Service manual

July 1, 2002 68P81076C25-C
3-12 Theory of Operation: Command Board
3.2.7 Synchronous Serial Bus (MOSI)
The synchronous serial bus is an internal bus used by the microcontroller for communicating with
various ICs. The serial bus, called MOSI (master out/ slave in), is used to program the digital-to-
analog (D/A) converter IC (U526), the serial-to-parallel shift register (U530) on the command board,
and the ABACUS II IC (U301) on the RF board. The MOSI data is sent from the VOCON board’s
microprocessor (U204) through the ADSIC input/output IC (U406) and enters the command board
through P501, pin 9. This serial bus has an associated clock and individual select lines for steering
the data to one of the three possible devices.
The clock and data are routed in parallel to all serially programmed ICs. The ICs are programmed
one-at-a-time by the microcontroller, with each IC ignoring activity on its clock and data lines unless
it has been selected.
3.2.8 Received Audio
The received audio is sent from the ADSIC D/A converter as the SDO signal. The audio enters the
command board at P501, pin 40, and is routed to the analog multiplex gate (U524, pin 1). The gate’s
output (U524, pin 2) is paralleled with the output of a second multiplex gate (U524, pin 9) and sent to
voltage divider R455 and R456. The voltage divider provides the required attenuation for minimum/
maximum volume control settings. Capacitor C454 provides a DC block and couples the audio into
U450, pin 2 for amplification.
The two multiplex gates provide control of either receive audio or vehicular repeater audio. These
gates are controlled by the inputs to U524, pin 13 and U524, pin 6 from the serial shift register, U530.
The independent inputs are software selected by the VOCON’s microcontroller.
The audio power amplifier (PA), U450, is a DC-coupled-output bridge-type amplifier. The gain is
internally fixed at 36 dB. Speaker audio leaves U450 on pins 11 and 13. For dash-mount models, the
audio is routed to the speaker via P503, pins14 and 16. The amplifier is biased to one half of the A+
voltage and connected directly to the speaker from the rear accessory connector (J2, pins 6 and 7).
The speaker outputs must NOT be grounded in any way. An audio isolation transformer must be
used if grounded test equipment (such as a service monitor) is to be connected to the speaker
outputs.
When the radio is squelched, the audio PA is disabled by the VOCON board’s controller, providing a
low output state to P501, pin 44 (speaker-enable input). The low input turns off Q401 and Q400,
removing SWB+ voltage to the audio PA, U450. When U450, pin 10 does not have SW+B applied,
the speaker is totally muted and the audio PA current drain is greatly reduced. Diode CR402 (not
normally installed) is used when a vehicular repeater is installed and audio muting is required.
A second output for filtered receive audio is provided to drive accessory hardware. The output of
P501, pin 49 (MOD IN/DISC AUDIO) is primarily used for transmitter modulation. In the receive
mode, the digital signal processor (DSP), via ADSIC, outputs audio at a fixed level (approximately
800 mV pp). This output can be connected to the accessory connector (P503, pin 21) by selecting
the appropriate jumper settings.
3.2.9 Microphone Audio
The mobile microphone connects to the front of the control head through connector P104.
Microphone high audio enters the command board via P502, pin 6 and is routed to differential
amplifier buffer U402. Resistors R414 and R415 provide 9.6-V bias voltage for the microphone’s
internal circuitry. Amplifier U402 pre-emphasizes and limits the incoming microphone audio through
components C462, R407, C463, and R408, which perform an active filter function. Components
R441, R442, C467, C465, R443, C466, and C568 provide de-emphasis, developing the required
clamped microphone audio, referred to as “mic audio in” (MAI).