A PLC and an HMI rarely arrive at the same dock on the same day. One vendor supplies the controller, another the panel, and the integrator discovers on commissioning day that nobody agreed on who owns the wiring, the data model, or the troubleshooting path. Before you issue a purchase order, walk through the six points below and put the answers in writing.
1. Match the I/O count to the machine, not to the datasheet
Start with a list of every sensor, valve, drive and safety device the machine actually has, then count the signal type for each one: digital input, digital output, analog input, analog output, encoder, thermocouple. Buyers often size the controller from the vendor's standard configuration instead of the real list, and the gap surfaces as spare slots that are the wrong type. Add roughly one spare channel per signal type so a failed output card does not stop the line while a replacement is shipped. Then check whether the machine will move or be rewired in the next few years — remote I/O stations mounted near the tooling reduce the length of field cable and the number of terminals you have to terminate on site.
2. Confirm the bus and the topology before the panel is built
The interface between PLC and HMI is a decision, not an assumption. Common options are Ethernet/IP, PROFINET, Modbus TCP, EtherCAT and a vendor-proprietary driver, and each one dictates cable type, connector, switch requirements and address planning. Ask the HMI supplier which drivers they support natively, and for which firmware version. Then decide the topology: a point-to-point cable between panel and control cabinet, or a trunk with drop cables and switches. If the machine sits several metres from the operator position, the cable route, the conduit fill and the shielding strategy all need specifying now, because retrofitting a route inside an existing enclosure is where integration budgets disappear.
Ask for the exact address plan in advance: IP addresses, device names, and whether the two devices must sit on the same subnet or be routed through a managed switch. A managed switch introduces a second supplier and a second set of configuration skills; if you do not have them in house, say so when you request the quotation and make the integrator provide it.
3. Check the electrical and panel requirements at the quotation stage
The mechanical reality of integration is usually smaller than expected but expensive to change: the cutout dimensions for the HMI, the mounting plate, the clearance behind the door for the bezel and connectors, and the separation between power and signal wiring inside the cabinet. Confirm the supply voltage available at the position, whether the HMI runs from the cabinet 24 V supply or needs its own feed, and how the PLC and panel are earthed. Ask whether the panel door can be closed with the cable in place — an HMI mounted on the outside of a door with a through-door gland is a common and avoidable surprise. Also ask for the terminal and wire numbering convention, and for the drawings in an editable format rather than a PDF, because you will need them when the machine moves plant.
4. Agree the tag structure and who owns the program
Integration effort is mostly a conversation about names. Before coding starts, agree a tag naming convention, and decide which side writes the HMI tags by importing the PLC tag database rather than retyping them. Retyped tags are the most common cause of a screen that shows nothing. Ask which device is the clock master and time source for alarm records, and how alarms are numbered and described, so an operator's message means the same thing on every screen.
Then get the commercial question out of the open: who delivers the PLC program and the HMI project file, in what format, and with what licensing. Ask whether the program is copyable, whether an offline editor is needed to make changes, and what the editor costs. A machine you cannot reprogram without the supplier's seat is a machine you do not fully own. If you expect your own engineers to maintain it, request a documented, commented program and a change log as part of the acceptance conditions, not as an afterthought.
5. Test the integration before the machine ships
Ask for a factory acceptance test that includes the control layer, not only the mechanics. A reasonable checklist: every input triggered at the machine and observed on the screen; every output commanded from the screen and observed at the terminal; an analog value forced at both ends and compared to the displayed figure; a safety device interrupted and the machine stopped, with the alarm shown and logged; the HMI power cycled and the PLC state checked afterwards; the communication cable pulled and the fail-safe behaviour observed. Agree pass criteria and units before the test, and require values to be read at both the sensor and the HMI rather than judged by eye. Fixing a mapping error in the factory costs a morning; fixing it after delivery costs a shutdown and two flights.
6. Plan for what happens five years later
Ask how long the HMI and PLC families will be ordered, and what the supplier's policy is on successor models. Proprietary operator panels that cannot be reprogrammed by your own staff become a dependency when a screen fails, so check whether the project file can be migrated and whether the editor is still purchasable. Check the licence model, too: some editors and driver add-ons are per-developer or per-machine, and an upgrade can quietly re-trigger a cost.
A short list to send with your enquiry
When you ask both suppliers for a quotation, attach the I/O list with signal types, the required bus and topology, the panel layout with available cutout and clearance, the supply voltage and earthing arrangement, the agreed tag convention, the acceptance-test checklist and the handover terms for the program files. Every item settled before the order removes one change order from the commissioning week. If a supplier will not commit to these points in writing, that answer is itself part of your evaluation.
PowerDe builds automation and cleaning equipment for production and maintenance environments and offers non-standard configuration, so where a machine's control requirements fall outside a standard offering, send the I/O list and the intended topology with your enquiry and the engineering team will confirm the configuration before manufacture. Details on automated production line solutions and on equipment manufacturing automation services; the company behind them is Shenzhen PowerDe Automation Precision Equipment.
