Wang 600 First Test Next Log Entry

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Dead machine, no responses

With all boards installed the machine was powered up. No smoke, bad smell or bad sounds. Prog Error and Mach Error were both lit and display was dark.

• Press Prime and Clear:
  • Good News: Prog Error and Mach Error turn off
  • Bad News: Display remains dark.

The Wang 600 is microcode driven, so almost all of its functionality is performed in software, by microcode. This includes keyboard scanning, operation processing and display generation. A blank display is therefore a very non-specific symptom and can be caused by any failure that prevents microcode from running correctly. Such failures could be basic and general such as no system clock or specific and local such as a single failed gate that corrupts a critical microcode step.

The correct response of Prog Error and Mach Error to Prime is of little help because these outputs come from flip flops that are electrically reset by the Prime signal. So the correct response is a direct electrical result and does not suggest any microcode is functioning.

Troubleshooting must proceed from the general to the specific - beginning with system clocks and timing signals.

System Clocks and Timing

Close

Wang 600 Main Timing

As a first step it is useful to check the fundamental system clock and the basic timing signals that are derived from the clock. All of these are found on the 5965 circuit board.

The HP16500C Logic Analyzer (see HP16500C in the Equipment section) was used to instrument the 5965 board. This board has the 4MHz master clock and a series of counters and gates that generate the ten-stage main sequencing signals for the microcode engine.

The trace capture shows that all basic signals are present and have expected sequences and relationships. This confirms that the power supplies are adequate to run the logic, the master clock is running and that the dozen or so ICs involved in the basic timing circuits are all working. This in turn provides some encouragement that the 50 year old logic ICs may still be in generally good condition.

With main timing signals verified, the next stages of troubleshooting becomes more involved.