Re: HP-71 Memory Expansion Message #2 Posted by Tony Duell (UK) on 7 Oct 2001, 7:03 p.m., in response to message #1 by Michael Meyer, M.D
The HP71 CPU bus (aka 'Saturn bus') is strange. For one thing, every memory chip keeps an internal copy of the program counter and data pointer so that the address doesn't have to keep on being sent to the memory chips. Using 'normal' memory chips with an HP71 is non-trivial.
There are basically 2 ways to simply expand the memory of an HP71. The first is to use the HP memory chips designed for the HP71. There are 4 * 4K RAM hybrids in an HP71. If you get a second HP71, you can take it apart, desolder the RAM hybrids and wire them into your existing HP71. It's basically a matter of connecting most of the pins to the corresonding pins on the exisitng memory hybrids apart from the configuration daisy chain. You need to break the latter at some point and wire daisy out on the chip in your 71 to daisy in on the first new RAM, then daisy out on that RAM to daisy in on the second new RAM, and so on.
Finally link daisy out on the last RAM you add to the daisy in line of the chip (or port connector) after the break.
The other way is to find an HP 1LQ4 chip. This chip was designed to link normal RAM and ROM chips to the Saturn bus, and was used in some third-party HP71 RAM modules. I have the data sheet for it somewhere, but I suspect your chances of finding such a chip these days is very small.
Since the HP71 hardware IDS (including schematics) was published, presumably it's not too hard to find. In which case I'd read that very carefully before attempting to modify your HP71. You need to understand the Saturn bus and in particular the configuration daisy chain.
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