repairing early HP calcs

[Tony Duell]

> 
> I dont own any. But according to a recent Circuit
> Cellar article, early ics can be easy to implement in
> a FPGA. Dont have the issue in front of me, but the
> guy needed to mimic if you will a crt controller.
> Therefore could the chips used in the early HPs (maybe
> up to and including the 41 series?) be readily
> emulated by an FPGA?

There are 2 main problems...

1) The triival one is that the HP calculators did not use standard logic 
levels. The Classics, Topcats and most of the Woodstocks (HP27 being the 
exception) are PMOS, for example

2) The more serious problem is lack of documentation for the ICs. Eric 
Smith knows more about this than me, having written emulators for these 
machines, but in many cases data sheets plain don't exist, and when they 
do, there are odd quirks. The NUT processor in the HP41 has a number of 
bugs compared to the data sheet I have, for example.

And of coruse fitting an FPGA in place of an HP ROM/RAM chip (which would 
cotnain, perhpas, 1K words of ROM and 16 56-bit 'registers' of RAM, with 
an odd internface to the rest of the machine) which comes in an 8 pin DIL 
package is another entertainment.

So yes, it can be done, but it is not trivial.

Shameless Plug Alert!

If you want to repair HP calculators, consider buying the HPCC Scheamtics 
CD-ROM. It contains scehmatics for almost 70 calculators, small 
computers, peripherals, etc. No, I don't get anything from the sales, 
other than supporting HPCC.

-tony