Wiegand interface
The Wiegand interface is a widely adopted de facto standard for transmitting data between access control card readers and controllers in electronic security systems, enabling the secure exchange of credential information such as facility codes and user IDs.[1] Developed based on the Wiegand effect discovered by inventor John R. Wiegand in the 1970s, it originally facilitated communication in magnetic stripe swipe cards but has evolved to support proximity cards, key fobs, and contactless smart cards.[2] The interface operates using a simple two-wire (Data 0 and Data 1) asynchronous serial protocol that sends low-going electrical pulses, with typical pulse widths of 20-100 microseconds and intervals of 200 microseconds to 20 milliseconds, ensuring reliable unidirectional data flow over distances up to several hundred feet.[2] The most common implementation is the 26-bit Wiegand format, standardized by the Security Industry Association (SIA) in 1996 as AC-01, which structures data into one even parity bit, eight facility code bits (allowing 0-255 unique sites), 16 cardholder ID bits (supporting 0-65,535 unique users per site), and one odd parity bit for error checking, yielding up to 16,777,216 total combinations.[1][2] This format powers the majority of legacy and modern access control systems, including those for building entry, time and attendance, and secure facility management, due to its low cost, ease of integration, and broad interoperability across manufacturers.[3] However, the protocol's limitations—such as its one-way communication, vulnerability to eavesdropping or spoofing without encryption, and finite code capacity—have prompted transitions to more secure alternatives like OSDP (Open Supervised Device Protocol) in contemporary installations.[2] Key components of a Wiegand system include the credential (e.g., a card with embedded Wiegand wire or coil that generates magnetic pulses when read), the reader (which detects and formats the signal), and the controller (which processes the data for access decisions).[3] The Wiegand wire itself, a proprietary ferromagnetic alloy known as Vicalloy, enhances durability and tamper resistance, making it suitable for high-security environments despite the protocol's age.[2][3] Despite these strengths, ongoing security concerns have led to recommendations for hybrid or upgraded systems that maintain backward compatibility while incorporating bidirectional communication and encryption.[2]Overview
Definition and Purpose
The Wiegand interface is a de facto wiring standard that facilitates the transmission of data from peripheral devices, such as proximity card readers and keypads, to central controllers in security and access control systems.[1] Developed as an industry convention rather than a formally mandated specification, it establishes a common electrical and signaling framework to ensure compatibility and interoperability among components from different manufacturers.[1] This standard, formalized in documents like the Security Industry Association's (SIA) AC-01 protocol for the 26-bit variant, defines the essential elements for unidirectional data transfer, including voltage levels, timing, and line configurations.[1] The primary purpose of the Wiegand interface is to enable straightforward, cost-effective serial communication of authentication credentials, such as facility codes and individual user IDs, without requiring complex addressing or bidirectional exchange.[1] By using a simple two-wire data line (plus ground and power), it minimizes wiring complexity and installation expenses while supporting reliable pulse-based signaling for short-distance connections typical in building security setups.[1] This design prioritizes ease of integration for system designers, allowing readers to relay encoded information from credentials like magnetic stripe or proximity cards to controllers for verification and authorization decisions.[1] The interface originates from the Wiegand effect, a nonlinear magnetic phenomenon discovered by inventor John R. Wiegand in the early 1970s, which involves specially processed ferromagnetic wires capable of generating high-fidelity pulses for data encoding.[4] Patented in 1974 as a bistable magnetic device, this effect was initially applied to create durable, tamper-resistant magnetic cards where data could be stored in segments along the wire, producing detectable signals when read.[4] The subsequent evolution into a communication interface bridged the gap between these early card technologies and broader system architectures. Key applications of the Wiegand interface center on physical access control, where it connects readers at entry points to backend controllers for door unlocking and logging.[1] It also extends to time and attendance systems, enabling the tracking of employee check-ins via compatible readers that transmit badge data for payroll and monitoring purposes.[1]History
The Wiegand interface originated from the Wiegand effect, a nonlinear magnetic phenomenon discovered by American inventor John R. Wiegand in the early 1970s while experimenting with specially annealed ferromagnetic wires that exhibit abrupt magnetization reversal when exposed to an external magnetic field.[5] This effect was patented in 1974 as a bistable magnetic device suitable for encoding data in thin wires embedded within plastic cards, initially intended for secure identification and access applications by leveraging the wire's dual-layer structure—a soft inner core and hard outer shell—to store and transmit binary information without traditional magnetic stripes.[4] The technology's early focus was on creating durable, tamper-resistant cards that could be read by proximity or swipe mechanisms, marking a shift from mechanical keys to electronic credentials in security systems. Commercialization accelerated in the 1980s through companies like HID Global (originally Hughes Identification Devices, founded in 1991),[6] which integrated the Wiegand effect into practical card readers and controllers, establishing it as a de facto industry standard due to the absence of formal international specifications.[7] Key milestones included the introduction of the 26-bit format in the early 1980s, featuring one leading parity bit, an 8-bit facility code, a 16-bit card ID, and a trailing parity bit, which became the benchmark for compatibility across vendors.[5] By the mid-1980s, the interface gained widespread popularity in access control systems for its straightforward two-wire data transmission (Data 0 and Data 1 lines), enabling reliable integration with emerging microprocessor-based panels in commercial buildings and facilities.[8] The Wiegand interface's dominance persisted into the 2000s, driven by its inherent simplicity in design—requiring minimal wiring and no complex encryption—low implementation costs compared to alternatives, and broad compatibility with legacy hardware that lacked advanced processing capabilities.[9] As of 2025, it remains embedded in over 90% of physical access control systems worldwide, supporting millions of legacy installations in sectors like corporate offices and healthcare, though its adoption is declining amid growing security vulnerabilities that expose unencrypted data to interception and tampering.[10]Technical Specifications
Physical Layer
The Wiegand interface utilizes a basic wiring configuration with three essential conductors: ground (black wire), Data 0 (green wire for transmitting binary 0 bits), and Data 1 (white wire for transmitting binary 1 bits). An optional red wire provides power, typically +5 V DC or +12 V DC, to the reader device, enabling remote powering without separate cabling in many installations.[11][12][13] Electrically, the interface operates on an open-collector configuration, where the Data 0 and Data 1 lines are pulled high to +5 V via resistors during idle states, ensuring a stable logic high. Data transmission occurs through momentary low pulses (pulling the respective line to ground) on either the Data 0 or Data 1 conductor, with typical pulse widths of 50–100 μs to represent bits. Access controllers incorporate pull-up resistors, commonly valued at 4.7 kΩ connected to +5 V, to detect these voltage transitions and interpret the signals accurately.[14][15][11] This design supports cable runs up to 500 feet (150 m) on 18 AWG shielded wire without signal degradation or repeaters, though for lengths beyond 500 feet, voltage drop must be evaluated, and thicker wire gauges (lower AWG numbers) may be necessary to maintain performance. The interface accommodates both 5 V and 12 V systems for power delivery to readers, while data line current draw remains low at approximately 1 mA per line during pulses, limited by the pull-up resistor value. Readers actively generate these low-going output pulses, whereas controllers passively monitor the lines through their integrated pull-ups for reliable detection.[16][15]Data Protocol
The Wiegand interface utilizes an asynchronous serial transmission protocol that is strictly one-way, directing data from the reader device to the controller without any return path. Bits are encoded and sent as sequential low-going pulses exclusively on either the Data 0 (D0) line to represent a logic 0 or the Data 1 (D1) line to represent a logic 1, ensuring that pulses on both lines never overlap to maintain signal integrity.[1][17] The encoding scheme resembles an inverted form of Manchester encoding in its use of pulse positioning for self-clocking but simplifies it to a single pulse per bit on one data line, with the lines otherwise held high in the idle state. To facilitate error detection, parity bits are included within the bit stream, typically an even parity bit for the first half of the data and an odd parity bit for the second half, allowing the receiver to verify the integrity of the transmitted information.[2][11] Precise timing parameters govern the protocol to accommodate varying hardware capabilities while preventing misinterpretation: each pulse must have a minimum width of 20 μs and a maximum of 100 μs, the interval between the trailing edge of one pulse and the leading edge of the next (pulse interval) ranges from a minimum of 200 μs to 20 ms, and complete messages are separated by at least 2 ms to allow the receiver to reset. These constraints yield an effective data rate equivalent to approximately 1 kbps, though the asynchronous nature means no fixed baud rate is enforced.[1][2] Messages follow a structured format that begins with an initial even parity bit covering the ensuing data segment, proceeds through the core data fields (whose length varies by application, such as 24 bits in the common 26-bit format including parities), and terminates with a final odd parity bit over the remaining segment, ensuring overall balance without any preamble sequence or postamble. The protocol lacks support for acknowledgments, flow control, or bidirectional exchange, relying on the reader's transmission of fresh messages upon card presentation.[17][11] Error handling is rudimentary and depends solely on the parity bits for detection; the receiving controller computes parity over each half of the message and discards any with mismatches, but no retransmission requests or advanced correction mechanisms are incorporated into the protocol.[1][2]Formats and Variations
Standard Bit Formats
The standard bit formats in the Wiegand interface refer to fixed-length data packets that encode facility identification and unique card identifiers, along with parity bits for error detection, transmitted serially from readers to controllers. These formats originated with early proximity card systems and remain foundational for compatibility in access control. The most prevalent are the 26-bit, 34-bit, and 37-bit variants, each balancing capacity for sites of varying scale while adhering to the protocol's simple binary structure.[18] The 26-bit format, designated H10301 by HID Global, consists of a leading even parity bit, an 8-bit facility code (allowing up to 256 unique sites), a 16-bit card ID (supporting 65,536 unique cards per facility), and a trailing odd parity bit, for a total of 26 data bits. This structure enables a maximum of 16,777,216 unique card-site combinations, suitable for small to medium installations.[18][19] The 34-bit format, commonly associated with Honeywell (formerly Northern Computers) systems and labeled N10002, expands capacity with a leading even parity bit, a 16-bit facility code (up to 65,536 sites), a 16-bit card ID (up to 65,536 cards per facility), and a trailing odd parity bit. This design addresses the limitations of the 26-bit format for larger enterprises requiring more granular identification without shifting to fully proprietary schemes.[20][21][22] The 37-bit format, an HID Global offering (H10304 with facility code), includes a leading even parity bit, a 16-bit facility code, a 19-bit card ID (up to 524,288 cards per facility), and a trailing odd parity bit, providing the highest capacity among standard formats. It represents the practical maximum for CR80-sized cards as defined in ISO/IEC 7810, optimizing data density on physical media without exceeding typical reader processing limits.[21][23]| Format | Total Bits | Leading Parity | Facility Code (bits) | Card ID (bits) | Trailing Parity | Max Cards per Facility |
|---|---|---|---|---|---|---|
| 26-bit (H10301) | 26 | Even (1 bit) | 8 | 16 | Odd (1 bit) | 65,536 |
| 34-bit (N10002) | 34 | Even (1 bit) | 16 | 16 | Odd (1 bit) | 65,536 |
| 37-bit (H10304) | 37 | Even (1 bit) | 16 | 19 | Odd (1 bit) | 524,288 |