Protocol
The communications protocol used on a Wiegand interface is known as the Wiegand protocol. The original Wiegand format had one parity bit, 8 bits of facility code, 16 bits of ID code, and a trailing parity bit for a total of 26 bits. The first parity bit is calculated from the first 12 bits of the code and the trailing parity bit from the last 12 bits. However many inconsistent implementations and extensions to the basic format exist.
Many access control system manufacturers adopted Wiegand technology but were unhappy with the limitations of only 8 bits site codes (0-255) and 16 bits for card numbers (0-65535) so they designed their own formats with varying complexity of field numbers and lengths and parity checking.
The physical size limitations of the card dictated that a maximum of 37 wiegand wire filaments could be placed in a standard credit card as dictated by CR80 or ISO/IEC 7810 standards before misreads would affect reliability hence most Wiegand formats used in physical access control are less than 37 bits in length.
An advantage of the Wiegand signaling format is that it allows very long cable runs, far longer than other interface standards of its day allowed.
Read more about this topic: Wiegand Interface