A PLC and an HMI often look like one line item on an equipment quotation, but the two components answer different questions. The PLC decides how the machine behaves; the HMI is what an operator, a shift supervisor, and a maintenance technician actually touch. The gap between "the machine runs" and "the machine runs on our line, on our shift, with our people" is usually closed with information that never appears in the PLC program itself.
The checklist below is the one we ask buyers to complete before we start writing sequences. Completing it does not make the integration cheaper, but it removes the change orders that make it expensive.
1. Start with the interface, not the controller
Most integration pain sits where two control domains meet. A production line that ties several machines together usually has one controller per station plus a supervising layer above it. Before selecting anything, record what the machine must hand to a neighbouring cell and what it must receive back — a part-ready signal, a recipe number, a fault code, an interlock, a batch counter. Each of those has a direction, a speed requirement, and an owner.
That record is also your safety boundary. An emergency stop that has to stop two machines in the same guard zone cannot be simulated by an HMI screen. Anything that protects a person belongs in hardwired safety circuitry with a defined safety category, and that decision has to be made before anyone writes a ladder rung.
2. Pin down the I/O list and the wiring reality
Write the I/O list as a table of real devices, not of signal names: sensor type, voltage class, whether the point is sinking or sourcing, whether it needs a relay, an isolator, or an analog input with a defined range. Then add roughly a quarter spare on every card type. Spare points cost almost nothing at panel-build time and are the difference between a two-hour fix and a weekend shutdown later.
Also record where the field wiring terminates. A remote I/O block at the machine shortens the cable run and moves the failure point; a hardwired backpanel at the cabinet does the opposite. Neither answer is wrong, but the choice belongs to the site survey, not to the panel shop.
3. Decide how much intelligence lives in the HMI
Decide, per screen, whether it is a display or a control. If an operator can change a setpoint from the HMI, that setpoint needs a range, a default, a units label, and a defined behaviour when the PLC loses the value. Alarms need the same treatment: define the classes (stop, warning, informational), who acknowledges them, and whether an alarm must survive a power cycle. A screen showing every raw fault in the program is not documentation — it is a reason operators stop reading alarms.
Language and units are part of the same decision. Note who will read the screens, in which language, and in which units, because retrofitting a second language into a screen that was authored with English text baked into graphics is a redraw, not a translation.
4. Fix the data model and the historian question early
Tag naming is the least glamorous and most durable part of the job. Agree on a convention before the program starts, one that carries the station, the device, and the signal type, and keep the tags the HMI reads identical to the tags the PLC owns. If the same value has two names, the mismatch will surface during commissioning rather than in design.
Then decide what has to leave the machine at all. If the line has to report cycle counts, downtime reasons, or traceability data upstream, that changes the PLC task list and the network load, and it should not be added after the program is frozen. Whether it travels over a fieldbus, an Ethernet protocol such as OPC UA, or a plain CSV export to a shared folder is a decision about IT security policy and IT ownership — bring your network administrator into the conversation at this point, not at the acceptance test.
5. Treat power, environment, and access as design inputs
Write down the supply the site can actually deliver: single or three phase, control voltage, available capacity. Then check what else lands inside the same panel and the same program. On lines that carry an in-line cleaning position, the cleaning equipment brings its own supply demand and its own interlocks into scope — the fixed steam cleaning machine listed in PowerDe's cleaning series runs on a 220 V supply with a heater rated in the several-kilowatt range — and both the load and the interlock chain have to be sequenced with the machine it serves.
Note the ambient conditions at each station: humidity, washdown, sunlight, and whether the screen has to work through gloves. Those conditions drive the enclosure rating, the bezel, the cable entry, and the screen technology, and they also determine whether a handheld device is a better interface than a panel-mounted one. A wash position changes the environment of everything mounted near it, so settle it before the panel layout is drawn.
6. Agree the engineering handover before the order
Before the purchase order is signed, get agreement on what you will receive: a commented program in a format you can open, a tag database the HMI can be rebuilt from, an electrical drawing set, a backup-and-restore procedure, and a named point of contact for the version of firmware you will be holding in three years. Also settle who holds the passwords. A machine whose program lives only inside a vendor laptop is a machine you cannot maintain.
Ask the vendor to run the sequence on paper with your own operators present. Their questions about shift change, product changeover, and cleaning routines surface requirements that no specification document catches.
What to send when you request a proposal
If you want a realistic proposal rather than a placeholder, send the interface list, the I/O list, the site power and network constraints, the screen-language and units requirements, and the handover terms you expect. For automated production line work that also carries a cleaning cell, we work from those inputs and come back with the control architecture, the panel scope, and the order of steps we would commission — along with the points we still need from you.
