Wiring question regarding Acorn Electron ROMs

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raybeers
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Wiring question regarding Acorn Electron ROMs

Post by raybeers »

Hi all,

I just joined the forum today. Although I've been reading the threads on here for a while. Hello!

I'm working on a breadboard project, which is connected to the expansion slot of an Acorn Electron. My initial objective is simple, to load a parallel ROM plugged into the breadboard.

Reading the documentation (Advanced User Guide) I see that ROMs have (or occupy) locations or slots. Probably due to a lack of foundational knowledge, I've completely failed to comprehend how to tell the Electron what location/slot my parallel EEPROM on the breadboard occupies.

I know enough to know that only one ROM can be putting data on the data bus at a time. Presumably setting the CE connection on the ROM (active low) is how it does this. Perhaps some of you good people can tell me how I wire my ROM to occupy a given slot/location.

Thanks in advance.

Best regards
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jms2
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Re: Wiring question regarding Acorn Electron ROMs

Post by jms2 »

This is the brief version: You’re right that CE is the correct way to control access to the rom. To do this, you need to build a circuit (probably using logic gates) to perform address decoding. The address you need to detect is &FE05. When you identify that this address is on the address bus, you then need to check the data bus to see if your rom number has been selected. I guess at this point you should implement a latch which records the rom number.

Hopefully someone will be able to show you a typical circuit which does this (I’m on holiday so it’s tricky!)
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danielj
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Re: Wiring question regarding Acorn Electron ROMs

Post by danielj »

Welcome!
You could do worse than taking a gander at the plus one circuit diagram:
https://www.4corn.co.uk/archive/diagram ... iagram.jpg

That has an 8k ROM built in to it which is located at 12.

You can also look at that in conjunction with this: http://mdfs.net/Info/Comp/BBC/Circuits/Master/eprom.gif (an eprom cartridge, these take two ROMs), to see how the cartridge slots work. /OE2 and /OE4 select which slot, then the logic on the cartridge determines which ROM of the two you're interested in by (these are 0/1 or 2/3 depending on which slot you've plugged the cartridge into), this is done using the connection through A16 which, if you have a look here:
http://chrisacorns.computinghistory.org ... AN/014.pdf
is ROMQA (which is the least significant bit of whatever's been latched at &FE05), hence it lets you detect between two adjacent ROMs :)

Sorry, that's a bit rushed, but I hope it gives you something to be getting on with? Someone more verbose than me is bound to be along shortly :D

d.
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davidb
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Re: Wiring question regarding Acorn Electron ROMs

Post by davidb »

There have been a few projects recently that do something like this. Take a look at Elminster's Modern Retro Hardware List for the BBC Micro Master and Acorn Electron for the links.

These two might be useful because they should have schematics available: However, they do rather more than just expose a single ROM over the bus.
Ramtop
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Re: Wiring question regarding Acorn Electron ROMs

Post by Ramtop »

Welcome Ray!

Not so long ago I was doing the same thing you are. I won't claim to know the best approach, but this is the board I knocked up to do the job: viewtopic.php?f=3&t=12737&start=30#p220210

I used a small CPLD that watches for writes to the ROM selection register at $FE05 and pulls CE low when the correct bank is selected, then monitors for any reads in the sideways ROM area ($8000-$BFFF) and pulls OE low when it sees one.

From my own trials I'd say you need two things to successfully interface a ROM to the Elk; an understanding of the 6502's read cycle, and a solid grasp of how the sideways ROM system works - Acorn put out a document called the 'Electron Expansion Application Note' which does a good job of explaining it. I've attached a copy for your enjoyment :)
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Gary
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1024MAK
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Re: Wiring question regarding Acorn Electron ROMs

Post by 1024MAK »

The system of adding extra ROMs is similar to the system used in the BBC B Micro. Hence it is well worthwhile having a read of this web page :wink:

Obviously the hardware is rather different. And in an Elk system, a different address is used for the latch/register. And due to the way the ULA works, the code for selecting a sideways ROM is slightly more complex. But the principles are the same for both the BBC B and the Elk.

Mark
Last edited by 1024MAK on Fri Aug 09, 2019 1:06 pm, edited 1 time in total.
raybeers
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Re: Wiring question regarding Acorn Electron ROMs

Post by raybeers »

Wow. Fantastic response. Thanks all. :D

There's a lot of stuff to look through. So I'd better get reading. I'm sure there'll be a lot of inspiration there.

I'll let you know how I get on. Although I'll be out and about for a few days now so it will be a while before I can come back to this.

Thanks again. This seems like a great community.
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