Serial Bit Order
A serial bit stream 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 expects. There are two things which could be unexpected:
- Since line drivers invert the signal prior to transmission (and line receivers do an inversion on reception), the signal that appears on the wires is usually upside down. That is a "1" or "idle" period is represented by a low voltage and a "0" or "start bit" is represented as a high voltage.
- Ever since the start of asynchronous communications, UARTs have sent bits down the wire with the least significant data bit first and the most significant bit last. All that this really implies is a change in order of the lines connected from the computer's data bus to the transmit and receive shift registers.
So, what one sees on the oscilloscope is:
Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2016)