HP-19C Repair Odysee

[PANAMATIK]

(05-03-2021 02:05 PM)teenix Wrote:  After playing around with the PIK chip myself a bit and realizing some (but not all) of its complexities, I have to say awesome work, well done :-)

Thanks Tony!

I want to show here one of these complexities, which I had to deal with today and to overcome. It is also an example of how crazy the HP-19C machine is constructed. Obviously IMO as the result of the idea to use the existing ACT chip and the PIK chip for the HP-19C, which were originally designed for the "Woodstock" calculators (ACT) and the HP-97 (PIK) by their engineers, and do not fit very well for this machine.

As mentioned the PIK chip constantly reads the 8x4 keyboard matrix (every 5-6ms) and determines the keycode. In case a button was pressed, it fills a 16 byte buffer with the keycodes, which will be read by the ACT processor and processed. But there is no way to read the information from the PIK chip, whether a button is still hold down. There are two buttons, SST and R/S, which show the actual program step, while pressed. How does this work?

The hardware designers decided to concurrently let do the ACT chip the keyboard scan as well. For this purpose they only connected ONE of the four keyboard rows to the ACT keyboard matrix inputs. This is the row where the buttons SST and R/S and 5 other buttons are located. For any other buttons it is impossible to detect, if they are still pressed. And indeed by analyzing the HP-19C ROM code, I found that only in case of the SST or R/S button the status flag S15 is tested, which tells the ACT that one of the keyboard inputs are at low level.

A crazy enable signal, switched by three transistors, makes it possible that the other ACT key matrix inputs can be used to detect the state of the PRGM/RUN and MAN/TRACE/NORM switch and doesn't interfere with the S15 flag.

Today in the morning I got the key matrix running and was able to enter keystrokes, which were detected by the PIK, and I could perform the first calculation: I added two numbers. But I could not yet test whether a key was hold down. Also the PRGM/RUN switch was not yet to detect. I needed the whole day to achieve this. Now the HP-19E ACT in conjunction with the newPIK can detect the state of the PRGM/RUN switch, and detect whether the SST and R/S key is still pressed, and I was able to enter my first program.

Code:


01  25 14 01 LBL 1
02        01 1
03        41 + 
04     16 64 f PAUSE
05     14 01 GTO 1

And it works! Also when using the SST key to see the program step, before it is executed.

Bernhard