Opening HP-9825 ROM Cartridges

[Vassilis Prevelakis]

If you are wondering whats inside, here is a picture

more info on how to open them at http://www.series80.org/HP9825

**vp

[Han]

Looks like these cartridges can be hacked quite easily if one wanted to add more ROM to them. The solder pads are already there!

[Eric Smith]

Sure, except that it's not an industry-standard ROM chip. It's a strange HP 16-bit wide ROM with multiplexed address and data, a built in address latch, decoder, and non-TTL signal levels. So you can't just add an EPROM or Flash chip; you'd need a daughter card with level shifters, and address latch, a decoder, and possibly some timing circuitry.

Edited: 18 June 2006, 4:48 p.m.

[Tony Duell]

That's how I open them too (and for that matter, 9810, 9820 and 9830 cartridges come apart in the same way). Melting a little solder onto the head of the 'nail' improves heat transfer and makes it easier to get them apart. The 'nails' are an aluminium alloy, and solder doesn't stick to them.

To put them back together, I normally use countersunk self-tapping screws. Not original, but it sure makes it easier to take them apart again.

There are at least 2 types of HP9825 cartridge. One uses the HP custom ROM as you've discovered. The other uses a normal ROM chip, and a little TTL (address latch, address decoder). Note that the PCB in one type is fitted components-up, in the other type it's fitted components-down. This is somewhere in the schematics you have.