Ryan Brooks's 6502 Nixie Clock

So, I wanted a Nixie clock and I really wanted to design the hardware and program it myself.  Most of the kits out there are based around a PIC or some other new, easy to use microcontroller.  I thought it would be much more interesting to make a clock powered by an old school computer. UPDATED Oct-06

The result is a 6502-powered Nixie clock.  I started this project about two years ago when large tubes (Z5680M in this case) were still available.   Since then, any large tube is 5x the price it was -- if you can find them.

The design consists of three boards: 

6502 controller

    8k of SRAM and clock, Dallas DS1742 timekeeper RAM
    8k of EPROM: 2764
    I/O: 6522 VIA
    Misc logic: 74LS00, 74LS138, DS1233 reset...

Shift board and HV drivers

    Daisy chain of 24 bits of shift registers: 74LS595
    BCD drivers: Russian 74141-alikes

HV power supply

    Switch-mode: MC34063A - A design from the NeoNixie group

The display consists of four East German Z5680 (beautiful) tubes for hours and minutes, and two Russian IN-18 tubes for seconds.   Each tube anode is fed via a 10kohm resistor and a pot mounted on the back of the clock chassis.   The individual digit cathodes are then wired into the output of the 74141s that decode a BCD input from the 74LS595s.  The 595s (serial shift registers with latches) are fed from three output lines on the 6522, which I use as serial clock, data, and output latch.   This allows you to use three lines of I/O to control the 24 bits necessary for 6 BCD digits.

Two of the boards, the 6502 controller and the shift board, were PCBs that I designed in Protel 99.   You can get the entire design database for these here, as a zip file.  The HV board was made with point-to-point wiring on a protoboard.  With these schematics and circuit board layouts with Gerber files, you are welcome to duplicate my design.   You can find the 6502 tasm source and S-record object code here.

Pictures of the construction are here

Outputs - Port B

0,1,2 - Data, Clock and Strobe
3 - Alarm annucator   (future: is 4k or so enough for a small sample?)
4,5 -neat LEDs (scrapped, kept everything neon)
6 -HV modulation (unused currently)

Inputs - Port A

0,1 - Hour +/-
2,3 - Minute +/-
4 - Second Reset

All inputs are active low and are connected to momentary switches (DPST); No hardware debounce. 
DC Power supply

All three boards are driven from the same low-voltage DC power supply.   The 120VAC input goes through an IEC connector/filter to a fuse holder and SPST switch.  From there, it enters a 10VAC transformer (sized for 2 amps) and then a bridge rectifier.   The ripply DC is filtered with a large cap and then made available on solder lugs.   This raw ~10VDC goes to the HV board and to the two logic boards through a standard 7805 for the digital circuits.

What I learned


6502 schematic
shift board

Thanks to everyone on Neonixie Yahoo! group and 6502.org!  Awesome people!   - Ryan K. Brooks