Remap a USB serial adapter COM port when software stops at COM4

Why does the panel software only list COM1 to COM4?

The list is fixed in the software. It offers four serial names, COM1 through COM4, and nothing inside the program extends it, so an adapter Windows named COM8 cannot be selected.

The number after COM is a label Windows hands the adapter when it detects it. Your PC had already handed out seven names, so the adapter took the eighth. Windows itself has no four-name limit; the software does. It was written with four slots in its port picker and ships no setting that widens them, so the fix lives on the PC side.

Match your symptom to its cause before you touch anything:

• The software's port list shows COM1 to COM4 and nothing else: the program's fixed picker, with no adapter fault and no missing driver behind it.
• The same adapter talks to the PLC from a terminal program on COM8: the adapter, cable and wiring are good, and the picker is the only barrier.
• A number inside COM1 to COM4 is missing or greyed in the drop-down: another device on this PC holds that name right now.
• There is no Ports (COM & LPT) section in Device Manager at all: Windows has not detected the adapter, and detection comes before any remap.

How do I remap the USB serial adapter to a port in COM1 to COM4?

Open the adapter's properties in Device Manager, choose the Port Settings tab, open Advanced, and pick an unused number from COM1 through COM4 in the COM Port Number drop-down. Close the dialogs and write the number down.

1. Close the panel software, so it rebuilds its port list when it reopens.
2. Plug the adapter in, open Device Manager, and expand Ports (COM & LPT).
3. Right-click the adapter's entry and choose Properties.
4. Open the Port Settings tab and click Advanced.
5. In the COM Port Number drop-down, choose a number from COM1 to COM4 that no other entry holds.
6. Click OK on every open dialog. If Windows asks you to reconnect the adapter, unplug it and plug it back.
7. Reopen the panel software and select the number you assigned.

This is the documented route in Windows itself: Properties, Port Settings tab, Advanced, then the new number. No download, script or driver change is involved.

Check it worked: Device Manager shows the new number in brackets after the adapter's name, and the panel software's list now contains it. If the number snaps back to COM8, another device claimed it in the meantime; pick a different one.

How do I open Device Manager and find the adapter?

Press the Windows key, type devmgmt.msc, and press Enter. The adapter sits under Ports (COM & LPT) with its current number in brackets after its name, so confirm the name before you change anything.

Confirm the entry by pulling the adapter out of its USB socket and watching which entry disappears, then putting it back. The test takes about 10 seconds. With a scanner, a scale and a printer sharing one PC, it stops you remapping the wrong device.

If there is no Ports (COM & LPT) section, choose View, then Show hidden devices, and look again. An adapter that stays invisible has a detection or driver problem, and the remap waits until Windows sees it; a walkthrough on restoring missing COM ports in Windows Device Manager covers that side of the job.

Which COM number is safe to pick?

Pick a number in COM1 through COM4 that no other entry in Ports (COM & LPT) holds. With four names in the list, check each of the four before you choose, because a number another device owns will fail when the panel software opens it.

A safe pick meets three conditions at once: it sits inside the software's range, no entry under Ports (COM & LPT) holds it, and no running program is using it when the panel software opens the port. Terminal programs, data loggers and other panel tools all count, so close them before the first test. A number another program holds fails at the open step, the same conflict behind a SCADA connection to a PLC when an HMI panel holds the port.

If all four names are taken, free one: unplug the device that holds it or remap that device upward, then take its old name for the panel adapter. The picker's range is the constraint, so one of the four has to become free.

Example: the adapter sits on COM8 and the list stops at COM4

Say the adapter sits on COM8, other devices already hold COM1 and COM3, and COM2 and COM4 are unassigned. The software's four-name list leaves 2 candidates, you assign COM2, verify it in Device Manager, and select COM2 in the software. The bench PC sits next to the panel, so the 2 m serial cable is no concern; at 9600 baud an RS-232 link of this length is well inside the roughly 15 m the signalling is generally considered dependable over.

The bench PC's Ports (COM & LPT) list reads:

Four names sit in range and two are held, so two are open: 4 - 2 = 2 candidates. Take the lowest, COM2. Right-click the adapter's COM8 entry, open Port Settings, then Advanced, choose COM2 in the COM Port Number drop-down, and click OK. The entry now reads COM2. Reopen the panel software, select COM2 from its list, and start the download; the software opens COM2 at 9600 baud, 8 data bits, no parity and 1 stop bit, and a small panel project transfers in well under a minute. If the transfer starts and then dies, the name is no longer the problem and the serial settings are, which the next section sorts out.

The port opens but the link still fails, what now?

If the software opens the number and the link still fails, the mapping is fixed and the fault sits in the serial settings or the cable. The common default for PLC and panel links is 9600 baud with 8 data bits, no parity and 1 stop bit, so start there.

Work the checks in this order:

1. Same name on both sides: the software's selection and the brackets in Device Manager match exactly.
2. Port free: every other program that talks serial is closed, because a held port fails at open.
3. Parameters match: set 9600 baud, 8 data bits, no parity and 1 stop bit, the default most PLC and panel links ship with, then correct whatever the panel's own document prints differently.
4. Com light: if the light on the adapter stays dark while the software tries, the link is not closing electrically; the HMI to PLC serial timeout with a dead com light checklist starts at the connector.
5. Transfer dies partway: timings and drivers move to the front there, and the USB serial adapter timeout when programming a text panel article covers that case in detail.

How do I keep the number from changing later?

The number you assign stays with that adapter on that PC. A move to a different USB socket or a driver update can bring it back on a new one, so check Ports (COM & LPT) after any change and keep one socket reserved for the panel adapter.

An ordinary reboot brings the same number back. The two events that move it are the adapter going into a different USB socket and a driver reinstall. devmgmt.msc shows the current number in one glance, so open it after any hardware or driver change and compare against the setup sheet before you blame the panel project. Put the number on the sheet, label the cable, and leave the panel adapter in its own socket.

Recommendation: remap the adapter at the bench, before the PC goes on the machine. Pick the lowest free number in COM1 to COM4, write it on the panel's setup sheet, dedicate one USB socket to the adapter, and recheck the number in Device Manager after any driver update or socket change. Leave the PLC-side settings untouched: the remap changes a PC-side name and nothing else.

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