Forward To The Past - 3


One tiny victory tonight:

If you tie all the data lines of a Z80 CPU low, then tie RST, BUSREQ, WAIT, INT and NMI high and apply a clock you get a rather large 16-bit counter.  Using an oscilloscope or even a fast logic probe, you will see the address lines cycling through

Why?  Well opcode 00 is NOP (No Operation), so the address counter just moves on to the next address.  Other useful signs of life will be:

/MREQ and /RD will be low (we're reading memory) while /IOREQ and /WR will be high.  HLT will also be high.

I would also recommend using a packaged crystal oscillator, as it's just one less thing to have to get working.

So for our first test, the wiring will be:

Pin  Signal  Tie-to
16   /INT         +5
17  /NMI         +5
24  /WAIT      +5
25  /BUSREQ +5
26  /RST          +5
  7  D4              GND
  8  D3              GND
  9  D5              GND
10  D6              GND
12  D2              GND
13  D7              GND
14  D0              GND
15  D1              GND

11 VCC           +5V
29 GND           GND

Here's a schematic for this simple stage.  Note that the schematic assumes a 14 pin oscillator, whereas the photo shows an 8-pin device.  The layout of the device is the same however.

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