Physical Layer
The Wiegand interface uses three wires, one of which is a common ground and two of which are data transmission wires usually called DATA0 and DATA1, alternately labeled 'D0' and 'D1' or 'Data Low' and 'Data High'. When no data is being sent both DATA0 and DATA1 are pulled up to the 'high' voltage level - usually +5 VDC. When a 0 is sent the DATA0 wire is pulled to a low voltage while the DATA1 wire stays at a high voltage. When a 1 is sent the DATA1 wire is pulled to a low voltage while DATA0 stays at a high voltage.
A high voltage level of +5VDC is used to accommodate for long cable runs (most reader manufacturers publish a maximum of 500 feet) from door readers to the associated access control panel typically located in a secure closet.
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