|Re: Interchangeable Parts for Pioneers|
Message #8 Posted by Paul Brogger on 28 Mar 2003, 6:57 p.m.,
in response to message #7 by Ron Ross
I've changed circuit boards. In fact, because I'm always wringing maximum $ out of every genuine 42s I can get working, my "personal" hot rod is a shiny new HP-17BII with a 32K HP-42S circuit board. I've painted the keys and keyboard black, except for those which are neutral or agree (STO, RCL, UP, DOWN, SHIFT, the numerals, the arithmetic operators, etc.)
I used to think I could memorize the layout, but I don't actually use it often enough for that. So, I've got a ~full-scale color print of the original MoHPC -42s keyboard image -- cropped, laminated, and attached to the back by a clear tape "hinge". I fold it out to use anything beyond the basics, and fold it back when storing the calculator in its case.
Despite my contrary report in an early repairs article (I should update that!), the ROM is actually part of the processor chip. There is a second chip, but it's RAM. (That's why we can upgrade the -42s from 8K to 32K.)
If I remember correctly, Tony Duell somehow discovered that by tying an "off-board ROM enable" line low (or whatever), he could cause the CPU to consult an external ROM (I think located in the extra set of pads on the PCB). So, if one had the right PROM programmer, intimate knowledge of 42s internals and Saturn Assembly Language, one might be able to hack the 42s operating code! (Potentially VERY cool, and about as esoterically geeky as one might get!) On a simpler note, one might convert -17BII PCBs to -42S functionality if one could produce the right PROM (without angering any copyright holders . . . )