Who Uses 'Clock & Data' Format In Electronic Access Control?

One aspect of readers/credentials seldom covered is the 'Clock & Data' format. In the obscure world of credential communication formats, it is another semi-obsolete serial bus that is not very secure and is not often used, especially since the advent of the ethernet network. However, like Wiegand, many current and new readers support the format. My curiosity is: why?

Maybe 'Clock & Data' formats are used outside the US, or are common in certain verticals. Is the cabling different? Any insights you can share?

Ive had to use it in situations where a customer had an existing base of mag stripe Dorado readers and they were factory ordered to send C&D. I didnt see any worse or better performance than the Wiegand readers at the same site. I'm curious, and I'm sure someone here knows the why/how C&D makes sense.

Many users require backward compatibility. It is just a communication protocol that can be switched either in programming, or physically at the controller board by means of jumper or DIP switch settings. What is not secure about clock and data that you think Wiegand is?

Magstripe signaling utilizes two wires, called "clock" and "data". The data line is used to send all the binary
data to the panel. This is accomplished by changing the state of the voltage on the line. Five (+5V) volts is
equivalent to a "1" bit, and zero (0V) is equal to a "0" bit. By changing the state from high to low, the coded
binary data from the recorded stripe on the card is sent to the panel.

The second wire used for clock/data signaling is used to tell the panel when to sample the data line. Each
time a bit of data is sent down the data line, a pulse is sent down the clock line, instructing the panel to take
a "sample" of the data line and record that next bit.

Wiegand signaling is significantly different. It uses two separate data lines to pass data to the panel in reference to a third ground wire. They are called "Data 1" and "Data 0". As the names convey, Data 1 is used to carry the "1" bits to the panel, and

the Data 0 line carries the "0" bits. For both data lines, the voltage is normally high (+5V). When a bit is being
signaled, the voltage on the appropriate line is pulsed to zero (0V) volts. Wiegand allowed very long cable runs. From wikipedia:

Wiegand wire is low-carbon Vicalloy, a ferromagnetic alloy of cobalt, iron, and vanadium. Initially, the wire is fully annealed. In this state the alloy is "soft" in the magnetic sense; that is, it is attracted to magnets and so magnetic field lines will divert preferentially into the metal, but the metal retains only a very small residual field when the external field is removed.

During manufacture, to give the wire its unique magnetic properties, it is subjected to a series of twisting and untwisting operations to cold-work the outside shell of the wire while retaining a soft core within the wire, and then the wire is aged. The result is that the magnetic coercivity of the outside shell is much larger than that of the inner core. This high coercivity outer shell will retain an external magnetic field even when the field's original source is removed.

The wire now exhibits a very large magnetic hysteresis: If a magnet is brought near the wire, the high coercivity outer shell excludes the magnetic field from the inner soft core until the magnetic threshold is reached, whereupon the entire wire — both the outer shell and inner core — rapidly switches magnetisation polarity. This switchover occurs in a few microseconds, and is called the Wiegand effect.

The value of the Wiegand effect is that the switchover speed is sufficiently fast that a significant voltage can be output from a coil using a Wiegand-wire core. Because the voltage induced by a changing magnetic field is proportional to the rate of change of the field, a Wiegand-wire core can increase the output voltage of a magnetic field sensor by several orders of magnitude as compared to a similar coil with a non-Wiegand core. This higher voltage can easily be detected electronically, and when combined with the high repeatability threshold of the magnetic field switching, making the Wiegand effect useful for positional sensors. Once the Wiegand wire has flipped magnetization, it will retain that magnetization until flipped in the other direction. Sensors and mechanisms that use the Wiegand effect must take this retention into account.

Wiegand, (1) provided for long cable runs, (2) enabled the Wiegand encoding of a card to be predetermined and embedded into the card. The card cannot be reprogrammed like a mag stripe card. However the Wiegand card's structure limited the amount of data to 37 bits. Many access control system manufacturers adopted Wiegand technology, but were unhappy with the limitations of only 8 bits for site codes (0-255) and 16 bits for card numbers (0-65535), so they designed their own formats with varying complexity of field numbers and lengths and parity checking often running into the hexidecimal stage.

Softwarehouse had a personaility module that was used at the door or CPU to read and decipher Wiegand signals and convert them to clock data for the panel. Mag stripe cards did not require this module. However double card readers per door did. The module could be changed to clock data. We have used them in line as a signal booster on extremely long runs.