times: a kiosk terminal, a maintenance laptop, a portable tester, or a common HMI, the operator screen or panel, in a plant room. In electrical and automation work, the meaning gets more specific. An IO controller is the PLC or controller that manages a remote I/O station, and a shared setup lets one physical device feed more than one controller without duplicating every signal. That is why I treat shared access as an engineering decision, not just an operational convenience. Siemens’ PROFINET guidance uses the term for this kind of partitioned I/O arrangement, where each module or submodule still has a clear owner.
The important distinction is that sharing does not remove responsibility. It changes how responsibility is divided. If the people running the system cannot say who owns the signal, who can write to the device, and what happens if a controller drops offline, the design is already too loose. That leads directly to the architecture question: how is the device actually split up?
How a shared I/O device is partitioned between controllers
In a PROFINET network, a widely used industrial Ethernet standard, the hardware may be shared, but the control relationship is not. One controller owns one module or submodule, another controller owns a different part, and the assignment is exclusive rather than collaborative. That is the practical benefit of a shared I/O station: you can keep the sensors or actuators physically close to the process while still feeding more than one control domain. In real terms, you are reducing duplication at the edge of the system, not creating a free-for-all on the bus.
- The station remains one physical device.
- Each module or submodule is mapped to one controller only.
- The engineering tool should know which controller owns which part.
- Diagnostics are easier when the configuration sits in one common project.
- The device manual or GSD file, the device description file used by the engineering software, should be checked for controller limits before commissioning.
Siemens also notes that shared-device configurations can be handled in a common project or across multiple projects, but a common project usually gives better consistency checks and cleaner diagnostics. I prefer that approach whenever the site structure allows it, because it reduces the kind of small configuration errors that waste hours later. Once that layout is clear, the best way to understand the concept is to look at where it appears on real plants.
Where you will see shared devices in practice
Shared devices show up more often than people expect, especially in retrofit-heavy UK sites where engineers are trying to improve visibility without rebuilding the whole panel. The table below covers the cases I see most often.
| Example | Why it is shared | What to watch |
|---|---|---|
| Common HMI or kiosk panel | Multiple operators use the same screen on different shifts. | Session locking, user roles, and audit trails matter more than the screen itself. |
| Remote I/O station near field sensors | One enclosure serves more than one PLC or automation domain. | Make sure module ownership is clear and diagnostics map back to the right controller. |
| Energy meter or power monitor | Maintenance, operations, and the SCADA layer all need the same measurement data. | Decide whether each system reads the meter directly or through a gateway. |
| Field device shared by control and safety | The same transmitter or valve signal supports both normal control and protection logic. | Independence between layers must be preserved, especially in hazardous processes. |
| Portable test equipment | Several technicians use the same calibrated device over time. | Calibration status, asset tracking, and custody control become essential. |
These examples look different, but they all force the same design question: what exactly is shared, and what remains separate? That is where the upside appears, and it is also where the trade-offs start to matter.
Why teams choose shared devices and what they give up
I usually see shared designs chosen for three reasons: lower hardware count, less cabling, and a cleaner physical layout. In distributed systems, moving I/O closer to the process and letting multiple controllers use the right parts of the same device can cut panel clutter and simplify expansion. In 2026, the attraction is often less about saving one piece of hardware and more about making the data model cleaner and the plant easier to extend.
| What you gain | What you give up |
|---|---|
| Fewer duplicate modules, less wiring, and less enclosure space | More configuration work and a higher chance of ownership mistakes |
| Better use of local I/O near the sensors or actuators | A fault can affect more than one consuming system |
| Cleaner integration between PLC, SCADA, and maintenance data | Diagnostics can become harder if the device is not documented properly |
| Less duplication during retrofit projects | Some teams overestimate how much complexity the sharing actually removes |
The pattern is straightforward: the more valuable the device is to several teams, the more discipline it needs. That is why the safety and cyber side cannot be an afterthought.
Safety and cybersecurity are the real constraints
Once a shared device sits in a process environment, it becomes both an engineering asset and a risk boundary. Emerson’s discussion of field-device sharing between a basic process control system and a safety instrumented system makes the key point clearly: the control layer and the safety layer may use related signals, but the safety function should remain independent. I agree with that approach. If a failure in the normal control path can compromise the safety path, the design is not ready.
For me, the practical rules are simple:
- Keep safety functions independent unless the hazard analysis proves otherwise.
- Use unique accounts, role-based access, and audit logs on shared operator endpoints.
- Segment the network so that every workstation does not have equal access to every device.
- Document the failover behaviour when one controller, user, or communication path is lost.
- Do not let convenience override change control on live systems.
This is especially relevant in UK plants where control, maintenance, and safety responsibilities are often split between different teams and contractors. A device that looks simple on the panel can hide a messy permission model underneath it. That is why the next step is not technical elegance; it is a sober decision about whether sharing is actually justified.
How I decide whether sharing is the right design
When I review a design, I ask five questions before I sign off on a shared device:
- Does the asset need to be shared by users, by controllers, or by both?
- Is the information or control path safety-critical, or can it tolerate a common failure domain?
- Can every module, signal, or account be traced back to one clear owner?
- Will diagnostics still point to the right place when something fails at 2 a.m.?
- Is the site team actually prepared to support the configuration over time?
If the honest answer to any of those is unclear, I would usually prefer a simpler boundary: duplicate the device, isolate the function, or split the system another way. That is not conservatism for its own sake. It is the cheaper choice over the lifecycle if the plant is going to run for years, not weeks.
The checks that keep a shared design worth the trouble
The cleanest shared-device designs have three things in common: one owner per function, one clear failure mode, and one documented access model. If I cannot explain those three things in plain language, I do not consider the design mature enough for handover.
- Owner per function means every module, signal, and write path has a named engineering owner.
- Failure mode means the team knows what happens if one controller, user session, or communication link fails.
- Access model means only the right people and the right systems can interact with the device.
That is the practical answer I come back to again and again: a shared device is useful when it reduces duplication without blurring responsibility. If the design stays explicit about ownership, safety, and diagnostics, it can be an elegant part of an electrical system. If not, it becomes a small convenience that creates a large maintenance problem.
