Why go serial at all

Sending a 4-bit (or wider) word over several separate wires works fine on one board, but becomes expensive and unreliable over any real distance. Serial communication instead sends the same word one bit at a time over a single wire, trading wires for time — exactly what a shift register is built to do.

Framing a word with start and stop bits

The receiver has no independent way to know when a new word begins, so a UART frame wraps the data in markers: an idle line sits at 1, a start bit (a single 0) announces that data follows, the data bits shift out one per clock, and a stop bit (a 1) marks the end and returns the line to idle. An edge detector watching for the 1-to-0 transition on an idle line is exactly what tells a receiver a start bit has arrived.

Transmitter and receiver

A transmitter is a loadable register that parallel-loads the word to send, then switches to shifting it out through a shift register one bit per clock, with the start and stop bits stitched onto either end. A receiver runs the reverse: an edge detector catches the start bit, a small counter times out the right number of clocks to shift each data bit into its own shift register, and once the count finishes the assembled word is read out in parallel.

Try it yourself

Build a transmitter in the circuit editor from a REG4 feeding a SHIFT4, driven by a CNT4 that stops shifting once all four bits (plus start and stop) have gone out, and confirm with the Truth Table tool that the bit sequence appearing on the single output wire matches the original 4-bit word, correctly framed.