Fix an HMI protocol error that pops up every 30 seconds

What each version of the symptom points to

The shape of the symptom decides where to look first:

• A popup that returns on a steady, short interval while the link otherwise works. An intermittent comms fault: one transaction at a time is being disturbed or refused. Work supply and wiring first, then mode, password and polling.
• The popup wakes the screen from the screen saver. The error itself raises the backlight, so a panel that lights up with nobody near it has just logged an event. Read the wake time as the event time.
• The same error sits on screen and never clears. A constant failure, which is a different path with its own checks: fixing HMI PLC timeout and invalid checksum errors covers it.
• Two to three errors an hour on some programs and almost none on others. Timing: the fault tracks what the program polls and how often. Diff the two projects' update lists.
• Errors cluster around a machine event: a weld start, a VFD ramp, a solenoid firing. Induced noise on the comms cable or the supply wiring. Go straight to the routing check.

Start with a timestamp log

Record before you change anything:

1. The exact text and code on the popup. One panel family prints 0x0008 Protocol error, and in the field that code has been read as a PLC sitting in RUN instead of TERM, or a PLC carrying a password. Your panel's own error table outranks any field reading, so look the code up in the manual first.
2. For each event: the time to the second, what the PLC was doing, what the machine was doing, and whether the screen woke from the saver. Note the backlight saver timeout setting in the baseline, because a wake-up with no one touching the panel is part of this symptom.
3. The communication diagnostic counters on the panel and the PLC, where they exist, with their values before and after an event.
4. Every reset and power cycle, marked in the log. A reset that quiets the screen pauses the record, and the interval clock starts again from there.

The spacing is the finding. Match it against what cycles on the line: a poll, a solenoid, a machine stroke at the same period; or, at two to three events an hour, something that cycles every 20 to 30 minutes.

Example: a 5 A, 24 V DC supply feeding only the panel

The supply is the first hardware check, because a panel that dips mid-transaction throws protocol errors.

Inputs. The panel's stated minimum draw is 1.2 A, quoted on this panel class with nothing else using the supply. The candidate is a DIN-rail control supply with a 120 W nameplate at 24 V DC, a common size on the shop floor.

Rule. The supply passes when its rated current covers the panel's stated minimum and the panel keeps the output to itself.

Arithmetic.

• Supply rating: 120 W ÷ 24 V = 5 A.
• Panel draw at the stated minimum: 1.2 A × 24 V = 28.8 W.
• Headroom: 5 A − 1.2 A = 3.8 A, which is 3.8 A × 24 V = 91.2 W.

Result. The supply clears the panel's minimum by 3.8 A (91.2 W).

Decision. Above 1.2 A and dedicated, the supply is correct, and the 3.8 A of headroom is the reserved margin the 'nothing else on it' condition asks for, so it stays empty. The moment a lamp, heater or solenoid driver lands on that output, the supply goes back on the suspect list: move those loads to their own supply, or size one supply from the full current list, control-power style (PLC CPU plus communication module plus I/O current, added). Finish the check with a meter: measure the voltage at the panel's supply terminals while the machine runs, and again if you are watching when the popup returns, and compare each reading with the operating range in the panel's manual.

Route the wiring away from the noise sources

The usual inducers of a one-transaction-at-a-time fault are unsuppressed solenoids, welders, VFDs and other high-current or high-frequency equipment. MOVs on the solenoid coils tame the spike at the coil; the wiring from coil to panel can still couple into your comms run, so the routing check stays on the list even in a panel with every coil suppressed.

• Trace the comms cable and the panel supply wiring along their full runs.
• Flag every parallel run beside high-current or high-frequency wiring: welder cabling, VFD output leads, solenoid circuits. Shared duct or tray is the classic case.
• Reroute out of the shared run, and cross power runs at right angles where a crossing cannot be avoided.
• Check the shield terminations against the panel's and PLC's installation instructions, and change nothing there on your own authority.

If your panel shares one serial line with several PLCs, wiring one HMI panel to three PLCs on an RS422 multidrop covers the layout side of that arrangement.

Check: the comms and supply runs no longer sit parallel to the sources you flagged, and the shields terminate the way the installation instructions require.

Test the mode and the password, one change at a time

On that field reading, two settings can put a protocol error on screen: the PLC sitting in RUN instead of TERM, and a PLC password. A constant error fits the reading neatly. An intermittent one still leaves both on the test list, because systems in the field have run entire seasons in RUN with a password set before the error showed up once.

Record both states on the faulting system and on the clean one: mode switch position, password set or not. A difference between the two is a finding. To test it, change one condition at a time, and only while the machine's procedure allows: move the mode switch, then watch a window that covers many expected events (30 minutes is 60 expected events at a 30-second spacing), restore the production state, then change the other condition.

Two lines you do not cross. A mode change is a machine state change, so make it with the machine in the state its procedure calls for, never live on a running line to see whether a popup stops. And a password that shows up in the comparison is handled through the vendor's documented configuration for the panel-to-PLC link; the password stays on the PLC.

Compare the clean twin, then the projects

Two same-family systems, one clean, is the best diagnostic setup you can have. Baseline both:

• panel and PLC project versions
• communication settings and addresses on both sides
• the polling and update lists: what is read, and how often
• firmware versions
• supply source, rating and shared loads
• cable routes and shield termination
• mode switch and password state
• the applications' operating patterns

Every line on that list is open; matching model numbers closes none of them. The differences between the two baselines are your suspect list, and you work it one change at a time with the log running.

Give the program-dependence clue its own pass. A fault that runs at two to three an hour on one program and almost never on another says the two projects do something different on the wire. Diff their update lists: what each polls, how often, and how much per transaction. Find what the busy project does that the quiet one does not, change that one thing, and let the log rule on it.

Firmware comes last

Firmware sits at the end for two reasons. Everything ahead of it is faster to check and more often the cause. And the twin gives a verdict: if both systems run the same firmware and only one faults, the difference lives in that installation or project, and a firmware flash there changes the subject instead of the fault.

If the versions differ, bring the faulting system to the clean system's version, following the manufacturer's release notes, and let the log deliver the verdict. The old rate here was one popup per 30 seconds: 3,600 s ÷ 30 s = 120 events an hour, and 120 × 24 h = 2,880 expected events in a 24-hour log. A log that stays clean for 24 hours after the change is a result.

The fix, in order

1. Log events with timestamps, the exact code, machine and PLC activity, and saver wake-ups.
2. Verify the supply: dedicated output, at or above the stated 1.2 A, voltage steady at the panel terminals.
3. Reroute comms and supply wiring out of parallel runs beside welders, VFDs and solenoid circuits; terminate shields per the installation instructions.
4. Record mode and password on both systems, then test one condition at a time where the machine procedure allows, and restore.
5. Diff the two projects' polling and update lists, and change the suspect item on its own.
6. Match firmware to the clean system only after steps 1 through 5 pass, then judge by a 24-hour clean log.

Recommendation: On a link that works apart from a popup every 30 seconds or so, run the supply and routing checks first, with the panel on a dedicated output at or above its stated 1.2 A and no parallel runs beside welders, VFDs or solenoid wiring. Then test mode and password one change at a time with the log running, and call firmware the repair only when a 24-hour log stays clean afterwards. If the link dies outright instead of flickering, that is a loss of communications, and HMI no communications when one PLC model fails covers that path.

One way out, when the board's only job is to show status: some of these screens exist to display a notice or status list, and the serial link exists only to update it. If that is your case, moving the content onto a display that plays from its own storage, like our shop-floor signage displays, takes the serial protocol out of the fault path. Keep the HMI for control; a signage display is not a control device and does not take its place.

Questions