Serial Bit Order

If one looks at the waveform for either an asynchronous or a synchronous stream of bytes (characters) using an oscilloscope, then one notices that things are not quite as one expects. There are two things which may be unexpected:

So, what one sees on the oscilloscope is:

Asynchronous Transmission

For an asynchronous transmission, this means that the line usually is at the low voltage (OV or -12V) and that a logic 0 is sent as a high voltage (+5V or +12V). The least significant bit of the character follows the start bit (rather than the most significant as one may have expected), followed by the rest of the bits in the byte, the parity bit (if present) and the stop bit.

Exception to the Rule

The bit order for all bits follows the above rules, with only one exception - that CRC appended to the end of some link layer frames.


Gorry Fairhurst - Date: 01/01/2001 EG3557