What it is
Wiegand is a wire communication interface between a reader (i.e. a card-, a fingerprint- or other data capture devices) and a controller. It is widely used in access control systems.
On the physical level, the Wiegand interface consists of three conductors: Data0, Data1 (transmission wires) and Ground wire.
How does it work
The interface works one way. It means that Wiegand sends a signal from a reader to a controller.
When no data is being sent, both DATA0 and DATA1 are pulled up to the "high" voltage level. 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.
There’s a number of Wiegand formats but the most popular are:
• 26 Bit Wiegand (being the most common). It consists of 24 code bits and 2 parity bits.
• 34 Bit Wiegand which consists of 32 code bits and 2 parity bits.
Wiegand System and Security Concerns
From a security standpoint, Wiegand systems present several challenges: here you can read more about how HID reader can be hacked due to vulnerabilities in the Wiegand protocol.