6-4. DECODER/TIMER (7A3). Continued
The SWITCH STATUS, MAIN/STANDBY, and CM PRES lines are checked and the information provided to the
microcontroller on the DATA BUS. The STATUS LINES OUT are latched as directed by the microcontroller.
A crystal oscillator on the decoder/timer provides several clock frequencies. A 3.84-MHz crystal output is
divided by two to obtain 1.92 MHz. It is used by the microcontroller and logic circuits on the decoder/timer. It is
divided further to produce 640 Hz and 320 kHz. These are also used by the microcontroller.
A delay circuit holds the CLR-N line at logic 0 for about 1 second when the control-monitor is turned-on. After
that, it is held at logic 1. It is used to reset the microcontroller and start the initialization routine.
Logic circuits convert the PROCESSOR Q signal into FSK DATA. Several control lines from the microcontroller are
required to accomplish this. See figure 6-3 and figure FO-17. The FSK DATA signal is a 2560/3200 Hz square
wave. 2560 Hz is a logic 1. 3200 Hz is a logic 0.
OUT 3 STROBE and DMA OUT-N are also generated for use by the microcontroller.
6-5. ANALOG MODULE (7A4).
The analog module performs receive and transmit functions. See figure 6-4. Three radio channels are available
for receive and transmit. A channel is selected by the radio switch. The transmit circuit shapes and buffers the
FSK square wave signal from the decoder/timer. The receive circuit converts the analog FSK signal into a digital
The three transmit channels are identical except for the input control signal. For radio 1, RADIO 1 XMT line goes
to logic 1. This closes the appropriate transmit switch. The FSK DATA signal is from the decoder/timer. It is a
2560/3200 Hz square wave. The shaper/filter circuit converts it into a sine wave. This FSK analog signal is routed
through the transmit switch to a buffer amplifier. It isolates the outgoing signals from the incoming signals, The
signal is transformer coupled to the radio. Radio 2 and radio 3 paths operate the same as radio 1.
The receive path is selected that corresponds to the transmit path. For radio 1, RADIO 1 RCV line goes to logic 1.
This closes the appropriate receive switch. The received signal is transformer coupled to the receive switch. It is
routed to a buffer amplifier. The buffered FSK signal is demodulated. The analog data signal is converted into a
digital data signal (RCV DATA). The RCV DATA signal is sent to the microcontroller. Radio 2 and radio 3 paths
operate the same as radio 1.
When the FSK demodulator circuit detects a carrier, the 2880 DET-N line is pulled to logic 0. The RCV DATA signal
is fed back into the analog module. It is used to maintain the frequency accuracy of the VCO and tracking of the
received analog signal.
The receive and transmit signals to a second control-monitor are also routed through the analog module. During
transmit, the CM XMT line goes to logic 1. This closes the control-monitor transmit switch. The signal transmitted
is the PROCESSOR Q signal. It is a 640-Hz digital data stream. It is routed through the transmit switch to a current
driver. The current driver increases the current to the level required to drive the CM OUT line. A signal from the
second control-monitor is input on the CM IN line. It is also routed through a current driver. The CM DATA signal
is output to the microcontroller.