# OSDP and Wiegand

*Open Supervised Device Protocol* · IoT

> OSDP is a two-way, encrypted protocol that connects card readers to access control panels. It is replacing the older Wiegand interface.

Wiegand has been the usual way to connect card readers to door controllers since the 1980s. The reader sends the card number one way over two data lines, most often in the 26-bit format. The data is not encrypted and the controller cannot tell whether the reader is still connected, so anyone with access to the cable can capture a card number and replay it. Cable runs are also limited to roughly 150 metres in practice.

OSDP was developed by the Security Industry Association (SIA) and adopted internationally as IEC 60839-11-5. It runs over RS-485, and several readers can be addressed on one line. Communication is two-way. The controller polls the reader continuously and notices if the cable is cut or the reader is removed. The reader's LED, buzzer and display are driven from the controller too.

OSDP Secure Channel encrypts traffic between controller and reader with AES-128 and lets the devices authenticate each other. Reader firmware can also be updated over the same line. New installations usually choose OSDP, and existing Wiegand readers are often connected through converter modules or replaced in stages.

In IoT-based access control, the reader or relay connects to an edge device on site. The door decision is made on that device from a local permission list, and access events are sent to the central system over a protocol such as MQTT. The door keeps working when the internet connection drops.

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Canonical: https://internetten.com.tr/en/glossary/osdp