The “Wiegand Interface”

The “Wiegand Interface” has been the most popular technology for connecting card readers to door controllers for years. Though the interface is widely used and understood to be an industry standard by many security system professionals, some of the finer details may be a mystery to others.  In this article we will take a deeper look into the technology and answer the question of “how does Wiegand work”?

History of the Wiegand Interface

Wiegand card readers were invented after a scientist named John Wiegand applied a principle called the “Barkhausen Jump” to discover the “Wiegand Effect”, where specially treated wires produce magnetic fields. These wires, when built into a plastic security card, are spaced apart in a pattern and arranged in two rows corresponding to 0’s and 1’s. They produce a data stream when passed or “swiped” through a slotted card reader containing a magnet.

The data stream is then transmitted over two wires. These wires are DATA0 and DATA1 (typically green and white) to a door controller that was designed to receive such an output from the reader. Note that only one bit is transmitted at a time, nominally with a pulse width of 100 micro seconds and a pulse interval of 1 msec.

Since the manufacturing of the cards was so specialized it required custom machines to make the wire in a precise manner. This way it would gain the properties to produce the Wiegand Effect. This also meant that cards were virtually impossible to duplicate. Hence, they were considered extremely secure, especially when compared against magnetic stripe cards.

Although actual Wiegand swipe cards and readers have disappeared from access control systems, the Wiegand Interface survived because virtually every controller made worldwide already supported the interface. Thus, the vast majority of proximity and smart card readers, biometric readers, push button transmitter/receivers, keypads, and other similar devices continue to be designed and built with a Wiegand Output.

The Wiegand Software Interface

Originally, most Wiegand swipe cards contained 26 Wiegand wires corresponding to 26 bits of data. The bits were broken up into Facility Code (FC) and ID Number fields, with Bit 1 and Bit 26 being reserved to calculate parity for error checking. these parity bits are known as the “lead parity bit” and “trailing parity bit”.

Two 26 bit formats existed:

• 8/16 (8 bit FC, 16 bit ID)
• 12/12.(12 bit FC, 12 bit ID)

8/16 allows for 256 unique facility codes (0-255) and 65,536 unique IDs (0-65,535) within each facility code, while 12/12 allows for 4096 of each.

“8/16” eventually became the main format. Access control system manufacturers designed their software to be able to support 26 bit Wiegand format, which is still widely used today.  An 8/16 “format sheet” is used to describe the data used by the manufacturers and would contain something like the following information:

A problem arose however because 24 data bits doesn’t allow for enough unique numbers to ensure card numbers are not duplicated.

Initially, engineers would provide 26 bit cards by spreading different facility codes in each geographic area. This was to minimize the chance of duplicate cards ending up in the same location and causing a potential security problems. Soon however custom card formats with larger bit counts to allow for greater number ranges were created. Custom Wiegand formats usually ranged from 32 to 37 bits, but with advancements in proximity and smart card technology, bit counts can now go much higher.

The Good and the Bad of the Wiegand Interface

The Wiegand Interface is a widely used term by many and has its pros and cons. The good news is that the Wiegand Interface is something that virtually all manufacturers of access control systems support. With Keri’s Doors.NET software, customers can either choose an existing format from the Credential Types library or create their own data format.  There are a plethora of reader manufacturers of all types whose products data output is via a Wiegand interface, giving customers a wide range of choice.

The bad news is that it is easily misused by customers who don’t manage their card sequences properly. Cards are often duplicated (especially 26 bit) posing a security risk, and customers sometimes mis-order cards and/or choose a format that doesn’t really suit their needs. As stated earlier, 26 bit Wiegand format still constitutes a large share of the market and is used on many “high end” projects. Ideally, It should only be used in the lowest security applications.

The Wiegand Interface allows for only one way communication, from the reader to the controller. There is no back and forth communication to provide the system operator with otherwise critical information about the reader, or to update the reader’s firmware.

Cable distance between reader and controller is limited to 300’ (90m) with standard #24 AWG wire (0.6mm) reader cable and 500’ (150m) with the more expensive #18 AWG wire (1.2mm) wire.