Serial Bit Order

A serial bit stream, sent along a communications line, is typically produced by a hardware or software Universal Asynchronous Receiver/Transmitter (UART). If one looks at the physical layer waveform for either an asynchronous or a synchronous serial bit stream using an oscilloscope, then one may notice that things are not quite as one might at first expect. There are two things that could be unexpected:

So, what when you look at an EIA-232 signal on an oscilloscope, you might see:

Differential transmission, such as EIA-485, uses two lines to carry each baud. This carries both the inverted and non-inverted waveforms. LSB-first byte ordering is still commonly used.

See also:

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2021)