Tech: HP35 anode driver

[Harald]

Hi Tony,

This is all described by late Jacques Laport, thanks to Eric you can still access his website here.

It's been a while since I programmed it, but hopefully I remember it correctly.
The anode driver generates the clock and decodes the segments for each digit. It clocks the cathode driver, which essentially is a shift register. The start bit is set by the A&R.
The anode drivers internal reset is generated from two of the five input lines containing segment data. It happens when segments B and D are turned on simultaneously. This means the synchronization happens automatically when the initial display containing a "0" is shown. And indeed it is required. With my board I can display whatever I want at startup. The display shows nonsense if non of the characters contain segments b and d simultaneously. So I always start with "0.00" for a split second. You can notice that if an HP35 is emulated because the HP35 startup display is not "0.00"

Cheers,
Harald