HP-29C ACT compatibility

[PANAMATIK]

(06-08-2017 10:16 AM)Paul Dale Wrote:  Is there any chance the newACT would be able to handle the card reader and printer in later calculators? E.g. replacing the ACT in a 67 can be done but the card reader doesn't work afterwards -- what would be required to make it operate again? I can donate at least one sacrifical board for experimenting with.

This is the question to which I do not have a final answer!

There is a chance to get the card reader (HP-67/97) and printing (HP-19C) working with the new ACT. But when I started this project and made the design the goal was just to repair an HP-25, nothing else. Later I myself had the idea of restoring also an HP-67/97 or 19C. But the timing for handling the serial communication line to the card reader and printer via the PIK chip and to the ROMs and display drivers simultaneously was very difficult for the chosen PIC processor. It is not just done with a few program lines. However still I know intuitively that it is possible to accomplish the goal even with the actual newACT hardware.

Going more into detail:

The PIC processor has only one serial SPI interface. I use this resource for sending and receiving serial data from/to the ROM chips and the anode display driver via the bidirectional ISA line. The SPI clock is used to generate the PHI1 signal. I use another peripheral resource of the processor for generating the PHI2 signal by synchronizing a PWM output with the SPI interface with same baud rate, but phase shifted. The baud rate is 180 kHz. For printing or reading the card reader, another serial line, the DATA line, must be driven simultanuously and there is no peripheral resource left for it.

A solution could be to drive all serial lines via normal I/O pins, 180kHz is a little bit too fast for normal I/O pins and would consume much processor time, but it is a way I can see for making the new ACT able to also handle the card reader and printer. It is not a good solution and some display segments would have different brightness as a side effect, because the timing is not accurate. Perhaps this is the reason why I did not follow this approach with too much energy. You see I did at least try some experiments in this direction.

The more promising solution would be to use the SPI interface for normal operation but, when detecting a PIK card reader or print instruction, to switch to the normal I/O pin method. This is definitely difficult to program, but would make the card reader eventually running with the actual newACT hardware.

Of course, a redesign and creating a completely new project with FPGA instead of the PIC processor would be able to do the task, but this would need to consume my engineering resources for months. In another parallel universe I would have done it already, but obviously I admit I couldn't find this parallel universe here, because I believe I'm living in a real world called planet earth.

However, I'm still working on it.

Bernhard