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SBX-32 THEORY OF OPERATION
SBC-9632 MICROPROCESSOR PCB
1. POWER SUPPLY
Power to the remote panel is supplied by a 9-12 VDC wall-pack power supply through J1.
Polyswitch RT1 provides fault current protection to the wall pack supply and the remote panel.
Diode D1 (in combination with RT1) also provides reverse voltage protection should a wrong
polarity power supply accidentally be connected to the remote panel. The 9-12 VDC input is
regulated to 5 VDC by U4.
2. MICROPROCESSOR, RESET, I/O
All functions of the remote panel are controlled by U1, an 8051-type microprocessor. The
microprocessor is complete with a serial port, ram, counters, I/O lines and program storage
EEPROM. Y1, C6 and C7 make up the 11.05 MHz oscillator clock for U1.
Port 0 (P0.0-P0.7) and Port 1 (P1.0-P1.7) control the front panel display board.
Switch S1 and S2’s status is input to the microprocessor by P2.0-P2.7.
U3 combines the individual microprocessor TXD and RXD lines into a single COMM line. It
also provides the necessary drive and level conversion between the microprocessor serial I/O lines
and the COMM line interface. Resistors R2 and R3 provide a bias for the COMM input signal
comparator. The direction of the COMM line is controlled by P3.7, and is normally in the receive
mode except when a panel transaction is made.
U2 provides the reset function to the microprocessor upon three different conditions. The first
is a power on reset. The second is at any time the +5 VDC power supply goes below ~4.5 VDC (a
brownout condition). Third is when the microprocessor fails to toggle P3.6 in a timely fashion
(watchdog). Under normal operation the microprocessor should toggle P3.6 many times per
second.
SYA-9632 DISPLAY PCB
The microprocessor via P1.0-P1.7 controls the front panel through IC U1. U1 is a high density
PLD. It is programmed with an address decoder for three 4-bit addressable latches, encoder logic
to drive the 7-segment LED displays and decoder logic for the row and digit drive signals.
Row driver transistors Q1-Q11 and Q28-Q29 buffer the segment and row data. Column driver
transistors Q12-Q27 buffer the digit data. The display functions by the microprocessor setting up
the segment drivers (row) with the data for a digit, and then enabling the appropriate digit (column)
driver. Each digit is enabled for 1mS out of 16mS.
The keyboard is scanned by turning off the row drivers and testing the KEYROW lines while
the column transistors are turned on one at a time.
SBX-32.DOC