HP Classic Series Display Driver

[Harald]

(09-03-2016 09:57 PM)PANAMATIK Wrote:  Hi Harald,
How did you manipulate the classic display driver to show alphanumeric characters? You would be able to emulate much more calculators than I could do with the woodstock display hardware.

Bernhard

Hi Bernhard,

The classic display drivers can display any combination of segments you like. The only exception is the decimal point:
- The decimal point always takes up a whole digit. It can't be combined with other segments
- The left most digit can't display a decimal point
- Two consecutive decimal points are not possible

This is to do with how the display drivers work. The cathode driver is a shift register that gets clocked by the anode driver. The anode drive decodes 5 data lines. There are 4 data words per digit on those 5 display lines. So in total there are 60 data words for the 15 digits. Every 4 data words the anode driver issues a step signal which clocks the cathode driver. But if the anode driver receives a decimal point it issues an extra step signal before the decimal point is displayed. Which means every decimal point reduces the the data length by 4 words.
After the last digit has been written a 1 needs to be clocked into the cathode drivers shift register.
You also have to be careful to only issue consistent data to the display driver. Otherwise it gets damaged. The problem is, that the anode driver must not charge any of the inductors if none of the cathode transistors are turned on. Because that would mean the inductors can't be discharged through the LEDs. That results in a high voltage on the driver transistors and they get destroyed.

How does the woodstock display driver work in comparison? I assume it has a fixed character set? How is that transmitted to the driver?

Which other calculators do you have in mind for emulation? Currently I am planning to just output display data received from Tony's PIC controller. I have the feeling writing an emulator is way over my head..

Cheers,
Harald