Most employee fobs leave the reader as Wiegand data, so a reader on the panel's USB port gives you nothing until a converter turns the output into Modbus registers or keystrokes. Decode the 26-bit credential, compare it against a table you maintain, and grant the screen from that lookup. Keep one administrator password as the recovery path.
An employee fob can open a protected HMI screen, but it will not type the password for you. Most access-control fobs and cards leave the reader as Wiegand data, 26 bits in a format no password field accepts. The build that works has three parts: a converter that turns the reader's output into Modbus registers or keystrokes, a pop-up prompt that asks the operator to present a fob, and a credential table you maintain that decides who gets in. The panel's built-in password screen stays as the administrator recovery path, nothing more. Here is the format, the interface choice and the numbers to check before you order anything.
What the reader sends
The common output is the 26-bit Wiegand format, the de facto standard of access-control readers: a leading parity bit, an 8-bit facility code, a 16-bit card number and a trailing parity bit. Eight facility bits give 255 facility codes. Sixteen number bits give 65,536 possible card numbers, 0 to 65,535, inside each facility code. The credential your panel compares is the 24 bits between the parity bits.
Longer formats exist, and 37-bit readers are common. A reader set to 37-bit output feeding a converter that decodes 26 bits hands the panel a number that was never enrolled, so every presentation fails. That is the symptom you get after a reader swap. Match both ends to one format before wiring anything else. Reader manuals lay out the bit structure of the Wiegand interface; read your reader's sheet for the format it ships in.
Some readers can emit the card's raw ID in place of the facility-plus-number pair; one documented example shows a 13.56 MHz contactless tag arriving on Wiegand as a 4-byte hex value, 009BC342. Write down which output your reader uses before you design the lookup.
Choose the interface between reader and panel
What decides the path is what the panel consumes, and the documentation says so, not the connector. Two converter families cover nearly every case.
A Wiegand-to-Modbus converter takes the reader's output and sends the card number to the controller over RS485 Modbus; the panel reads the value as register data through the controller. This is the default for an access screen, because the credential lands where your logic already lives and the reader stays a reader when the panel is eventually replaced. For the controller side, the page on integrating Wiegand card readers with PLC systems goes deeper than this article.
A Wiegand-to-USB converter presents the card number to the host as keyboard input, so the value lands in a numeric entry field like typed digits. Take this path when the panel's documentation names keyboard-class devices on that USB port. A USB host port accepts the device classes its documentation lists, the same documentation question as which USB drives an HMI panel accepts.
If your panel documents native support for badge or barcode readers, use that and skip the converter entirely; choosing a USB badge reader for gloved operators covers that route. If the user list is short and stable, a PIN-protected settings screen means less hardware and no enrollment table.
| Path | Reader side | Panel side | Pick it when |
|---|---|---|---|
| Wiegand to Modbus | Wiegand output | RS485 Modbus into the controller, value read as registers | The panel or PLC speaks Modbus and the lookup lives in the controller |
| Wiegand to USB | Wiegand output | USB host port with documented keyboard input | Documentation names keyboard-class devices on that port |
| Panel-documented reader | A USB device the panel lists | Native support in the HMI software | Your panel names readers it supports |
| PIN screen | None | Built-in password or PIN objects | Few users, rare changes, no reader hardware wanted |
Recommendation: Use a Wiegand-to-Modbus converter into the controller, prompt for the fob with a pop-up, and decide access in your own lookup table. Take the Wiegand-to-USB converter only when the panel documentation names keyboard input on that USB port. Stay with a PIN screen when the user list is short and changes rarely.
Example: one fob through the chain
Take a reader set to the 26-bit format and a test fob whose enrollment data reads facility code 37, card number 4231.
Check the ranges first. Facility 37 fits the 8-bit field, 0 to 255. Card 4231 fits the 16-bit field, 0 to 65,535.
Treat the 24 credential bits as one integer:
37 × 65,536 + 4,231 = 2,424,832 + 4,231 = 2,429,063
That is the number the converter hands over, in whatever representation its manual documents: one decimal value, a hex string, or words you reassemble in a fixed order. Your table holds 2,429,063 against that operator's permissions.
Now the mismatch case. Swap in a reader outputting 37 bits and change nothing else: the converter still decodes 26, the same fob arrives as a number matching no row, and every presentation fails. Set the reader back to 26-bit output and the same fob reads 2,429,063 again. One fob, one stable value on every read is the acceptance test for the converter, and the number belongs in the commissioning record.
Example: sizing the enrollment table
A plant with 140 operators and a 30 % yearly turnover adds and removes about 42 records a year, under one a week. That volume is table maintenance, not a project: one person with the editor open handles it. The same plant using a shared password instead would reissue and re-teach one password to 140 people every time anyone with it leaves.
Build the access flow on the panel
This is the screen-side half of HMI screen access control for a protected operator view, with the credential arriving from hardware.
- The protected-area button opens a pop-up reading "Present your fob". Build it as shown in building an HMI pop-up screen with a screen window.
- The reader event lands the credential in the agreed register, or the keyboard converter types it into the entry field on the pop-up.
- Your logic compares the value to the table. A match with permission opens the protected area. An unknown fob, a disabled record or an empty read leaves it closed and re-prompts.
- Clear the read value after each decision, so a second presentation cannot inherit the first result.
- Keep one administrator password as the documented recovery path for reader or converter failure. This is also the fix for the shared-password problem you started with: a fob record is removed in one table edit, while a shared password lingers until someone changes it.
Size the prompt for arm's length, about 3 ft. At the planning figure of 30 ft of viewing distance per inch of capital height, 3 / 30 = 0.1 in capitals is the floor. The 20 ft per inch figure for fast reading gives 3 / 20 = 0.15 in, and that is the size to use for the prompt line.
Use the gate for screen access only. An interlock or a stop function never routes through a credential lookup.
When to pick something else
If the HMI project cannot hold a credential table, move the lookup to the controller and let the panel ask for a single pass bit. If your operators already wear gloves and carry badges, the badge-reader route above avoids issuing fobs. Keep the fob gate on the HMI itself: shift notices, the authorized-user procedure and the quality board belong on a separate shop-floor signage screen, which plays from a memory card with no subscription.
What to ask for before you order
From the reader documentation: the output format, 26-bit, 37-bit or raw ID, and the fob technology it reads, with 13.56 MHz contactless the common one.
From the converter documentation: the data direction, the register or keystroke mapping, and the number representation. A converter sheet that shows none of these is the wrong converter.
From the panel documentation: the Modbus support, and the USB device classes the host port accepts.
From your own project: who maintains the table, how a leaver's record is removed the same day, and who holds the recovery password.
Order the reader and converter only when all four answers are on paper.
Questions
How many different fobs can a 26-bit format distinguish?
Multiply the fields: 255 facility codes times 65,536 card numbers is 16,711,680 combinations, about 16.7 million. The facility code keeps two sites' overlapping card ranges distinct; your HMI table only needs the combined 24-bit value to be unique.
Should the fob number itself become the screen password?
No. A Wiegand credential is an identifier, not a secret: it is a number the reader sends on every presentation. Treat it like a username and keep the decision in your table, where a record can be deleted in one edit. Leave the built-in password screen active for a single administrator account, used as the recovery path.
What should happen when the reader or converter fails?
Design the resting state closed: no read, no grant, and the protected area stays shut. Recovery is the administrator password, used deliberately and noted in the shift log. After a restart of the panel or converter, present a known test fob and check it still returns the enrolled value before trusting the gate again.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.