Archive for July, 2015

Apple II Address LED Display using the Prototyping and Debugging Board

Saturday, July 4th, 2015

I created an Apple II Address BUS LED Display using our Prototyping and Debugging Board for Apple II. The Display is based on an old document I found with schematics for the Address BUS LED display and single stepping for 6502.

Components:

For the Address LED Display

1 x NKC Electronics Apple II Prototyping and Debugging Board
16 x Red 5mm LED
1 x Blue 5mm LED (for SYNC display)
17 x 100 ohm resistor
4 x 100nF ceramic capacitor
3 x 74LS174 (we are using 74ALS174)

For single stepping circuit

1 x 74LS74 (we are using 74ALS74)
2 x SPST switch
1 x SPDT momentary switch
1 x 270 ohm resistor

So far, I’ve built only the Address LED display and it works very well. I still need to test the single stepping circuit, as the schematics I found is very old and the SYNC signal from the 6502 microprocessor is not available on the Peripheral bus.

Update (04-Sep-2015): I found out that the SYNC signal from the 6502 microcontroller is available on the Peripheral slots on the Apple //e, so today I implemented the single step circuit and it works great. It has the option to do single cycle or single instruction. A second switch is used to define if Apple //e is running continuously or single stepping and a momentary push button is used to do the single stepping. I also connected the BLUE LED to the SYNC signal.

Update (14-Sep-2015): Schematics for the Address Display and Single Step circuitry. It was extracted from the Apple I manual. Single Step uses only one 7474 IC, one resistor and 3 switches… it shows Steve Wozniak’s genius design skills. I found a similar schematics from the 6502 team and it is way more complex.

6502 Single Step and address display schematics from Apple I manual