Wiegand26 protocol

Hi rookie,

There are actually two questions wrapped up together;

1) How are the physical protocols different (signal voltage, signal rate, data pins/lines, etc.)?

2) How are the data schema different (packet format, bit meaning, etc.)?

Let's take a look at #1 - the physical. Basically, the Wiegand protocol is messed up. There is no official baud rate as there is in typical TTL or RS232 serial communication. There is often a clock rate, which is used to time against the bits coming out of the data wires. Some readers that emulate Wiegand will output the data at a pre-set baud rate (2400 or 9600 usually). Also, if the reader is following the Wiegand interface standard, there will be two data lines. Here's what Wikipedia has to say about it;

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 Low and Data High. 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 bottom line is, Wiegand as a physical interface, is all screwy. It came about in the 80's and was based on a proprietary access control/security application using "Wiegand wires". The "Wiegand effect" however is kind of fascinating, and worth a read.

Ok, let's move on to #2 - the data. Again, from Wikipedia;

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

A reader emulating Wiegand (no true "Wiegand sensors" are used anymore) will simply read other tag formats and output  24 bits of the tag ID data across the data0 and data1 wires according to the physical interface requirements. The problem with a 24 bit identity is that it is very small, and other types of tags have much larger identity bit layouts. For example, the EM4102 family has a 40 bit identity tucked into it's 64 bit schema, and many "Wiegand compatible" readers will read them just fine. However, when the reader outputs the first 24 bits of the tag ID, it truncates or drops the rest. This means a massive amount of EM4102 tags will appear to have the same tag ID.

For more information, search this forum for other posts about the Wiegand interface.