Why the bridge speaks OSDP, not Wiegand

Wiegand sends card numbers in one direction and in plain text. OSDP runs over RS-485, checks the reader constantly, talks both ways and can encrypt. That is why the KLOKL Bridge uses OSDP.

If you connect a card reader to Home Assistant, the first question is how the reader talks to the controller. Most cheap readers offer Wiegand, many professional ones also OSDP. For the KLOKL Bridge we deliberately decided for OSDP only.

Wiegand: one way, in plain text

Wiegand dates from the 1980s. The reader sends the card number as pulses on two data lines, D0 and D1. Nothing comes back, nothing is encrypted, and the controller cannot tell whether the reader is still there. Small devices hidden behind a reader can record card numbers and replay them later; they are sold openly.

OSDP: a supervised conversation

OSDP (Open Supervised Device Protocol) is an open standard of the Security Industry Association and is published as IEC 60839-11-5. It runs over RS-485. The controller polls the reader several times per second; if the reader is removed or the cable is cut, this is noticed immediately.

Because the conversation goes both ways, the bridge can also control the reader. In practice this means:

• green LED and beeper when access is granted, red when it is denied
• keypad input arrives as its own value, separate from the card
• the reader status is visible in Home Assistant as a diagnostic sensor

Encryption where the reader supports it

OSDP Secure Channel encrypts the traffic between reader and controller with AES-128. Our firmware supports it for readers that offer it. Not every reader does: the Hikvision DS-K1107AMK we test with, for example, does not. Even without encryption, OSDP is ahead of Wiegand because of supervision and feedback.

Long cables, four wires

RS-485 transmits differentially and is made for long cable runs in buildings. The reader needs four wires: +12 V, A+, B− and GND. The bridge supplies the reader with power, so no second power supply is needed at the door. If the bridge sits at the end of a long bus, switch on its 120 Ω termination.

At a glance

• Wiegand: one way, plain text, no supervision, no feedback
• OSDP: both ways, supervised, LED and beeper control, optional AES-128

What this means in Home Assistant

The bridge translates OSDP into MQTT. Card numbers, keypad input and the PIN result arrive as sensors; your automations send the feedback back to the reader. How that looks in practice is shown in our guide to card and PIN; all topics are listed in the documentation.