A buyer searching for HMI is usually trying to stop a bad substitute. An automation component is specified by the signal, the network, the count of points, and who maintains the program. The notes below are selection criteria for an HMI. They are not a catalog promise. Confirm every rating on the current datasheet, drawing, or test report for the revision you will receive.
What has to be true of HMI
An automation component is specified by the signal, the network, the count of points, and who maintains the program. For an HMI, start from the duty, then the interface to the rest of the machine or the room, then the document that proves the claim. A lower price that leaves one of those blank is a different product.
HMI is easy to confuse with SCADA Software. If the quote could be read as either, add the line that separates them: the rating, the fitment, or the standard named below. Link the drawing number to that line so receiving inspection can reject the wrong one.
Numbers that change the quote
| Line | What to write for HMI |
|---|---|
| Network | The protocol the plant already speaks. |
| Capacity | Points, axes, or scan needs if the process cares. |
| Software | The license and who owns the source. |
| Environment | Panel ambient and the IP code if it is outside the panel. |
| Function | Controller, I/O, drive, or HMI. They are not interchangeable. |
| Signals | Voltage, PNP or NPN, or analog range. |
How to compare two offers
Lay both quotes for HMI on one sheet and line up network, capacity, and software. If a cell is empty, the offer is not finished. Ask for the missing line in writing. When those match, the honest difference is lead time, the test evidence, and who carries the warranty. Do not average a fully specified HMI with a cheaper line that omitted network.
Documents worth having before the deposit
Ask for the protocol and the I/O type in the same line. The file should name HMI and the revision, not only the supplier’s homepage. A factory quality certificate such as ISO 9001 can sit beside that file. It does not replace it.
Failures that show up after delivery
- Mixing PNP sensors with NPN cards.
- Leaving the software license out.
- Buying an HMI and expecting it to be the safety controller.
On an HMI, the expensive version of the first mistake is discovering it after the goods are in the crate. Put the rejection condition in the purchase order while the supplier can still restate the offer.
A worked RFQ line
Use this shape, then replace the brackets with your duty: “HMI — State function, signal type, network, capacity, and software.” That sentence is a template, not a rating of any brand. If a supplier’s datasheet uses different units, convert them onto this sheet so the next offer can be compared.
Where words like RoHS / CE belong
Searches often add a constraint to HMI: RoHS / CE, or waterproof / IP65. Keep those on this page. RoHS is a material limit. CE is a declaration that lists the rules relied on. Ask for the document that names this part number and revision. IP65 in IEC 60529 means dust-tight and protected against water jets. Write that code if hoses are the real exposure. The word waterproof alone does not name a test. If you need a third constraint, add it as another line on the same drawing. A second URL that only swaps the adjective does not give a buyer a second fact.
Related in this section: how to specify SCADA Software.
Questions buyers actually ask
What should an RFQ for HMI include?
State function, signal type, network, capacity, and software.
Is a cheaper HMI the same part?
Only if network, capacity, and the documents match. A missing line is a different offer.
Do waterproof, OEM, or IP words need their own page?
No. For HMI they are extra lines on this specification. Ask for the test or the drawing that gives the word a value.