SC102, v1.1, Fault Finding

First do the following:

  • Carefully inspect all soldering.
  • Check all the components are in the right place.
  • Check all ICs for bent legs which are not making contact with their socket.
  • Check all ICs are inserted the right way round.

Simple Voltage Tests

Remove all jumper shunts from the CTC module. Plug the CTC module in to the RC2014 compatible backplane with no other modules fitted. Power the backplane and perform the following checks with a volt meter:

  • Check the supply voltage on the CTC module, between, say, U1 pin 10 and U1 pin 20. This should be 4.5 to 5.5 volts, preferably 4.75 to 5.25 volts.
  • Check each input (CT0, CT1, CT2 and CT3) on connector P3 is being pulled up to at least 4.5 volts. The pull-up resistors are 470k so if your measuring device is not high impedance you will likely not see as much as 4.5 volts due to loading.
  • Check the outputs (ZT0, ZT2 and ZT2) on connector P3. These should each be a low digital level, less than 0.5 volts.
  • Check the interrupt enable input (IEI) on connector P2 is being pulled up to at least 4.5 volts.

Input Clock Signals

With the set up the same as above, do the following:

Check the clock signals CLK and CLKX at jumper header JP6. The set of pins nearest the on-board oscillator are the ones carrying these signals.

If you only have a meter, the voltage should be around 2.5 volts. If it is outside the range of 2 to 3 volts you may have a problem with the clock signal.

If you have a logic probe it should indicate the signal is oscillating.

If you have an oscilloscope you should see sensible digital clock signals on these pins. The frequencies should match the bus clock and the on-board clock oscillator.

Basic Operation

Set the address selection switches shown below (base address 0x88, giving a range of 0x88 to 0x8B).

Fit a single jumper shunt as indicated below (in yellow). CTC channel 1 should now be clocked from the primary bus clock (CLK).

Install a working set of RC2014 compatible modules into the backplane, together with the CTC module.

From the Small Computer Monitor, enter the following commands to set CTC channel 1 to generate a 0.6144 MHz clock on its output (assuming the bus clock is 7.3728 MHz) .

  • O 89 55
  • O 89 6

If all you have in the way of test equipment is a meter, then measure the output voltage of channel 1 (signal ZT1 on header P3). You should see a significant change when the above commands have been entered. A measurement here with a digital multi-meter showed a change from 0 volts to 0.5 volts.

A logic probe could be used to check the output of channel 1 (signal ZT1 on header P3) changes from a steady state to oscillating.

If you have an oscilloscope, verify he output of channel 1 (signal ZT1 on header P3) is 0.6144 MHz (assuming the bus clock is 7.3728 MHz).

If there is no output, check the input clock to channel 1 (signal CT1 on header P3).

If you are unable to verify the CTC output using test equipment, then check channel 1’s counter is functioning by repeatedly reading the counter’s value using Small Computer Monitor’s Input command (I, not 1):

  • I 89

As the counter should be changing fast, the value displayed by reading the counter should be random, in the range of 0 to 6. If the same value is displayed each time, then the CTC is not counting (or you are beating the odds of ‘random’).

Address Decoding

If the above tests show there is a problem, then check the address decoding:

Check the output of the address comparator U1, pin 19, is a logic high level of at least 4 volts.

Enter the following code with the Small Computer Monitor’s Assemble command (A):

  • a 8000
  • in a,(89)
  • jp 8000

Exit assemble mode with the Escape key and then run the program with the Go command (G):

  • g 8000

This program continually reads from the CTC, so you should see activity on the address comparator’s output. With a simple meter this will show as a voltage drop. In my case from 4.9 volts when the code is not running to 4.3 volts when it is running.

If the address decoding does not seem to be functioning as described above, do the following:

  • Check each address switch input (Q2 to Q7) on U1 is being pulled up to at least 4.5 volts when the appropriate switch (SW1) is Off (open) and drops to less than 0.4 volts when the appropriate switch (SW1) is On (closed).
  • Check there is activity on the comparator’s address bus inputs (P2 to P7).
  • Check the comparator’s inputs P0, P1, Q0 and Q1 are all logic low, of less than 0.5 volts.

If all the above checks look good, but the comparator’s output does not appear correct, then it is likely the comparator chip (U1) is faulty or there is a short on the output signal.


None of the above tests use interrupts, so the system’s /INT signal should remain high.

Homebrew 8-bit retro computing