Buying a PLC and an HMI from different suppliers is often the source of a project's problems, not the coding. Most commissioning delays, warranty disputes and change-order arguments trace back to questions that were never put in writing before the purchase order was issued. A good request for quotation does not just ask for a price. It forces each vendor to state, in comparable terms, what they will deliver, what they will not deliver, and who carries the risk when something does not work.
If you are sourcing control panels, retrofit packages or complete non-standard automation equipment, the questions below are the ones that separate a usable quote from a low number that will grow later.
1. Which hardware does the integrator actually supply?
Ask each bidder to list, line by line, what is inside their scope: PLC CPU and I/O modules, power supplies, relays and contactors, terminal blocks, enclosure, cooling, HMI panel, industrial PC, network switches, cable, and labour. Then ask what is excluded.
Two quotes that look identical on a summary sheet can differ by a whole safety circuit, a remote I/O drop, or the software licence. Request an itemised bill of materials with manufacturer, model number and quantity. Anything written as "equivalent to" or "or similar" should be flagged and pinned down before award.
2. What is the plant's existing standard, and will you match it?
State your installed base in the RFQ: PLC brand and family, HMI brand and software version, programming environment, tag-naming convention and any library or function-block standard you already maintain. Ask the vendor to confirm, in writing, that they work within that standard and to name the engineers who will.
If you already own programming seats, say so and ask whether they need their own licence. If you do not, ask them to price the licence and state who owns it afterwards. This single question avoids the situation where a new machine arrives with a control platform nobody in your maintenance team can open.
3. Who writes the functional specification?
This is the highest-value question in the whole document. Ask whether the integrator will produce a written functional specification, sequence descriptions and interlock list before coding starts, or whether they expect you to supply a finished one.
Ask how specification changes are handled: what counts as a change, how it is priced, and how the schedule is adjusted. Vague answers here produce most disputes on integration projects. A vendor who can show you a sample specification and an interlock list is demonstrating a working method, not just intent.
4. What are the I/O count, panel size and mounting constraints?
Give the real numbers: point count by type (digital in/out, analogue in/out, high-speed counting, thermocouple), spare capacity you want reserved, field wiring method, and the physical envelope the panel must fit into. Include whether the HMI is door-mounted, remote-mounted on an arm, or in a separate station.
Ask the vendor to state the resulting enclosure dimensions and heat rise calculation. A panel that fits a narrow maintenance aisle on paper can be impossible to open in the field. If the machine sits outdoors, near washdown, or in a dusty or oily area, name the required ingress protection explicitly and ask for the component selection that supports it.
5. How will the PLC and HMI talk to the rest of the plant?
List every interface the controller must handle: fieldbus or industrial Ethernet protocol, drives, robots, vision systems, weighing instruments, barcode scanners, safety relays or safety PLC, and the supervisory layer above (SCADA, MES, historian, cloud gateway).
For each interface, ask the bidder to confirm the protocol, the direction of data flow, which side owns the mapping or database configuration, and whether any gateway hardware is required and priced. Drive and robot integration is a common source of unbudgeted engineering hours because it sits between the machine supplier and the control integrator unless the RFQ assigns it.
6. What network and security requirements apply?
Ask how the controller will be accessed for remote support, whether the vendor needs a VPN or remote gateway, and who configures and owns it. State your rules on USB ports, wireless links, remote access accounts, password policy and whether the control network must be segmented from the office network.
Require the vendor to document firmware and software versions for every programmable device at handover, and to state how long they will support those versions. A machine delivered with undocumented firmware versions becomes a sourcing problem the day a part fails and nothing on the market matches it.
7. What safety functions are in scope, and who signs them off?
List the safety devices and functions: e-stops, light curtains, safety gates, interlocks, two-hand controls, muting, safe torque off, and any required performance level or category. Ask the vendor whether they design the safety circuit, verify it, and provide the validation documentation, or whether that sits with your engineers or a third party.
Also ask how bypass and maintenance modes are handled in both the PLC program and the HMI. If a bypass function is allowed, require that it be logged, time-limited and access-controlled.
8. What does the HMI actually display?
Ask for a screen list, not a screen count: number of screens, navigation structure, alarm handling, alarm history, recipe management, user levels, trend and logging, and whether the HMI supports your operators' language. Confirm whether alarm text is editable by your staff or locked in a library.
Ask about screen development effort as a priced item. "Standard HMI package" means nothing comparable across vendors. A screen mock-up or prototype of two or three representative screens, supplied with the quote, tells you more about the integrator's design discipline than any line of text.
9. What testing happens before the panel leaves the workshop?
Ask for the factory acceptance test content and ask to witness it. At minimum, expect a point-to-point I/O check, an interlock and e-stop test, a power-up and fail-safe test, an HMI screen walkthrough, and a documented test report you sign before shipment.
Then ask what happens on site: how many commissioning days are included, who provides the operator training, how punch-list items are closed, and what defines provisional and final acceptance. Acceptance criteria written into the purchase order are worth more than a verbal promise of support.
10. What documentation and source code do you receive?
Ask explicitly for ownership of the PLC program and HMI project in editable native format, not only a compiled download. Also request electrical drawings in editable CAD or E-Plan format, terminal and BOM lists, cable schedules, the version record, and the operator and maintenance manuals.
Some integrators quote lower and retain the source. That is a legitimate commercial position, but it must be visible before you award, because it determines what your team can do in year three when a process changes.
11. Spares, support and the version question
Ask the vendor to identify obsolete-risk items and recommend a spares kit with prices: spare HMI, spare CPU, power supplies, fuses, drives and critical sensors. Ask their response commitment for a line-down call, whether they hold the spares or you do, and what support means after the warranty period ends.
A supplier who has built control panels for a range of machine types will usually tell you which parts they stock and which they have had to chase. That answer is often a better predictor of support quality than the warranty wording.
12. How is the quote priced and what does the schedule assume?
Ask for the split between hardware, panel build, software engineering, commissioning and travel, and for the assumed number of commissioning days and site visits. Ask what is excluded, what the payment stages are, and how long the price is valid.
Compare quotes on scope, not on total. A bidder with an itemised scope and named exclusions is usually the lower-risk choice, even when their number is higher than a competitor who quoted a lump sum against an undefined specification.
Turning the answers into a decision
Score responses on the completeness of the answers rather than the attractiveness of the price. A vendor who returns an itemised BOM, a draft functional specification, a clear change-order method, named commissioning days and editable source code has already told you how they will behave during the project. A vendor who returns a number and a lead time has told you very little.
If your work includes complete line builds or retrofits, ask the supplier about their automated production line solutions and how they structure integration scopes across mechanical, electrical and control work. Where the control package is part of a larger machine build, that structure determines who is accountable when interfaces fail — and it should be settled at RFQ stage, not at commissioning.
For custom machinery, also confirm what the supplier handles in-house versus subcontracts: panel build, wiring, PLC programming, HMI development and on-site commissioning. Shenzhen PowerDe Automation Precision Equipment describes its capability set as equipment R&D and manufacturing, system integration, customised configuration and technical support. Ask any bidder the same question you would ask that supplier, and require the answer in writing before the order is placed.
Send the same questions to every bidder
The value of this list is that it is fixed. Send the same questions, in the same order, to every integrator on your bid list and require written answers. You will find that the differences between bidders become visible before you commit money, and that the questions you asked in the RFQ are the same ones you will ask again at the factory acceptance test — which is exactly what you want.
