Computer Concept PALPROM redesign.

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KenLowe
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Computer Concept PALPROM redesign.

Post by KenLowe » Tue May 14, 2019 11:49 am

Following on from this thread, I recently had some PCBs fabricated to see if I could recreate any of the CC 128k PALROMs, like SpellMaster and Mega 3 ROM on a new carrier. Surprisingly (for me), it worked first time!

One the goals I set myself was to create a carrier board that was no wider than a standard DIP28 socket. This, however, came with a bit of a challenge, as I needed to use the same socket header extension to interface with both the PAL (GAL) and the 128k PROM. Finding a suitable header was a bit difficult, but I eventually tracked down something suitable from SAMTEC (and supplied by toby.co.uk).

I then discovered that 256k PROMs were actually cheaper to buy than 128k PROMs, so decided to modify the carrier board to use the full capacity of the 256k PROM. The board has a jumper that allows you to select between 2 separate 128k images on the PROM. Again this came with a bit of a challenge, as the pin on the PROM used to switch between the 2 banks was being connected to a 5v supply from the main ROM socket. I had to 'split' the header pin so that lower half on the pin would pick up power from the BBC mainboard (to power the GAL & the PROM), and the upper half of the pin connected an address line on the PROM to a jumper on the carrier board which allows selection of the two 128k images.

I now have a PALPROM that I can switch between the Mega 3 ROM and SpellMaster ROM. However, only one ROM can be active at any one time. It's not possible to have both visible at the same time. If you want both, then you would still need two modules.

If anyone's interested in getting hold of one of these, please let me know. I don't intend making a large batch, as they're quite fiddly to make, but I could certainly make a few. Note that the parts (SIL headers in particular) are quite expensive, so £15 for a module is probably where I would pitch it.

Here's a couple of photos of the completed module (this one is using Flash in place of a PROM)...
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Elminster
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Re: Computer Concept PALPROM redesign.

Post by Elminster » Tue May 14, 2019 12:26 pm

I thought I had caught up with the hardware yesterday, and now you release something else I need to add to the list :)

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Elminster
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Re: Computer Concept PALPROM redesign.

Post by Elminster » Tue May 14, 2019 8:17 pm

Both this and Integra-B remake added to hardware list. Keep it up :D

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