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second mixer board compares the control voltage to the received signal level and
provides feedback to the DSP via connector J73. The two signals on J73 tell the DSP if
the applied AGC voltage is greater than or less than an AGC voltage derived from the
received signal. The DSP then uses this information to control shaping of the analog
AGC response. Four user-programmable control outputs are provided on the rear panel
J8 and are programmed by commands sent from a controller or PC. These logic level
outputs are capable of providing approximately 10 mA of current. External circuitry
should be added if more power is required. They may be connected to external devices to
provide additional control capability to the system.
3-11 FRONT PANEL CPU (81819): Refer to Figure (10-43). This PC board
subassembly contains the CPU complex, support logic and an audio-amplifier section.
The microprocessor (U3) is a Philips 80C552 with an oscillator frequency of 22.11 MHz.
There is 8k of RAM (U9, HY6264A) and jumper-selectable 32k to 128k of ROM (U4,
27C256 to 27C010) on this board. An analog-to-digital converter in the 80C552 reads
the position of the manual gain control mounted on the keypad board.
The three rotary encoders on the front panel are of the two-phase type. The main encoder
is connected to the CPU board on cable 88, while the two auxiliary encoders are mounted
directly on the board. XOR circuitry U2 and U6 on the CPU board generates an interrupt
when any of the three encoders is moved. Latch U5 is then addressed by U10 logic and
read by the CPU to determine which encoder changed, and by storing the last-known
latch value, the direction. Some portions of logic gates U2 and U10 are unused.
The main display is a 16-character-by-1-line alphanumeric display with serial data
transfer over SCLK and D0 on cable 84. The auxiliary displays are 16 character-by-2-
line ASCII-type displays; data is transferred in parallel mode on four data lines D0-D3.
These displays are mounted directly on the CPU board. The signals to both auxiliary
displays and the keypad board are buffered by U7 and U8. The 80C552 also includes
pulse-width modulation generators, which are used to drive a Darlington-pair dimming
circuit Q200 and Q201 for the meter (connector J97) and a dimming signal to the keypad
board (385) for dimming of the LED's via the BLANK pin of U2, U3 and U4. Dimming
of the displays is performed by command instead of hardware. The dimming function is
accessed by holding in the Setup button and turning the Memory/Scan knob.
The audio section of the CPU board is unrelated to CPU operation and includes the
power amplifier U11, a TDA1013B, for speaker audio. The speaker, headphones volume
controls and the headphones jack are mounted on the front panel; they are connected to
the audio section of the CPU board. Headphones audio arrives from the I/O converter
board on cable 8 as a stereo signal. Main monophonic speaker audio arrives on cable 34.
After amplification, audio returns to the rear panel switching-type speaker jack on cable
96. There is no amplification of the head phones audio before traveling to the
headphones volume control and jack. The meter is driven by the logic board via cable
72.
3-10
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