# Building a pyserial Plug for Serial Devices
URL: /guides/pyserial-plug

Talk to a DUT or instrument over a UART from an OpenHTF plug with pyserial — port discovery, command/response with timeouts, line framing, reading a serial number and firmware version into measurements, and capturing a boot log as an attachment.

The most common DUT interface on a production line is a UART: a debug console, a bootloader, an AT-command modem, a proprietary ASCII protocol. pyserial opens the port; an OpenHTF plug owns it for the duration of the test.

Prefer selecting by USB `serial_number` (the adapter's, not the DUT's): `/dev/ttyUSB0` and `COM3` change when cables move.

**Timeouts everywhere.** `serial.Serial(timeout=1)` bounds each `read`; the plug adds a per-command deadline on top. A hung DUT then fails the phase in seconds, not forever.

**Framing.** Decide what ends a response — a prompt, a newline, a fixed length, a checksum — and implement exactly that. `readline()` alone is only right for single-line replies.

**Echo.** Many consoles echo the command; strip it.

**Binary protocols.** Use `bytes`, `struct.pack`/`unpack`, and `ser.read(n)`; skip the `decode`.

**Flow control and DTR.** Some boards reset when DTR toggles on open. `serial.Serial(port=None)` then `ser.dtr = False; ser.port = port; ser.open()` avoids the pulse.

**Boot logs as attachments.** Capture the whole boot with `read_until_pattern` and attach it; when a unit fails later, the log is already in the record. For continuous capture in the background, the bundled `SerialCollectionPlug` does it in a thread.

**Windows** ports are `COM3`; above `COM9` use `\\\\.\\COM10`.

<Card title="Bundled plugs" href="/reference/bundled-plugs" description="SerialCollectionPlug for background capture." />

<Card title="Device Under Test" href="/dut" description="Setting the DUT ID from the device instead of a scan." />

<Card title="PyVISA plug" href="/guides/pyvisa-plug" description="For instruments that speak SCPI." />

<Card title="pyserial.com" href="https://www.pyserial.com" description="Full pyserial reference, maintained by the same team as this site." />