Elan Digital Systems Ltd.
8
HD717 USER’S
GUIDE
The waveshapes for the two protocols are summarised below. Refer
to the ARINC specification for a more complete definition.
BIPOLAR R-Z
LINE A to LINE B
+10V
-10V
0V
1
1
0
0
0
+5V
-5V
0V
+5V
-5V
0V
LINE A to GND
LINE B to GND
BIT-CELL
HARVARD BIPHASE
LINE A to LINE B
+5V
-5V
0V
1
1
0
0
0
+5V
-5V
0V
+5V
-5V
0V
LINE A to GND
LINE B to GND
BIT-CELL
As can be seen, there is sufficient clocking information in both
waveforms to allow complete asynchronous reception without the
source clock present. The BRZ code is inherently simple to decode
due to its “transition-per-bit” nature. Harvard Biphase is more
complex to decode and relies on there being at least one zero in the
data stream to ensure the receiver is correctly in sync with the
transmitter. The HD717 provides a status bit and various control
