User's Manual

RPT 633
TM1184 Issue 1 Page 3 - 7
Decoding
20. The VOX keying module decodes the incoming encoder serial data into the two
logic signals shown in Table 1.
Table 1: VOX Keying and Mode Signals
NOTE: Tx KEY is active low.
Tx KEY MODE TRANSMITTER
(PL1-2) (PL1-3) STATE
Low Low Data Mode
Low High Speech Mode
High Low/High Off
OPERATION
General
21. Components IC1a and IC1b are negative edge retriggerable monostables having
output pulse widths of 4.5ms (OA) for the Mode signal and 2ms (OB) for the CLK
signals to IC2. TR1 and associated components are not used in these transmitters
and are therefore by-passed via LK1 and LK2.
Power-Up
22. At power-up R4 and C5 hold IC2 CLR input low. This causes IC2 pin 3 (QC) to
route a low signal to IC3-6. Integrated circuit IC1a-6 routes a low signal to IC3-5,
which results in the Tx Key signal at PL1-2 remaining high and holding the
transmitter keyed off.
Data Path
23. To key the VOX keying module for data mode, the incoming data stream, via PL1-
4, LK1, LK2, TP2 to IC1a-5, retriggers IC1a for the duration of the data. IC1a-7
routes a low Mode signal via PL1-3 to set the transmitter into data mode whenever
data is present. Integrated circuit IC1a-6 (OA) routes a high signal to IC3-5. This
results in a low Tx Key signal via LK4 illuminating D3 and, via PL1-2, Keying
on the transmitter.