IC-3 on the control board is 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 control the PLL Oscillators, and interface with the two data selector chips (IC-4 and IC-5) as well as the static RAM chip IC-6. It also provides BAND data to the RF board for control of filters.

8243 Pinout

8243c_Datasheet

IC-2 Pinout Functions

Pin 1: Data Selector & Ram I/O Bit 0

Pin 1: Data Selector & Ram I/O Bit 0

Pin 2: Band Data Bit 0

Pin 2: Band Data Bit 0

Pin 3: Band Data Bit 1

Pin 3: Band Data Bit 1

Pin 4: Band Data Bit 2

Pin 4: Band Data Bit 2

Pin 5: Band Data Bit 3

Pin 5: Band Data Bit 3

Pin 6: Chip Select

Pin 6: Chip Select

Pin 7: PROG

Pin 7: PROG

Pin 8: Control I/O Pin 3

Pin 8: Control I/O Pin 3

Pin 9: Control I/O Pin 2

Pin 9: Control I/O Pin 2

Pin 10: Control I/O Pin 1

Pin 10: Control I/O Pin 1

Pin 11: Control I/O Pin 0

Pin 11: Control I/O Pin 0

Pin 12: Ground

Pin 12: Ground

Pin 13: Port 7 PLL Control Bit 0

Pin 13: Port 7 PLL Control Bit 0

Pin 14: Port 7 PLL Control Bit 1

Pin 14: Port 7 PLL Control Bit 1

Pin 15: Port 7 PLL Control Bit 2

Pin 15: Port 7 PLL Control Bit 2

Pin 16: Port 7 PLL Control Bit 3

Pin 16: Port 7 PLL Control Bit 3

Pin 17: Port 6 PLL Control Bit 3

Pin 17: Port 6 PLL Control Bit 3

Pin 18: Port 6 PLL Control Bit 2

Pin 18: Port 6 PLL Control Bit 2

Pin 19: Port 6 PLL Control Bit 1

Pin 19: Port 6 PLL Control Bit 1

Pin 20: Port 6 PLL Control Bit 0

Pin 20: Port 6 PLL Control Bit 0

Pin 21: Data Selector & Ram I/O Bit 3

Pin 21: Data Selector & Ram I/O Bit 3

Pin 22: Data Selector & Ram I/O Bit 2

Pin 22: Data Selector & Ram I/O Bit 2

Pin 23: Data Selector & Ram I/O Bit 1

Pin 23: Data Selector & Ram I/O Bit 1

Pin 24: Vcc +5 volts

Pin 24: Vcc +5 volts