User Manual

“Halo” System: Communication Protocol 970-00002-000
EXI Wireless Systems, Confidential Page 18 of 33 July 27, 1999
Table 2 Weigand Message
Field Length Description
Even Parity Bit 1 Parity Bit + next 16 bits = 0
Control Code 4 Defines message type
Door Bit 1 1 = Door Open, 0 = Door Closed
Maglock Bit 1 1 = Maglock On, 0 = Maglock Off
Data 1 8 Dependant on message type
Data 2 8 Dependant on message type
Data 3 8 Dependant on message type
Odd Parity Bit 1 Odd Bit + previous 16 bits = 1
Total 32
5.2.1. Parity Bits (Even and Odd)
These two one-bit fields are used for error detection and they are starting and terminating the packet.
The “Even Parity Bit” is calculated for the first 16 bits. They are added in the Boolean algebra without
Carry (0+0=0, 0+1=1, 1+0=1, 1+1=0) and if this result is 0 the “Even Parity Bit” is set to ‘1’ and vice
versa.
The “Odd Parity Bit” is calculated for the last 16 bits and if this result is 0, the “odd Parity Bit” is set to ‘1’
and vice versa.
Figure 8 illustrates how are these bits calculated. The same calculation is conducted on the receiving side
(RIM) and if received and calculated parity bits match, the transmission is considered successful.
Figure 8
Σ + EPB = 0
EVEN
PARITY
BIT
Σ
+
OPB
= 1
ODD
PARITY
BIT