Quick answer: A USB to RS232 converter contains a USB bridge, a UART and an RS232 transceiver that swings plus and minus 5 to 12 V. The host sees a COM port, the device sees a DB9 connector. It works only if the pinout matches the device’s DTE or DCE role, so a null modem adapter is often needed as well.
Serial ports refuse to die because industrial controllers, scales, label printers and network gear still use them. The adapter is cheap; the configuration is where projects stall, and the same five faults account for almost every failure.
What the adapter replaces
It replaces the UART and the line driver that used to live on the motherboard. Nothing about the protocol changes: 9600 8N1 is still 9600 8N1. What changes is that the operating system now assigns a virtual port number, and that number can move.
Voltage levels
RS232 uses plus and minus 5 to 12 V, with a mark at negative voltage and a space at positive. TTL serial uses 0 and 3.3 or 5 V. Connecting a TTL device to an RS232 port destroys it, which is why the adapter must be a real transceiver, not a bare wire.
Handshake lines
A full implementation drives RTS and DTR and reads CTS, DSR and DCD. Cheap adapters implement only TX, RX and ground. Devices that wait for CTS will hang forever on a three wire adapter, which looks like a software bug.
Chipset families
FTDI FT232
The most compatible option and the most expensive. Drivers exist for every operating system, the chip has a unique serial number so the COM port can be pinned, and latency timing is consistent. Buy this when a machine must work every day.
Prolific PL2303
Widely cloned. Genuine chips work well; counterfeit ones are disabled by newer drivers, which is why an adapter that worked last year may stop after an update.
Silicon Labs CP2102 and WCH CH340
Cheap and generally reliable for TX and RX. The CH340 is common on budget boards and has weaker handshake support. The CP2102 is a good middle ground with stable drivers.
Pinout and null modem wiring
| DB9 pin | Signal | Direction on DTE | Null modem link | Symptom if wrong |
|---|
| 2 | RXD | Input | To pin 3 | No data at all |
| 3 | TXD | Output | To pin 2 | No data at all |
| 5 | GND | Common | Straight through | Garbage characters |
| 7 | RTS | Output | To pin 8 | Device waits forever |
| 8 | CTS | Input | To pin 7 | Flow control stalls |
COM port faults that recur
The port number changes when the adapter is moved to another USB socket. Pin it in Device Manager under advanced settings, or address the device by serial number in software. A port already open in another program produces an access denied error that looks like a driver failure. Long runs with no ground reference produce framing errors that disappear when a ground wire is added. A laptop on a switching power supply can inject noise into the ground, which is why an isolated adapter is standard in industrial cabinets. Finally, adapters that derive power from the USB bus can brown out during a port reset and lose buffered bytes.
Practical scenarios
- PLC programming: FTDI adapter, DB9 straight cable, port pinned, and a null modem adapter in the bag because half the controllers are wired as DCE.
- Label printer: CH340 adapter at 9600 8N1, hardware flow control off, and a fixed IP-to-serial gateway if several users need access.
- Scale in a packing line: isolated adapter, shielded cable, ground bonded at the cabinet only.
- Legacy router console: adapter plus a rollover cable, 9600 8N1, no flow control, terminal software set to send line endings correctly.
FAQ
Why does the device not respond at all?
Check TX and RX orientation first, then flow control. A straight cable between two DTE devices cannot work, and a device that waits for CTS will never answer on a three wire adapter.
Do I need a null modem adapter?
Only when both ends are DTE, which is common between a PC and a controller. If the device is DCE, a straight cable is correct. When in doubt, try the null modem adapter; it cannot damage anything at RS232 levels.
Which chipset should I buy?
FTDI for production or daily use, CP2102 for a good balance, CH340 for occasional bench work. Avoid unbranded PL2303 clones, because driver updates can disable them.
Next step
Buy the chipset that matches the duty cycle, keep a null modem adapter and a DB9 gender changer in the toolkit, and pin the COM port before deployment. The RS232 cable guide covers pinout and cable types, the USB to serial adapter page compares adapter classes, and the USB adapter series lists the isolation and handshake options.
