History

• The system is based on the Wiegand effect, a physical phenomenon discovered by John R. Wiegand, who noted that a specially constructed wiring system could detect slight magnetic fields.
  
  

Function

• The discovery led to the development of security access systems that make use of small magnets in cards and other access keys through a binary code. The interface uses three wires, one grounded and two used for data transmission labeled "DATA0" and "DATA1" or "Data Low" and "Data High." Zeros are impulses on DATA0 line, while ones are impulses on DATA1 line.
  
  

Binary Protocol

• DATA0 and DATA1 are at high voltage when no data is transmitted. DATA0 is at low voltage while the DATA1 wire stays at high voltage when a 0 is sent. Sending a 1 converts DATA1 to low voltage while DATA0 stays at the high voltage.
  
  

Communications Protocol

• The Wiegand protocol originally consisted of 24 bits: eight bits of facility code and 16 bits of ID code prefixed and post-fixed with parity bits. The first and last parity bits are calculated from the first 12 and last bits of code respectively. Modern version have varying length and data bits can vary from 24 to 40 bits.