COM / Serial Port
Communication Errors.
A serial device that stops talking is rarely a dead port. Windows reassigns COM numbers, RS-232 and RS-485 are not interchangeable, and one wrong pin-out can silence a scale or a customer display. This guide covers COM, USB, LAN and HDMI faults in a single order of operations.
Which Port, Which Layer?
Port faults cluster around four interfaces. Matching the symptom to the layer narrows the search before any tool comes out of the bag.
| Symptom | First check | Likely layer |
|---|---|---|
| Port shows a warning mark in Device Manager | Missing or wrong driver | Driver |
| Port exists but no data comes through | Baud rate, data bits, parity | Software |
| COM number changed after a reboot | Enumerating order in Windows | Software |
| RS-485 device never answers | A and B wires reversed | Cable |
| Scale works, customer display does not | Different port, same connector gender | Cable |
| Device works only when nothing else is plugged in | USB power budget exceeded | Power |
| LAN link LED dark on one side | Cable or switch port | Cable |
| HDMI reports no signal but the PC is running | Cable or monitor input source | Cable |
The Trap: RJ45 Is Not Always Ethernet
This single misunderstanding accounts for a large share of “the port is dead” reports, because the fault produces no error at all — just silence.
Two different signals, one physical connector
On a till, a peripheral or a mini PC, an RJ45 socket is used for one of two completely unrelated purposes:
- 1 Labeled LAN — a genuine Ethernet port, wired to the TIA/EIA-568B standard.
- 2 Not labeled LAN — the same 8P8C housing used to carry RS-232 serial signals for a customer display or a printer.
Using the wrong cable, or plugging a serial cable into the network port, produces total silence with no diagnostic message. It is the hardest class of fault on this page to identify, because every check appears to pass.
Pin Definitions
Identify which of the three interfaces you are working with before ordering or making up a cable.
RJ45 serial (RS-232)
| Pin | Signal | Direction |
|---|---|---|
| 1 | NC | — |
| 2 | DSR | Input |
| 3 | TXD | Output |
| 4 | RXD | Input |
| 5 | DTR | Output |
| 6 | DSR (paralleled with pin 2) | — |
| 7 | GND | Reference |
| 8 | DTR (paralleled with pin 5) | — |
RJ45 Ethernet (labeled LAN)
Wired to TIA/EIA-568B:
- · Pin 1 — TX+
- · Pin 2 — TX−
- · Pin 3 — RX+
- · Pin 6 — RX−
Pins 4, 5, 7 and 8 are reserved for gigabit; on a 100 Mbit link they are typically unused.
RJ11 cash drawer interface
Pin numbering: latch facing down, contacts facing you, counted left to right 1–6.
| Pin | Signal | Description |
|---|---|---|
| 1 | Chassis / structural ground | Shielding and interference control |
| 2 | Drawer drive signal 1 | DC 12 V pulse triggering the first drawer latch |
| 3 | Drawer status signal | Input; reports open or closed state |
| 4 | Drive power | DC 12 V on every receipt printer in our range |
| 5 | Drawer drive signal 2 | Pulse for a second drawer; not connected on some models |
| 6 | Signal ground | Common reference |
Some simplified 4-pin RJ11 fittings retain only pins 2, 4, 5 and 6, omitting the chassis ground and the status line.
Cable conclusions
- · Connecting to a PC does not require a homemade null-modem cable.
- · What ships with the hardware: POS terminals, kiosks and all-in-ones include the power cable. A cash drawer includes its drawer cable, and a printer includes its data cable.
- · The common third-party cable mistake is using a standard network patch lead in an RJ45 serial socket, or a customer-display RJ11 lead in an RJ45 socket. Both fit, and neither works.
Work Through the Layers in Order
Serial faults sit in exactly one of these layers, and the ordering is chosen so that each check eliminates the widest set of causes first.
Layer 1 — Power
For USB devices, move the device to a different port — rear ports usually supply more current than front ones. On models with a powered USB hub, check the hub's own adapter first. An unpowered hub carrying three peripherals produces intermittent dropouts that look like a software fault.
Layer 2 — Cable
Serial cables are not interchangeable just because they fit. Two cables can carry the same connectors and the same pin count while being wired differently.
- · Straight-through versus null-modem wiring
- · DB9 male versus female
- · Pin assignment is standardized as pin 2 RX, pin 3 TX, pin 5 GND — but manufacturers do depart from it
- · RS-485 with A and B reversed produces no communication and no error
- · Practical length limits: around 15 m for RS-232, and up to roughly 1200 m for RS-485 at low baud rates
Layer 3 — Connector Integrity
A serial port has no consumable, so this layer stands in for physical condition. Inspect the pins for bending or retraction, particularly on DB9 connectors that have been mated repeatedly. For LAN faults, substituting a known-good patch lead is the fastest single test available.
Layer 4 — Driver
Open Device Manager, expand Ports (COM & LPT), and open the target device's properties to check the driver version.
Industrial machines frequently require a dedicated serial driver rather than a generic USB-to-serial one. Installing or reinstalling a driver also re-enumerates the device, so the COM number will usually change — which means the application configuration has to be updated to match.
Layer 5 — Software
Baud rate, data bits, stop bits and parity must all match the device manual. Four settings, and any one of them being wrong produces the same symptom: no data.
A serial port is an exclusive resource. Two applications opening the same port means neither gets data, and both may report a healthy connection. Common retail settings are 9600 8N1 for a scale and 115200 8N1 for some customer displays.
Layer 6 — Hardware
Serial controllers fail most often after lightning events or hot-plugging at the peripheral end. Test a different port: if COM1 fails and COM2 works, the port is the fault. If both fail, the problem is the software or the cable rather than the controller.
Three Steps That Separate Port, Cable and Device
These three steps are ordered so that each one removes an entire category of cause. Skip them and you end up replacing parts by elimination.
Separate the port from the device
Short pins 2 and 3 with a loopback plug and use a serial terminal to send and receive on the same port. If the characters come back, the port is good. If nothing returns, the port is the fault. It is the least expensive way to make that distinction, and it needs no second machine.
Separate the cable from the device
Connect the same cable to a known-good device of the same model. If that device works, the cable is sound and the original peripheral is the fault.
Separate the driver from the hardware
In Device Manager, enable Show hidden devices and look for a greyed-out ghost port. A ghost port means the driver installed correctly at some point but the device is not currently enumerating — which points to power or cabling rather than to the driver.
The Same Logic on USB, LAN and HDMI
| Port | Isolation action | What the result means |
|---|---|---|
| USB | Test the same device on another machine | Works elsewhere means local power or driver |
| LAN | Change the cable, change the switch port, watch the link LED | Dark LED means a physical-layer fault, independent of IP settings |
| HDMI | Change the cable, select the monitor input manually, try another display | A lit keyboard means the machine POSTed, so the fault is in the display chain |
Repair or Replace
Nearly every fault on this page is resolved with a cable, a setting or a driver. Board-level port damage is the exception.
✓ You can do this on site
- · Replace serial, USB or network cables
- · Reinstall or roll back the serial driver
- · Change the COM number, baud rate or application settings
- · Add a powered USB hub
- · Confirm port health with a loopback plug
! Return to supplier
- · On-board serial controller replacement
- · Mainboard-level repair
- · Repeated failure of the same port after lightning or surge events
Safety boundaries
- · Never hot-plug serial, cash drawer or scale cables on a live till. Several of these carry switched 12 V lines that can damage the port or the peripheral.
- · Power down before changing any cable at the peripheral end.
- · Do not probe a live serial port with a multimeter unless you know the pin assignment for that specific model.
Prevention
- · Label every serial peripheral with its COM number, baud rate and cable type. Swapping a machine then becomes a copy operation rather than a diagnosis.
- · Power down before plugging or unplugging. Hot-plugging is the most common cause of serial port damage.
- · Add surge protection where cabling runs long distances or where lightning is common.
- · Make the COM number a configurable setting in the application rather than a fixed value.
Consider a replacement unit when a board-mounted port has failed more than once, when you need RS-485 but the model provides only RS-232, or when the port count no longer matches the peripherals attached. If the port is fine and the device still delivers nothing, the fault is upstream in a scanner that beeps but sends no data, or browse the full troubleshooting index.
Related Troubleshooting Guides
The customer display runs over the same serial path, and the same data-link logic applies to scanners.
Upgrading RAM and Storage in Fanless Mini PCs
Slot tables and the upgrade procedure, including which models take mSATA rather than M.2.
Customer Display Shows No Signal
The display is a serial device, so most of the checks above apply directly.
Scanner Beeps But Transmits No Data
The same “reads but does not transmit” pattern, on a scanner interface.