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EPROM and extender board


Van
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Hi all

So in the process of a larger project, I routed a EPROM Pak with a 40 pin connector for external circuits. Much credit and insperation goes to NF6X's EPROM Pak, https://github.com/NF6X/CoCoEPROMpak.

I ended up starting from scratch on the board due to the extra space needed for the 40 pin header. I dropped the connector for 26 pin ROMs, 27256 devices are easier to find for me.

This EPROM/Ext board hosts a 28 pin ROM and provides jumpers to allow stand alone, manual bank switching or can be controlled externally. With a 27512 this would allow 4 16K images.

The external header is laid out almost one to one with the card edge, but the two 12V and one of the GND pins (pins 1 & 2 and pin 34) are re-purposed for ROM banking control.

The 40 pin header is un-buffered so take care when handling. Due to the limits of Eagle free, the buffer circuits will have to be a separate board to either be mated to the back of the EPROM/Ext Pak or connected via a short ribbon cable. I've yet to route this board so it's WIP for now.

Here is the Schema; The first boards are at fab; after I get the boards and verify them I'll open the files on OSH Park. The cost runs <$40 for a run of 3. So if you just want an EPROM pak use NF6X's design it's on OSH and cheaper :)

Yogi

 

 

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  • 4 weeks later...

Well, got the first board run back, yea!

During continuity testing found a little weirdness. Some of my GND fills are not contiguous. Bad when the EPROM's GND is isolated :( Now the weird thing is I can't find an un-routed ratline for the GND net. Weird.

Simple fix on these boards, but will re-route the design a bit to correct this. Also some minor changes to the parts selection and placement.

Yogi

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