a classiccmp electronics project?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Jul 29 19:08:50 2004

>
> Ok, I don't know if this is possible, but it would be handy so I'll throw
> the idea out and see if the more electrically inclined have any ideas.
>
> My specific issue is the HP2100A, 2100S, and 21MX M/E/F systems. However, I
> suspect this problem is present in other machines... DEC, DG, etc.
>
> The loader roms and microcode roms (two different chips) for each system are
> pretty much impossible to find. I'd like to make copies of them both for
> myself to use in other systems I currently have, and also to have a set of
> spares around. In addition, I'd like to make copies for other classiccmp'ers
> who may have systems that want/need a particular firmware or loader rom
> option. Finally, there are some roms that people have posted images for
> online that I'd like to burn myself because I don't have those particular
> roms.
>
> Since the blank chips are nigh impossible to find anymore... is it possible
> to use something like a PIC chip on a small DIP carrier card, that could be

I would doubt it. Those old fusible-link PROMs were actually quite fast
with access times of <50ns in many cases. And the designs depended on it.

I've ben looking at the HP98x0 calculator processor with a view to
designing a diagnostic PCB for it. It became pretty obvious that the
microcode PROMs could not be replaced, for example, with most common
EPROMs suitably wired -- they were just too slow.

Using a microcontroller to simulate a PROM is going to be even slower, I
would think.

Bootstrap ROMs -- that is to say ones that are mapped into the normal
memory space, as opposed to microcode PROMs which are used to control the
CPU, could generally be slower, and you might get away with EPROMs here.
In which case you could use, say, a 32K EPROM to store 128 256 byte
images and use a 7 position DIP switch (connected to the high 78 address
lines of the EPORM) to select between them.

-tony
Received on Thu Jul 29 2004 - 19:08:50 BST