IC-2 on the control board in an I/O expander chip.  It is a uPD8243c.  This chip is directly compatible with the 8049 processor, and uses instructions received from the processor on pins 8-11 to control 4 output ports. These 4 ports drive the multi-digit florescent display unit.

8243 Pinout

8243c_Datasheet

IC-3 Pinout Functions

Pin 1:  I/O Port 5 Bit 0 (1 MHz digit)

IC-2 Pin 1: Port 5 Bit 0

 

Pin 2:  I/O Port 4 Bit 0 (100 Hz digit)

IC2 Pin 2: Port 4 Bit 0

Pin 3:  I/O Port 4 Bit 1 (1 KHz digit)

IC-2 Pin 3: Port 4 Bit 1

Pin 4:  I/O Port 4 Bit 2 (10 KHz digit)

IC2 Pin 4: Port 4 Bit 2

Pin 5:  I/O Port 4 Bit 3 (100 KHz digit)

IC2 Pin 5: Port 4 Bit 3

Pin 6:  Chip Select (low)

IC2 Pin 5: Chip Select

Pin 7:  Clock Input (PROG)

IC2 Pin 7: Clock Input

Pin 8:  CPU Interface I/O Port 2 Bit 3

IC2 Pin 8: Port 2 Bit 3

Pin 9:  CPU Interface I/O Port 2 Bit 2

IC2 Pin 8: Port 2 Bit 2

Pin 10: CPU Interface I/O Port 2 Bit 1

IC2 Pin 10: Port 2 Bit 1

Pin 11:  CPU Interface I/O Port 2 Bit 0

IC2 Pin 11: Port 2 Bit 0

Pin 12:  Ground

IC2 Pin 12: GND

Pin 13:  I/O Port 7 Bit 0 (segment output ‘e’)

IC2 Pin 13: Port 7 Bit 0

Pin 14:  I/O Port 7 Bit 1 (segment output ‘f’)

IC2 Pin 14: Port 7 Bit 1

Pin 15:  I/O Port 7 Bit 2 (segment output ‘g’)

IC2 Pin 15: Port 7 Bit 2

Pin 16:  I/O Port 7 Bit 3 (Buzzer Output ON=Low)

IC2 Pin 16: Port 7 Bit 3

Pin 17:  I/O Port 6 Bit 3 (segment output ‘d’)

IC2 Pin 17: Port 6 Bit 3

Pin 18:  I/O Port 6 Bit 2 (segment output ‘c’)

IC2 Pin 18: Port 6 Bit 2

Pin 19:  I/O Port 6 Bit 1 (segment output ‘b’)

IC2 Pin 19: Port 6 Bit 1

Pin 20:  I/O Port 6 Bit 0 (segment output ‘a’)

IC2 Pin 20: Port 6 Bit 0

Pin 21:  I/O Port 5 Bit 3 (digit output M.CH)

IC2 Pin 21L Port 5 Bit 3

Pin 22:  I/O Port 5 Bit 2 (10 Hz digit)

IC2 Pin 22: Port 5 Bit 2

Pin 23:  I/O Port 5 Bit 1 (10 MHz digit)

IC2 Pin 23: Port 5 Bit 1

Pin 24:  VDD +5 volt supply

IC2 Pin 24: VDD +5 volt supply