If you are a procurement engineer, plant maintenance lead or project manager comparing quotations, the checklist below is the practical way to find those gaps while they can still be closed on paper.
Gap 1: The tag list arrives after the screen design
The most common failure point is the data interface itself. Purchase orders usually say "PLC plus 10-inch HMI", which is a hardware description, not a data description. The real interface is a tag table: every name the HMI reads or writes, its data type, its address, its unit, its update direction, and who created it.
- Tag names and data types, written once and used identically by both sides
- Units and scaling — a counter delivered in raw counts, displayed as litres per minute
- Direction of every write: which values the operator may change, and where those values are stored after a power loss
- Reserved spare tags, so a change request mid-project does not force a rebuild of the tag table
A useful test at the quotation stage: ask the integrator to deliver a tag list, not only a bill of materials. If nobody can produce one, the interface has not been engineered yet — the schedule will absorb the difference.
Gap 2: Communication is specified as a protocol, not as a behaviour
Naming a protocol such as Modbus TCP, PROFINET or EtherNet/IP settles the wiring question and leaves the harder questions open. What happens when the HMI loses the link: does the process keep running, does it hold, does an alarm appear, and how quickly? Which values freeze on screen, and could an operator act on a stale number? Who owns the IP scheme, cable routing and switch ports, especially on a line that already carries other devices?
- Defined behaviour on link loss, including the displayed message and the state of the equipment
- Polling rate per tag group, so a slow screen does not mask a fast-changing machine value
- Network ownership in writing: which party assigns addresses and provides the infrastructure
Gap 3: Control modes and interlocks are described in prose
"Automatic, manual and maintenance modes" sounds agreed until someone asks what automatic mode permits, who may enter maintenance mode, and which physical interlocks stay active there. Prose descriptions hide three separate decisions: the mode transitions, the conditions that block a transition, and which mode a machine returns to after a power failure.
Ask for a mode table — one row per mode, one column per permitted action — plus a written list of the safety interlocks that operate independently of the HMI. This is also where duty-holder logic belongs: local versus remote control, and the handover between them.
Gap 4: Alarms have codes but no defined response
An alarm list copied from a drive manual is not an alarm specification. For each alarm the project needs a source, a severity, an operator-facing message in the language of the shift, whether it stops the machine or only warns, and whether it must be acknowledged before restart. History matters too: how long events are kept, and how they are exported for maintenance review. A machine that raises a message nobody has to confirm gives maintenance nothing to review afterwards.
Gap 5: Recipe and parameter ownership is left open
Product changeover is where PLC and HMI meet most often. Specify how many recipes exist, where they are stored, which fields an operator may edit, which are locked to an engineer level, and what happens to a modified recipe on power loss. Unmanaged recipe access produces the two complaints that surface months later: products processed with the wrong parameter set, and settings that quietly drift because several people can write to the same value.
Gap 6: Access levels and traceability are treated as an afterthought
If your buyers or auditors ask who changed a setting and when, that requirement has to be in the specification before the program is written. Retro-fitting user management onto a finished HMI project is expensive and often only partially possible. State the number of access levels, who administers accounts, whether parameter changes are logged with a timestamp, and how passwords are handed over at the end of the project.
Gap 7: Acceptance tests both screens and interfaces
A screen that "works" in the integrator's workshop is not acceptance. A usable test set is short and specific: every write from the HMI observed at the PLC, every PLC state change observed on the screen, a deliberate link break with its defined response, a power failure during operation, and one full product changeover performed by an operator, not by the programmer. Anything written as "smooth operation" in the contract is untestable and will be signed off anyway.
Gap 8: Source files and future changes are not in the contract
Ask which engineering tools and licenses were used, who will hold the editable PLC program and HMI project files, and what the price of a later change looks like. Integrators are entitled to protect their intellectual property; what the buyer needs is the ability to maintain and extend the system. Clarify in advance whether you receive full sources, a documented export, or a maintenance agreement, and put the version-handling routine in writing.
How PowerDe handles the interface question
PowerDe builds automated production line solutions, including the Industry 4.0 demonstration factory, and supplies non-standard automation equipment designed around a customer's process. Because the line and its control layer are engineered together, the tag structure, mode logic and operator screens are defined during equipment design rather than patched together at commissioning — which is where most of the gaps above disappear before hardware is ordered.
Alongside complete lines, PowerDe supplies standalone industrial steam cleaning equipment for maintenance work around a plant. The detail page for the Fixed Steam Cleaning Machine PowerDe HW02 lists 220 V working voltage, 6 kW heating power, a 160° steam temperature, 0.8 MPa vapour pressure and an 80 L SUS304 tank, with dry and wet steam modes. On an automated line this kind of unit supports upkeep of the machinery and its surroundings rather than the control layer; whether a specific model suits a food, kitchen or machining duty should be confirmed with the engineering team rather than assumed from the datasheet.
PowerDe also offers engineering services — steam cleaning equipment product development, hardware testing and integrated engineering team services — which can cover the controls scope of an integration project where your own team supplies the line.
A short pre-award checklist
- Is there a tag list with data types, units and write permissions, agreed by both parties?
- Is link-loss behaviour defined, including the displayed message and equipment state?
- Is there a mode table plus a list of interlocks independent of the screen?
- Does every alarm have severity, operator text and defined stop behaviour?
- Are recipe storage location, editable fields and access levels specified?
- Are acceptance tests written as observable pass/fail items?
- Is the source-file and change-request arrangement agreed before award?
Each item is cheap to close on paper and expensive to close on the shop floor. If a quotation cannot answer these seven questions, the gaps are still open — and they will be found during commissioning, at the buyer's cost.
To discuss the control layer of a line, an integration scope, or cleaning equipment for the plant around it, contact PowerDe at sales@powerde.net or +86 13023198376.
