Wiegand interface

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The Wiegand interface is a de facto wiring standard which arose from the popularity of Wiegand effect card readers in the 1980s. It is commonly used to connect a card swipe mechanism to the rest of an electronic entry system. The sensor in such a system is often a Wiegand wire based on the Wiegand effect discovered by John R. Wiegand. A Wiegand-compatible reader is normally connected to a Wiegand-compatible security panel.

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 but sometimes also labeled Data High and Data Low. When no data is being sent both DATA0 and DATA1 are at the high voltage. When a 0 is sent the Data Low wire (also called DATA0) is at a low voltage while the Data High wire stays at a high voltage. When a 1 is sent Data High is at the low voltage while Data Low stays at the high voltage.

The high voltage level is usually +5VDC to accommodate for long cable runs (most reader manufacturers publish a maximum of 500 feet)from the door readers to the associated access control panel typically located in a secure closet.

The communications protocol used on a Wiegand interface is known as the Wiegand protocol. The original Wiegand format had one 1 parity bit, 8 bits of facility code, 16 bits of ID code, and a stop 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.

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.