USB serial adapter disconnects after going online to the PLC

Symptoms and what each one points to

• Drop lands a few minutes after going online, every time, and the COM number disappears from the software: Windows suspended or re-enumerated the converter. USB branch.
• The Ports list in Device Manager flickers at the drop, or the converter comes back with a new COM number: the physical USB link, the hub or power management. USB branch.
• The port stays listed while the activity LED goes dark and the software reports a communication error but keeps the port: the RS-232 path, its routing, or a second program on the same port. Serial branch.
• The drop tracks a big motor start, a welder, or a panel door being swung: induced noise or a loose connection moving with the disturbance. Serial branch.
• The download to the operator panel completes while online sessions drop: a bounded transfer sends its bytes once and stops. A completed panel download is not a test of a sustained session; run the watch below.

A port that answers slowly and times out while its entry stays listed the whole time is the cousin case: Fix USB serial adapter timeout when programming a text panel covers that one.

Watch Device Manager during the drop

1. Leave the converter in its usual socket. In Device Manager, under Ports (COM & LPT), note the COM number Windows assigned to it.
2. Open the programming software and confirm it points at that COM number with matching port settings.
3. Go online and keep the session running. Keep Device Manager visible on a second screen, or film the screen with a phone, so you can see the Ports list at the moment the link dies.
4. Change nothing else during the attempt. An entry that disappears or refreshes during the drop puts the fault on the USB side. An entry that holds while communication stops puts it on the serial side.
5. After any replug, compare the Ports list with the software's selection again before you go online.

One watch answers the question the quick fixes circle around: whether Windows still owns the port at the moment the link dies.

The USB branch: power management and the physical link

Two Windows controls can idle a converter that is connected but quiet between polls, and both have exact names:

• In Device Manager, open the converter's Properties, then its Power Management tab, and uncheck Allow the computer to turn off this device to save power.
• In the power plan, set USB selective suspend to Disabled: Control Panel > Power Options > Change plan settings > Change advanced power settings > USB settings > USB selective suspend.
• Change one control per attempt and rerun the watch between changes. If you clear both at once and the drop stops, the next laptop on this job inherits a mystery.
• Put the converter on a powered hub with its own supply and a short known-good USB cable. The USB 2.0 specification gives a high-power port up to 5 V at 500 mA, and a weak motherboard port or an unpowered hub shares that budget with everything else plugged in. A rear motherboard socket beats a front panel header fed by an internal cable.
• Leave the driver reinstall until last. A fresh driver over a power-management fault reproduces the original failure and costs you the afternoon.

The serial branch: cable path, noise, port selection

Before you move serial wiring in a live panel, lock out the panel's control power; RS-232 terminals sit close to parts of the circuit you do not want to brush while it is energized.

Confirm the software points at the COM number Device Manager assigned, with matching port settings, and compare the two again after any replug. If a second application polls the same port, sessions break in ways that look exactly like noise, because the port serves one program at a time. That case has its own walkthrough: connect SCADA to a PLC when an HMI panel holds the port.

Length has a hard ceiling. The RS-232 standard (TIA/EIA-232) caps the load capacitance at 2500 pF, and ordinary cable contributes about 50 pF per foot, which is where the classic 50 ft (15 m) ceiling comes from. A run near the ceiling works on a quiet bench and loses the margin inside a panel full of switched loads, so keep a programming run well under 50 ft and on its own path. Separate the cable from drive output leads and welder feeds; where two paths must cross, cross at right angles. Secure the converter and both connectors so nothing dangles on the rail.

Then swap parts one per attempt: a known-good cable, retest; a known-good converter, retest. A fault that follows a part is found.

An activity LED that stays dark while the port sits listed is its own case: HMI to PLC serial timeout with a dead com light walks through it. And if sessions hold but messages drop through them, compare notes with dropped PLC-triggered messages on a serial HMI panel.

Scenario: a 9600 baud session that drops a few minutes in

Inputs. Port speed 9600 baud, 8 data bits, no parity, 1 stop bit, so 10 bits to the character including the start and stop bits; read your own values off the port setup in the software, since 9600 8N1 is the common PLC serial default. Watch window 10 minutes, chosen because the typical complaint is a drop within a few minutes of going online, so ten minutes either reproduces the drop or clears the USB side. Port budget 5 V at 500 mA, the high-power port rating in the USB 2.0 specification.

Rule. Characters per second equals baud divided by bits per character. Traffic over the window equals that rate times the window in seconds. An online session moves data at that rate for the whole window; a bounded download sends its file once and stops, which is why the panel download keeps passing while the session keeps failing.

Arithmetic. 9600 baud ÷ 10 bits per character = 960 characters per second. 960 characters per second × 600 s = 576,000 characters moved in one 10-minute watch, about 576 kB of traffic. 5 V × 500 mA = 2.5 W available from the port.

Result and decision. Either the converter's COM entry disappears or refreshes inside those 10 minutes, or the entry holds while the data stops. An entry that vanishes in the window is a USB-side fault: apply the power-management changes above, add the powered hub where the 2.5 W budget is in doubt, and rerun the same watch. An entry that survives all 576,000 characters while communication dies is a serial-side fault: work the cable path, the routing and the port selection. The threshold is the window itself: 10 minutes of polling with no port event clears the USB branch, and a shorter clean run only means run it longer.

Confirm it stayed fixed

Rerun the online session for the full 10-minute watch with Device Manager open and the Ports list visible. Then unplug the converter, replug it in the same socket, confirm the same COM number returns, and go online a second time. Note which single change held: the checkbox, selective suspend, the hub, the cable or the port selection, and write it on the panel's maintenance card so the next laptop starts from the answer instead of the whole watch.

The decision

Recommendation: watch Device Manager through one 10-minute online session before you touch any setting. If the converter's COM entry disappears or refreshes in that window, uncheck Allow the computer to turn off this device to save power on the converter's Power Management tab, set USB selective suspend to Disabled, and retest one change at a time; finish with a powered hub if the port budget is in doubt. If the entry holds while the data stops, keep the RS-232 run short of the 50 ft ceiling, separate it from switched power wiring, secure both ends, and confirm the software points at the COM number Device Manager assigned. Close the case with the replug test and a second clean 10-minute watch. More shop-floor serial and display fixes are gathered under shop-floor screens and andon boards.

Questions