Information

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ZACWIRE
TM
DIGITAL OUTPUT
1.2 Bit Encoding
The bit format is duty cycle encoded:
Start bit => 50% duty cycle used to set up strobe time
Logic 1 => 75% duty cycle
Logic 0 => 25% duty cycle
Perhaps the best way to show the bit encoding is with an oscilloscope trace of a
ZACwire™ transmission. The following shows a single packet of 96Hex being
transmitted. Because 96Hex is already even parity, the parity bit is zero.
1.3 How to Read a Packet
When the falling edge of the start bit occurs, measure the time until the rising edge of
the start bit. This time (Tstrobe) is the strobe time. When the next falling edge occurs,
wait for a time period equal to Tstrobe, and then sample the ZACwire™signal. The
data present on the signal at this time is the bit being transmitted. Because every bit
starts with a falling edge, the sampling window is reset with every bit transmission.
This means errors will not accrue for bits downstream from the start bit, as it would
with a protocol such as RS232. It is recommended, however, that the sampling rate of
the ZACwire™ signal when acquiring the start bit be at least 16x the nominal baud
rate. Because the nominal baud rate is 8kHz, a 128kHz sampling rate is
recommended when acquiring Tstrobe.