A panel that shows no communications against one PLC model while it works against another has a settings or download software fault before a hardware fault. Read the serial port settings from both ends and set both to the FX2N programming port default of 9600 bd, 7 data bits, even parity, 1 stop bit. If they already match, download the same logic again with a current programming software revision; revision 7.0 restored a link that revision 5.0 had left dead.
A panel that reports no communications against one PLC model while it works against another is pointing at settings and download software before hardware. Work three checks in order: match the panel driver and serial port settings to the exact PLC model, isolate the failing side by benching the panel against the PLC that communicates, then download the same logic again with a current revision of the PLC programming software. One documented field case ran two days of phone support without a result: the panel connected about once in 100 tries against an FX2N, worked every time against an FX1N, and the link came back only when the identical relay logic was downloaded from revision 7.0 of the programming software instead of revision 5.0. Change one variable per test, and write down every setting you read.
What the symptom tells you
Match the pattern before you pick up a screwdriver; each one points to a different layer.
- No communications on every poll. The two ends disagree on a serial parameter, or the cable does not match the port's signaling. Nothing decodes, so the panel never reports a link.
- A connection about once in 100 tries. A marginal link: a parameter the ends disagree on, a loose conductor, or a port configuration that a recent download changed. Read settings before you swap parts.
- Works with one PLC model, fails with another. The strongest clue on the list. The panel, cable and driver can complete a link, so the variable is that PLC's port configuration or the driver's fit to that model.
- Connects, then fails with timeout or checksum errors. Frames are arriving and failing their checks, which is a different fault; fixing HMI PLC timeout and checksum errors works that pattern.
- Invalid address errors after the link opens. The link is up and the tags are wrong, which is a project problem, not a link problem; fixing HMI invalid address errors when tagging PLC memory covers it.
Match the driver and port settings to the exact PLC model
Start at the nameplate. Record the full model number off the PLC: an FX1N, an FX2N and an FX-2nc are different machines with different port arrangements, and a driver selected for one family is a guess on another. In the field case the working comparison was an FX1N, and that single fact drove the whole diagnosis.
Check the panel project's driver selection next. The driver must name the exact PLC family, because the driver fixes most of the serial parameters on the panel side.
Then read the port settings from both ends. The FX2N programming port's documented default is 9600 bd, 7 data bits, even parity, 1 stop bit. The FX2N-422-BD communication board, the optional programming port, documents 9600 and 19200 bd. Driver lists for FX-series serial links usually offer 9600, 19200, 38400 and 115200 bd. One field case suspected that a software revision change had moved a port to 19200 bd; that suspicion is a value to read back from the configuration, never a value to set from memory.
Underneath the specifics sits one rule every serial manual and RS-422 itself share: the two ends of an asynchronous link must run the same bit rate, data bits, parity and stop bits, or no frame decodes. The FX2N programming port uses RS-422 signaling, differential pairs with separate send and receive lines, so the cable has to match the driver's wiring diagram pin for pin; wiring one HMI panel to three PLCs on an RS422 multidrop shows that discipline on a shared port.
If the panel project exposes the port fields, correct the mismatch the read-back shows, download the project, and test. If the fields are locked, the fix moves to the PLC side: in one documented case, version 3.1 of the panel software kept the PLC communication edit box grayed out, and the box would not open with the panel online either.
Power the panel and the PLC down before you move or reterminate any wiring.
Isolate the failing side with the PLC that communicates
Keep the panel project, the cable, the power arrangement and the test method identical, and change one variable per test. Bench the panel against the PLC model that communicates, the FX1N in the field case.
If the FX1N connects on every try and the FX2N connects on none, the panel project, the driver and the cable can hold a link end to end. What remains is the FX2N's port configuration and the driver's fit to that model, so the next step is reading the FX2N port back with the programming software. If both PLCs fail with the same project and cable, the fault sits earlier in the chain: recheck the driver selection and the cable against both manuals before touching a PLC setting.
| Observation | What it isolates | Next step |
|---|---|---|
| FX1N connects on every try, FX2N on none | Panel project, driver and cable are sound | Read the FX2N port settings back and compare field by field |
| Both PLCs fail with the same project and cable | The fault is in the project or the link | Recheck the driver selection and cable wiring against both manuals |
| The failing PLC connects after one change | That change is the resolving condition | Repeat the test, record the change, stop editing |
The first row is the one that saves the most time: it turns a two-device mystery into a single-configuration problem.
Example: read both ends, then check the arithmetic
Scenario: a panel polls an FX2N through its 422 programming port every 250 ms. The driver is set to 9600 bd, 7 data bits, even parity, 1 stop bit, which matches the port's documented default. During a long upload someone raises the port to 19200 bd, leaves it there, and the panel starts reporting no communications.
Read the settings back before changing anything: the programming software shows the FX2N port at 19200 bd, the panel driver polls at 9600 bd. Now the arithmetic. Each character in this format carries 10 bits: 1 start, 7 data, 1 parity, 1 stop. At 9600 bd a bit cell lasts 1/9600 s = 104 µs; at 19200 bd it lasts 52 µs. The panel's receiver waits 104 µs per bit against a line that changes every 52 µs, so its sample points drift half a cell further into every character. Start-bit timing, parity and the stop check all fail, the panel never sees a valid response, and it logs a failed poll on all 100 tries.
Match the ends and the same exchange works. Set both back to 9600 bd, 7 data bits, even parity, 1 stop bit. A 12-character poll plus a 20-character response is 32 characters, and 32 characters at 960 characters per second (9600 bd ÷ 10 bits per character) takes 32 ÷ 960 = 33 ms of link time per exchange, about 13% of the 250 ms cycle. One number, equal at both ends, is the whole fix. If the driver also offers 19200 bd, that matched rate works too; the documented default simply needs no other change.
Re-download the same logic with a current software revision
The field case ended here. The settings read identical field by field, the FX1N comparison had cleared the panel, cable and driver, and the link still failed about 99 tries in 100. The programming software in use was at revision 5.0. The identical relay logic was then downloaded from revision 7.0, and communications worked from then on: no wiring change, no panel change, no parameter change.
A second documented case points the same direction. An FX-2nc had talked to a similar panel until a programming software revision change; the maker described the new revision as backwards compatible, and the suspected cause was a port rate that had moved to 19200 bd. Why a newer revision leaves the port configured differently is not documented in the FX-series material available for this article. The outcome is documented, and that makes the revision a variable to control.
Record the revision your current program was downloaded from, and keep a verified backup of the logic. Get the newest revision the PLC maker publishes for your PLC family through its own support or download channel, download your own backup logic, and test with nothing else touched. Never download a program file of unknown origin, and schedule the download for a planned stop if the machine is in service. If the link comes back, the revision was the fault; write it into the job record as the resolving condition.
Verify the link after the change
Cycle power to the panel and the PLC the way their manuals prescribe, then run the reads and writes the application actually uses, not just a status poll. The fault being closed out connected about once in 100 tries, so the pass bar is 100 clean poll cycles with zero failed exchanges before the job is done. At one exchange every 250 ms that is 25 seconds of running link; at a slower application rate, let it run an hour.
Write the record down: PLC model and its read-back port settings, panel model and driver, programming software revision used for the final download, the one change that resolved the fault, and the pass count. Whoever owns the machine next needs exactly that page.
Recommendation
Recommendation: when a panel shows no communications against one PLC model and works against another, read the serial settings from both ends first and set both ends to the FX2N programming port's documented default of 9600 bd, 7 data bits, even parity, 1 stop bit; the default needs no other change. If the read-back already matches field by field, download your own verified logic again with the newest programming software revision the PLC maker publishes; in the documented case, revision 7.0 restored a link that revision 5.0 had left dead, with no other change. Use the working PLC model to clear the panel side before touching anything, change one variable per test, and close the job on 100 clean exchanges.
Questions
Can I run the FX series link at 19200 bd?
On the FX2N-422-BD communication board the documentation lists 9600 and 19200 bd, so the rate exists on that hardware. Move the link to 19200 bd only when the panel driver offers it and both ends are set to it. The gain is small: a 32-character exchange takes 33 ms of link time at 9600 bd and 17 ms at 19200 bd. On the base programming port the documented default stays 9600 bd, 7 data bits, even parity, 1 stop bit.
Does a working FX1N give me the settings to copy to the FX2N?
It gives you the target values. The FX2N documents 9600 bd, 7 data bits, even parity, 1 stop bit as its programming port default, and a driver that communicates on this PLC family is set to that same set of values. Read the FX2N port back with the programming software anyway: a configuration left changed by an earlier download is exactly what this symptom suggests, and one read-back pass shows it.
Where do I get a newer programming software revision?
From the PLC maker's own support or download channel for your PLC family, or from its authorized distributors. Do not download from a third-party mirror and do not reuse a project file of unknown origin; download your own verified backup logic only. The documented repair moved from revision 5.0 to revision 7.0, so note the revision number shown in the software the current program came from before you start.
The PLC communication settings box is grayed out in the panel software. Where do I change the settings?
From the PLC side. In one documented case, version 3.1 of the panel software kept that box grayed out, and it would not open with the panel online either. When the project fields are locked, the driver's fixed settings become the target: set the PLC port to match them with the programming software, and if the port still refuses the new settings, move to the programming software revision check, which is where the documented repair landed.
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