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HP-41 From Inside

Posted by Russell Nelson on 2 Apr 2000, 10:10 p.m.

I worked at HP from '81 to '83, during which period the HP-41CX was produced. My part it in was very small; mostly it consisted of not strangling Hank when he tested the alarm on the timer module Yet Another Time. My job was designing a CMOS version of the Harris Bipolar power supply chip. Curious job for an electrical engineer turned software hacker, but in my naivete that's what they hired me for. That lasted for a year, when Harris improved the quality of the chips they were sending HP.

My G-Job* (everybody at HP had a G-Job or two) was working with HP-IL. I scarfed a 1LB3 and interfaced it to the Radio Shack Color Computer. Cool chip. It interfaced with 8 I/O ports in a manner very similar to a UART. So I got my hands on an HP-IL digital tape drive. I wrote some code for the Color Computer which let me save files on the tape drive. Mass storage in the days when hard drives were hard to find.

I still have the ISA board which makes a 1LB3 available to PC compatibles. I ought to write a Linux driver for it, just for hack value. And I would if my HP-41's still worked. They both died a few years ago. Don't remember what I did with them. Unless they're at the bottom of some box in the garage, they're gone. It's just as well, since they were both hacked-up machines. Originally HP-41's, one was hacked into a CV and the other into a CX.

So, has anybody noticed the similarities between USB and HP-IL? About the only improvement in USB is power to the devices. Otherwise HP-IL was much simpler to use.

The only product I did that ever made it out the door was the HP-41 HP-IL Development ROM. I got to learn HP-41 microcode. Very bizarre. Only 4 levels of code stack, so you have to store state in flags instead of your calling address.

The best hack I did was to create a power indicator. I needed to know when the HP-41 was on, sleeping or off, so I took the plastic from an HP-IL module, cut a hole in the top, and put a two-color LCD (stolen from some other HP product) into it, along with some logic to twiddle the LCD's (they need AC, not DC) using the power signals from the module connector. Voila'! A simple power state indicator.

Russell Nelson

* G-Job = Government Job


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