Classic Display Notes

[teenix]

Hi all,

Caution: Techy stuff follows :-)

Has anyone figured out how the Classic cathode driver is actually stepped to the next digit? I've looked around and no info. I know it's just a simple pulse, but how that pulse is generated is confusing.

I know there are errors shown on the HP45 patent document. One in particular (I think) is the RCD line that resets the cathode driver scan. My scope sees the pulse as 20uS during T4, whereas the HP diagram (Fig 18) shows a 1.25 uS pulse on T1.

The C CL pulse is supposed to step the cathode driver to the next display digit on its rising pulse edge and is shown doing this in Fig 18. It occurs just as the first LED segment of the digit is about to be excited (segment E).

I'm not sure how this works as it suggests that after a scan reset, the first digit is stepped over before it gets displayed. This obviously doesn't happen so I'm a bit confused.

This pulse is also tied to the anode driver [E] line from the ARC chip which seems to suggest that the LED G segment being lit or not affects the stepping also, which to me doesn't make sense.

PS Not expecting an answer, but you never know :-)

cheers

Tony