2. Meter Outputs -HD9
HD9 is capable of driving up to nine meters, output 15 to output 23. Port C -C0 to C7 of IC26 is used to
generate eight meter drives, output 16 to output 23. Port C drives an Octal darlington array IC23 to give the
required current drive. Output 15, for the ninth meter, will be covered in section 5, since this is driven from a
separate 8255 device.
Detection of the removal of the meter plug is achieved by IC27.lf the plug is removed INT4 is asserted and
generates a system interrupt.
3. Undefined Direct Outputs -HD8
Six open collector outputs are interfaced into HD8. These are generated by IC26 port 8 - B1 to B6, which
drive an octal darlington array IC33.
4. Switch Inputs -HD11
Switch inputs are interfaced into the system via IC21. Operation of the 8255 peripheral devices is covered in
section 1, detailing the generation of SEL 0 and SEL 1, and section 2, detailing the internal register operation.
Complex Sound and Switched Inputs
The final eight switch inputs, input 11 to input 18, are interfaced into IC22 Port A -A0 to A7. The ninth meter
drive is generated on port C - C3, which drives IC33 on sheet 4.
IC22, Port B - B0 to B7, is used to interface an 8 way dil switch into the system, SW3. A hex rotary switch, SW1
is interfaced into the system on port C - C4 to C7.
The complex sound generator, IC6 -SN76489, is also interfaced on Port B - B0 to B7 and controlled by Port C -
C2. Data is set up on Port B and latched into the sound chip by setting and resetting port C - C2. Since the device
is a slow peripheral and requires 32 clock cycles to complete an operation, logic within ULA2, is used to keep
the chip enabled, thus Port C - C2 may be removed early, without affecting the timing of the sound chip, and
thus not putting software overheads on the main processor .
The sound output, IC6 pin 7, is amplified by an on-board amplifier, IC3 - LM380, whose volume is controlled by
VR1.An alarm input is also interfaced in to the amplifier, via R29 and C15, which is not controlled by VR1 and
therefore is always enabled independent of the volume setting. The speaker outputs are interfaced into DH3 pins
8 and 9.
To Next Page
Loading...