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3-12 KEYPAD BOARD (81820): Refer to Figure (10-48). The keypad board contains
the LEDs, LED drivers and the keypad matrix circuitry. U2, U3 and U4 are serially-
loaded UCN6810 LED driver IC's. LED data is transferred from the CPU by
synchronous serial transfer over clock, data, and enable lines KCLK, D0 and OENA on
connector 85. Two LEDs are driven directly by the Logic board: TXD and SEL via cable
67.
The keypad matrix is comprised of the 74HC154 column decoder, U1 and 43 keys on a
3-row-by-15-column matrix. The column decoder uses the four encoded scan lines KS0-
KS3 and generates a 16-line output to drive the columns of the switch matrix. A pressed
key is read by the CPU on the row input lines, KR0-KR2. The keypad matrix is only
scanned when a key is pressed.
The manual gain port is mounted on the keypad board and the wiper connection returns
to the CPU board on connector 85.
3-13 LOGIC BOARD ↔ CPU INTERCONNECTIONS: The RX-340 main logic
board is connected to the front panel CPU board by a synchronous Inter-Integrated
2
Circuit (I
C) bus (cable #81) which transfers commands and data bidirectionally at a rate
of approximately 80,000 bits per second. The bus signals are CLK, DAT and ALERT.
When the front panel is in local mode, the front panel controls the receiver over this bus.
During Remote mode, however, operation is limited to RS-232 control only. Likewise,
during Local mode (Remote OFF) the RS-232 is limited in control and the front panel
takes priority. Since the logic board and the CPU board are very interdependent, they
2
must be connected by the I
C bus to function properly. The partition of tasks falls with
the front panel performing all user interface functions, encoder polling, and display
formatting, and the logic board controls the receiver functions and communications over
the RS-232 port.
3-11
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