Always wire from the reader's own installation guide. Colors below are a common convention, used by HID and many others, not a standard. Manufacturers and older readers differ, and a swapped data line produces bad reads rather than an obvious fault.

Wiegand: common conductor colors

Color	Function	Notes
Red	Power, +V	Usually 5–16 VDC; check the reader's range.
Black	Ground	Common with the panel's reader ground.
Green	Data 0	Signals a binary 0.
White	Data 1	Signals a binary 1.
Orange	Green LED control	Pulled low by the panel to show access granted.
Brown	Red LED control	Present on many readers; often unused.
Yellow	Beeper control	Optional.
Drain	Shield	Ground at the panel end only.

OSDP: four conductors on RS-485

Conductor	Function
Power pair	+V and ground
Data pair	RS-485 A and B (sometimes marked + and −, or TR+ and TR−), on one twisted pair
Drain	Shield, grounded at one end

Keep the data on a single twisted pair. Splitting A and B across two pairs, a common shortcut when reusing old Wiegand cable, defeats the noise rejection RS-485 depends on. On long runs, terminate the ends of the bus as the reader and panel manufacturers specify.

The differences that matter

	Wiegand	OSDP
Direction	One way: reader to panel	Two way: panel and reader talk
Typical maximum run	About 500 ft	About 4,000 ft
Readers per cable	One	Many, addressed on a shared bus
Supervision	None. A cut or tapped cable is not reported.	The panel polls each reader and reports one that stops answering.
Encryption	None. Card data crosses the wire in the clear.	Secure Channel encrypts traffic with AES-128.
Standard	Industry convention, documented as SIA AC-01	SIA OSDP, published internationally as IEC 60839-11-5

When you replace a Wiegand reader with OSDP

Confirm the panel or reader interface supports OSDP, and which version, before ordering readers.
Existing six- or eight-conductor Wiegand cable will usually work if one twisted pair can carry the data. Test it; old cable is often untwisted or damaged.
Set each reader's address and baud rate to match the panel's configuration. Two readers at the same address on one bus will both fail.
Turn on Secure Channel and record that it is on. An OSDP reader running without it gives up most of the security benefit.
Bench-test one door end to end before you change the rest.

The protocol specification is maintained by the Security Industry Association.