Wigand (interface)

Initially, the interface was used in magnetic card readers and was maximally optimized for the simplest readers. In essence, it was a simple reading amplifier output. Due to the prevalence of magnetic cards, this interface has become standard de facto. Later, magnetic cards were supplanted by contactless cards ( RFID ), but the interface was kept unchanged for hardware compatibility.

There are several variations of the Wiegand interface, which differ in numbers in the interface name. This number in the title indicates the number of bits in the package.

The most common access control readers and controllers support the following Wiegand variants:

• Wiegand-26. The most common. Consists of 24 bits of code and 2 bits of parity.
• Wiegand-33. It consists of 32 bits of code and 1 bit of parity.
• Wiegand-34. It consists of 32 bits of code and 2 bits of parity.
• Wiegand-37. It consists of 35 bits of code and 2 bits of parity.
• Wiegand-40. It consists of 40 bits of code, there is no control over parity.
• Wiegand-42. Consists of 40 bits of code and 2 bits of parity.

There are varieties of readers with a Wiegand length up to 128 bits. In fact, the number of bits in the Wiegand format can be almost any reasonable. When you meet a new figure in the name, it remains unclear only the presence of check bits of parity and the method of their calculation.

There are some other names for similar interfaces. Common, for example, is the KSF interface which is actually a Wiegand-32.

For communication between the reader and the ACS controller, a three-wire bus is used - two signal wires, one common. The figure shows the classical wiring diagram, and the electrical parameters of the interface are obvious from it.

The maximum length of the communication line is highly dependent on the correct choice of cable (the main criteria are low linear capacitance, low ohmic resistance) and proper construction of a decoupling circuit for the reader and controller power supply. The usual values ​​given by equipment suppliers are up to 150 ... 250 meters.

As a cable, you can use a twisted pair of the 5th category. In this case, the signals "Data0" and "Data1" should be transmitted in different pairs (wire "a"), the second wire of the pair (wire "b") is connected to the terminal "common".

Data Transfer

Transmission is short pulses. The presence of a pulse in the line "Data0" means that log.0 was transmitted, the presence of a pulse in the line "Data1" means that the log.1 was transmitted. The width of the pulses and their period varies greatly depending on the manufacturer of the reader. The pulse width is usually in the range of 20 ... 200 μs. The pulse repetition period is 300 ... 3000 μs.

The communication is one-way; at the moment of card detection, a single frame transmission occurs with a card code from the reader to the ACS controller. Transmission is the high bit of the code forward.

Separation of frames is carried out on timeout. Actually the minimum time between frames is 0.5 seconds, the recommended timeout for the ACS controller is 50 ... 250ms.

Facility concept

Historically, many access control systems and card manufacturers conditionally divide the card code into two unequal parts called faciliity and number . Usually, the number includes the lower 16 bits of the code, everything else is to the facilitity. The reason for this division is to save memory in very old access controllers. When mounting the object, cards with the same facilitation were selected and only the lower 16 bits of the tag code ( number ) were recorded in the controller's memory. Much time has passed since then, and similar savings are long gone, but many systems still operate on these concepts, showing the card code divided into parts. In our time, such a division has no meaning.

Parity check

If there is a parity check, then two bits are added to the bits of the code card — one before the code, the other after. Accordingly, the entire card code is divided exactly in the middle into two parts. The parity of the high half of the code is controlled by the first bit, the low half is controlled by the last. If the number of bits in the code is odd, then the central bit of the code is included in both parity checks.

The first parity bit (the upper half of the code) is set to 1 if the number of units in its half of the code is odd. The last bit of parity (the lower half of the code) is set to 1 if the number of units in its half of the code is even.

It should be noted that there are readers who do not obey this parity rule. Therefore, in reality, most universal ACS controllers simply ignore parity control. In addition, some contactless card formats carry information on the number of code bits and parity directly on the card, respectively, the reader can in no way affect the actual output data format. Such, for example, is the HID format of ProxPass, Indala ASP, and others.

Wiegand and PIN Keyboard

Many access control systems support identification by dialing a PIN code on the keyboard. In this case, the dialed code can serve as both the main identification feature and the additional one. Typically, the dialed code is also transmitted via the Wiegand interface. There are different approaches to its transfer, the most common are:

• Wiegand-26. At the same time, the entered numbers are buffered on the reader, and at the end of the set, all are transmitted in one Wiegand package. The way of encoding numbers into a parcel is not standardized, but it is often BCD coding , which allows transmitting a code of up to 6 digits (6 = 24/4).
• Wiegand-4, Wiegand-6, Wiegand-8. At the same time, the entered numbers are sent separately on Wiegand as you enter. There is no generally accepted standard for encoding numbers in a package, but variations that are called Wiegand-HID (Wiegand-6) and Wiegand-Motorola (Wiegand-8) are most common.

In both approaches, in addition to digits as such, readers can often transmit Wiegand service characters, such as # and *, if they are present on the keyboard.