User Manual

“Halo” System: Communication Protocol 970-00002-000
EXI Wireless Systems, Confidential Page 17 of 33 July 27, 1999
5. Weigand Communication Protocol
The Controller communicates to the Host PC through the “RS-485 Interface Module” (RIM). The RIM
sends information to the Controller via control lines and receives information from the Controller via the
Weigand lines. These two communications could take place at the same time due to multitasking nature of
the Controller’s and RIM’s firmware.
5.1. General
Weigand communication protocol is unidirectional protocol developed for card readers. Information is
transferred over two data lines, one used to transfer ‘zeroes’ and the other for ‘ones’. Timing is strictly
defined and is shown in the Figure 7.
Both lines are pulled high in the idle state so that receiver can detect line failure (if it stays low for more
then 50 µs). The signals in the Figure 7 represent one byte of data (CB Hex). Only one line can be pulled
low every 1 ms.
Because the receiver of this signal is sampling these two lines within tightly controlled 47 µs loops (once
every 47 µs), it is very important to keep the timing of these signals on target, especially duration of the 50
µs low pulse. If this pulse would happen to be shorter then 47 µs it could be missed by the receiver.
5.2. Weigand Messages
All information that Controller needs to pass to the Host PC is packed into the 32 bit long, predefined
packets and sent to the RIM over the Weigand lines. The format of these packets is shown in the Table 1
and each one of these fields will be discussed separately.
Figure 7 Weigand Protocol
WEIGAND 1
WEIGAND 0
50
µ
s
1 ms
1 1001 110
1 ms