---
tags:
  - computer-architecture
  - micro-controllers
---

A serial port/device transmits data sequentially, one bit at a time over a
single channel/data line.

This is in contrast to parallel transmission, where there are multiple
channels/data lines, used simultaneously.

With serial transmission, one bit is sent after another:

![Serial transmission](../img/serial-transmission.jpg)

With parallel transmission, each bit of a byte travels on its own channel in
parallel:

![Parallel transmission](../img/parallel-transmission.jpg)

Although we say a serial device transmits over a single channel, this is a
logical distinction. The actual electrical signals will be transmitted
physically via multiple wires.

## Serial devices on Linux

Most serial devices today connect via [USB](./USB_protocols_and_connectors.md).

On Linux, `/dev/ttyUSB*` is a [device](./Devices.md) file representing the
physical USB connection to a serial device like, for example, an
[ESP32](./ESP_32.md) microcontroller.