Wiegand 26/34 vs RS-485: How to Choose an Access Control Reader Interface
Quote an access control or canteen POS project and the interface question comes first: Wiegand 26 or 34 out of the reader? RS-485 or TCP/IP between devices? They are not alternatives to each other — they sit at different stages of the same system. This article sets out how to choose.
The short answer first
Wiegand carries the card number from the reader to the controller over a short run. RS-485 is the bus that links controllers to one another and to the host computer. A normal project uses both at the same time.
What Wiegand actually is
It is a three-wire parallel signal: DATA0, DATA1 and ground. When a card is presented, the reader pushes the card number bit by bit onto those two data lines and the controller decodes it. The 26 or 34 refers to how many bits make up one frame.
Wiegand 26 vs 34 — what actually differs
26-bit format: one parity bit at each end, and of the 24 bits between them the first 8 are the facility code and the remaining 16 the card number. Card-number space is 0–65535, roughly 65,000 cards under a single facility code.
34-bit format: again one parity bit at each end, with 32 bits between. Most vendors split it 16-bit facility code plus 16-bit card number, though some use 8 + 24. The card space stays at 65,000, but the facility code space is far larger — which matters when you partition a multi-plant factory or a campus and want to avoid card-number collisions.
One warning: more bits does not mean more security. Wiegand transmits the card number in plain text, and a logger clipped onto the line can capture and clone it. Encryption was not a consideration when the protocol was written.
Where RS-485 belongs
RS-485 is a differential serial bus: two wires, A and B, carry the signal as a voltage difference, which rejects common-mode noise very effectively. Standard reach is 1200 m, with 32 nodes on one bus, extendable past 128 with repeaters.
Its value is multi-drop, bidirectional and long distance. Controller-to-controller cascading, POS terminals uploading transaction records, lift floor controllers — nearly all of them run on RS-485.
How to choose — match the interface to the link
· Reader to controller: Wiegand. Simplest wiring, universally supported, lowest reader cost · Controller to controller, or when you need blacklist download and real-time record upload: RS-485 · Across buildings, beyond 1000 m, or electrically noisy sites: RS-485 with repeaters, or go straight to TCP/IP · Remote management, multi-site, cloud platform: TCP/IP
Three mistakes that cause most call-backs
· Never run Wiegand cable in the same conduit as 220 V mains. It is TTL level with poor noise immunity; past roughly 50 m you should shorten the run or switch to shielded cable — the 90 m in datasheets assumes ideal conditions · RS-485 must be daisy-chained, never star-wired. Star topology causes signal reflections and intermittent faults that are the hardest to trace. Fit a 120 Ω terminator at each end of the bus · RS-485 is half-duplex: only one device may transmit at a time. Device addresses must be unique and the polling interval generous, or you get sporadic communication timeouts
Bottom line
The interface is not the hard part: Wiegand from reader to controller, RS-485 for device networking, TCP/IP when you need remote management. What really decides whether a project runs stable is the cabling — star wiring and sharing a conduit with mains are the two most common reasons for a site revisit.