When the error appears only as the screen opens, the panel's burst of tag reads for that screen is hitting a subset of bad references, here 15 of 18 logged memory locations returning code 0x0002. Read the error log, bring the controller to the firmware version that cleared the symptom, then compare each failing address with a reference that reads clean and retest the whole screen.
An HMI protocol error that shows once as a screen opens, clears within a few seconds and comes back only when you reopen that screen is a screen-open tag-read failure, and the fix lives in the tag references, the controller firmware or both. In the case behind the numbers here, the panel raised error PLC-499 with return code 0x0002 every time one screen opened, and the error log held a 4-byte error for 15 of that screen's 18 memory locations. Work in this order: read the log and count the failing references, check the controller firmware against the version that cleared this symptom, then compare each failing address with a reference that reads clean. Replacing or rebuilding screen objects is the last step, and many cases never need it.
Match your symptom to the right case
The timing pattern decides where to start. Match yours to one of these:
- One error at screen open, cleared within seconds, back on every reopen. The panel's read burst for that screen's tags is being rejected. This article is that case.
- The message returns on a timer, for example every 30 seconds. A recurring scan is failing. That has its own fix in an HMI protocol error that pops up every 30 seconds.
- The panel answers a ping but reads keep failing. That points at the protocol driver, and the check for a panel that pings the PLC but reads fail walks it.
- One numeric display stays blank while the rest update. That is one object's tag address, and the tag-address check for a numeric display that shows nothing covers it.
- Address errors appear while you tag controller memory for the first time. That is an import and tagging problem: invalid address errors when tagging PLC memory.
The first pattern is the one here. The fault travels with one screen and with the act of opening it, so that screen's own tag references are the suspects.
Count the failing references in the error log
The log holds the two numbers that decide the whole job: how many tag references the screen reads, and how many the controller rejected. On this screen, 18 numeric input objects and a 4-byte error against 15 of the 18 memory locations.
Before you change anything:
- Write down the exact address text of every logged location, as the log prints it.
- List the locations with no logged error. Here that is 18 - 15 = 3 references, and they are your comparison set.
- Read the width in the entry as the width of the rejected read. A 4-byte entry means the read asked for four bytes; take the tag's data type from the controller project.
- Copy the log out if your panel can export it. Some logs cycle and drop their oldest entries first.
Then leave the clean references untouched. Changing all 18 tags in one pass erases the comparison the rest of this repair relies on.
Check the controller firmware first
The message on the panel is the controller's answer. PLC-499 with return code 0x0002 means the controller returned a rejection and the panel displayed it, so a controller-side fix lands before any panel-side edit. On this panel and controller family, the firmware version that cleared this exact symptom was V1.0.10.21.
- Read the installed firmware version from the controller's identification or diagnostic screen.
- Compare it with V1.0.10.21.
- If the controller is older and your site's change control allows it, update during planned downtime, following the vendor's official update procedure.
- Reopen the screen and check the error log for new entries.
A firmware update restarts the controller, so do it with the machine safely stopped and under your change control.
If the controller already runs V1.0.10.21 or later, skip the update. That was the second case in this story: current firmware, a direct export and import, and the error still logging at every screen open. The address comparison decides it. If your panel or controller model differs from this one, take the corrective version from your own vendor's release notes.
Example: 15 of 18 tag references fail when one screen opens
Inputs. One screen with 18 numeric input objects (panel project). Error PLC-499 with return code 0x0002 from the controller, shown as the screen opens, cleared within a few seconds, back on every reopen (panel behaviour). A 4-byte error logged against 15 of the 18 memory locations (panel error log). Corrective firmware V1.0.10.21 (vendor release documentation for this symptom).
Rule. Divide the logged failures by the number of tag references the screen reads. A count of zero sends you to the driver and the link. A partial count, anything from 1 to one short of the total, keeps you here and clears the cable, the network, the driver and the panel hardware, because the unlogged references read clean over the same path at the same moment.
Arithmetic. Failing references: 15. Screen references: 18. Clean references: 18 - 15 = 3. Share failing: 15 / 18 = 0.83, so 83% of the screen's tag references were rejected in the open burst and 17% read through.
Result. Three tags crossing the same cable, protocol and controller at the moment of the fault shows the path itself works. The fault sits in the 15 rejected references, or in the controller's handling of them.
Decision. 0 of 18 logged: the tags all read, so follow the driver and link case. 1 to 17 logged: check the firmware, then compare addresses; this case sat at 15. All 18 logged: check firmware first, then test the import for a global offset, because an address-format mismatch moves every reference together. Why tag address formats change between protocols explains how that happens.
Compare each failing address with one that reads clean
Take one logged location and one clean location and set their definitions side by side. Work down this list:
- The address text. What the panel stores for the tag, next to the same tag in the controller project. One character off can land the read in another tag or in unallocated memory.
- The width. The log says 4 bytes, so each failing read asked for four. Check where each failing tag starts and how wide it runs in the controller. Some controller families allocate a minimum of four bytes per tag however small the data, so read the tag's footprint from the controller's memory map before you edit the panel side.
- The register or memory type. Vendor troubleshooting documentation for device-no-response messages names the wrong register type for the intended register as a documented cause, so check the type as well as the number.
- The program's currency. If the controller program was edited or downloaded after the panel's import ran, the panel may be reading addresses the program has since moved. Check against the program as it runs now.
- The panel's tag-address guidance. This panel family's help file covers tag address problems under topic CM516. Your panel's help has an equivalent section; work it against the failing tags.
Correct the mismatches one tag at a time, download the panel project and reopen the screen. Each correction that removes one location from the log tells you the method is working.
Retest with the whole screen in place
The error came from opening the screen, so the deciding test is the screen open, with all 18 objects in place. A solo object test that passes tells you less than one open of the real screen.
- Open the screen.
- Leave it open for 30 seconds. The message cleared within a few seconds, so 30 covers the interval with margin.
- Check the error log for new entries.
- Close the screen and reopen it, then check the log again. Run the open-and-check cycle three times.
- Exercise each numeric input and confirm the value it reads and writes matches the controller.
Pass: three opens with no new log entries, and all 18 inputs reading and writing their intended values.
If production must run before the repair
Swap in an approved alternate screen, or remove only the references the log named, and record exactly what you took out. Leave the clean references in place, because they keep the comparison alive. A screen that skips its failing reads opens without the error, which buys time while it hides the fault, so the temporary state comes out as soon as the firmware and address work passes the full-screen retest.
The decision
Error once at screen open with a partial count in the log: bring the controller to the corrective firmware version, compare the failing addresses one tag at a time, and judge the result on a full-screen retest. Firmware current and no mismatch found, error still logging: preserve the exact error text, the log addresses, the installed firmware version and the panel project, and hand the case to the vendor's support channel. Error on a timer or every read failing: those are the driver and link cases, and the articles linked above carry them.
Recommendation: Read the error log and count the failing references before you touch anything. On a partial count, bring the controller to the firmware version your vendor documents as corrective for this symptom, then correct the failing tag addresses one at a time and call the repair done only after a clean full-screen retest. Never correct every tag on a screen at once: that erases the difference between the logged failures and the references that read clean.
Questions
What does return code 0x0002 from the controller mean?
In HMI error-code tables of this kind, a code in this class means the read or write request was sent to an invalid memory area: the controller rejected the address the panel asked for. The rejection is address-level, so the cable, the network and the protocol driver can be healthy and the code still appears. Treat the code as a pointer to the logged references, then compare those addresses against the controller's tag definitions.
Can the export and import functions still leave a wrong address?
Yes. Vendor support documentation for address errors includes cases where the tag exists in both projects and the connection is correct, yet the error still logs. An export and import copies the addresses as they stood when the export ran, so any controller program edit or download after that moment can leave the panel reading from where a tag used to be. Re-check the failing addresses against the controller project as it runs now.
Does this method apply to a different panel or controller model?
The method travels; the numbers are specific. The signature, one error at screen open cleared within seconds with only some references logged, and the order of work, log then firmware then address comparison then full-screen retest, apply to any panel and controller pair. The version V1.0.10.21 and the help topic CM516 belong to the family in this case, so take your own versions and topic numbers from your vendor's documentation.
When should I stop and hand the case to support?
Stop when three things line up: the controller already runs the firmware your documentation calls current, the comparison against the clean references finds no address, width or type mismatch, and the log still records failures on every reopen. Preserve the exact error text, the log entries with their addresses, the installed firmware version and a copy of the panel project, and pass them to the vendor's support channel.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.