Serial communication basics: synchronous, asynchronous, duplex, differential

Before memorising protocol names, fix four criteria in your head. UART, SPI, I2C and CAN then fit into one table.

Four criteria

Four questions that separate communication methods.

Criterion	Options	Meaning
Clock	Synchronous / asynchronous	Is a clock line sent along, or is the speed agreed in advance?
Direction	Full duplex / half duplex	Send and receive at once, or take turns?
Signal	Single-ended / differential	One wire referenced to ground, or the difference between two wires?
Topology	1:1 / bus	Only two connected, or several sharing a line?

In one table

Protocol	Clock	Direction	Signal	Topology
UART	Asynchronous	Full duplex	Single-ended	1:1
SPI	Synchronous	Full duplex	Single-ended	1:N (selected by CS)
I2C	Synchronous	Half duplex	Single-ended (open-drain)	Bus (addresses)
RS-485	Asynchronous	Half duplex	Differential	Bus
CAN	Asynchronous	Half duplex	Differential	Bus (ID arbitration)

3D labRun what you have read so far→

Why serial

Parallel communication, which sends several bits at once, needs many wires, and at high speed the arrival time differs from wire to wire. Serial communication, which sends bits one at a time over a few wires, became the standard for connecting boards and chips.

Differential signals

Differential signal. Even if the same noise rides on both wires, it disappears when you read the difference.

Opposite signals are sent on two wires and only the difference is read. External noise enters both wires equally and disappears in the difference. That is why noisy, long-distance environments such as vehicles and factories use differential links like CAN and RS-485.