Elk Cartridge EEPROM

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MartinB
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Elk Cartridge EEPROM

Post by MartinB » Sun Sep 28, 2014 11:02 pm

Daniel wrote: :D - so I asked about rom cartridge to see if we could work out fairly quickly how to adapt it to take eeproms, which you could reprogram within the machine. I don't have one of the slogger cartridges though to check the circuit. Does anyone else?
I've put this into a new thread coz it didn't seem to fit where it was spawned under "EPROM Programmer Advice Requested" 8)


As you know (?), my EUP board has a 32k, multi-chip, 28-pin socket which is independent of the User Port functionality and, if you followed that thread in any detail, you'll know it schematically boiled down to this...
EUP(2a).png
Now, I know others have implemented 62556 and similar static ram chips by commoning nOE and nCE but strictly (IMHO), it isn't correct to have the chip output enabled (nOE low) when writing, i.e. when RnW is low. Indeed, I know already know that an eeprom can't be used in this way (hence the LS04 on EUP) but, as a stepping stone, initially getting a 62256 working in the simplest configuration possible seemed sensible so I decided to just go with the flow :D

So, with the latter sentiment in mind and largely copying my EUP arrangement, I first removed the 139 from the cart PCB as it adds no value when we're going to use a single 32k chip. That done, it was simply a case of adding a couple of patch links as required to route the required cart edge connector signals to the 62256 and the vacant 139 pads readily lent themselves to this task as can be seen in the photo. The only snag was that the cart ties pin 1 of both sockets high (Vpp on the designed-for 27128 chips) by hard-tracking the socket pin 1 positions to the +5v rail and it's been done so comprehensively on this particular cart PCB design that it was just impossible to isolate socket pin 1 leaving me no other choice than to lift pin 1 of the 62556 and add an external wire to pick up ROMQA.

Anyway, that done, I gave it a quick SWR test and everything worked fine although I didn't linger looking for signs of the corruption issue since I knew was proceeding straight to a 28C256 fit. The picture below shows this simple intermediate stage but note that I'd already swapped the 62256 for a 28C256 before I decided that a photo would be useful...
Step 1 - SWR.JPG
The next stage then was to add the 28C256, confirm it didn't work with nOE and nCE commoned and then add an LS04 as per EUP. This of course required a double-sided tape dead-insect arrangement but at least there's plenty of room for another chip in these carts. Finally, I added switches for write-protect and output-disable and the results can be seen in the photo and video below...
Step 2.JPG



So, changes made, the new eeprom-based cart works absolutely fine and when buttoned up it'll be just as neat as any other cart. It's clearly not quite in the category of a 'straightforward' mod but as far as I can see, there isn't any lesser way of implementing an eeprom device. The precise changes required will vary between cartridge types but the minimal target schematic will always be as per the EUP design above. Since I haven't cut any tracks, the changes can easily be reversed but, apart from 'originality', why would you want to revert back? It still does everything a twin-rom cart can do but is infinitely more flexible.

I've just noticed that Daniel asked the original question in the context of Slogger carts and it's possible that they can be adapted more easily but since I don't have any of the 'new batch', that's maybe a follow-on question for Dave H then... :wink:
Last edited by MartinB on Mon Jul 09, 2018 8:18 pm, edited 1 time in total.

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daveejhitchins
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Re: Elk Cartridge EEPROM

Post by daveejhitchins » Mon Sep 29, 2014 7:16 am

I don't, knowingly, have a slogger ROM cartridge. I will check-out my own ARA and sort out the necessary mods for that, along with ABR. I was worried that I may not have R/nW available, however, a quick look says I do :D

I say I don't have a Slogger Cartridge knowingly . . . I have a box of Slogger PCBs from Dave (Arcadian) . . . There may be some in there - Tuesday evening for that, along with the EpromRyter 'stuff'.

Of course, if anyone has a Slogger Cart. they could loan me . . . ?

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ARA III, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com

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1024MAK
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Re: Elk Cartridge EEPROM

Post by 1024MAK » Mon Sep 29, 2014 8:53 am

How many versions / types or makes of "plug in your own ROM chips" cartridges are there?

If Acorn published the details fairly early on, did everybody else just copy the electrical design?

If the cartridge that you have uses a 74LS139, it is likely that modifications similar to that described above by Martin will be needed.

Mark

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Re: Elk Cartridge EEPROM

Post by danielj » Mon Sep 29, 2014 9:10 am

=D>

So I asked in that context as that's what jjthacker had, and it seemed like it might be a neater and more flexible solution for him than going down the eprom programmer route. I believe he was using the cartridges with the M128 - isn't there something funky with the cartridge slots there and the R/W line, or should this work happily with everything?

:)

d.

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1024MAK
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Re: Elk Cartridge EEPROM

Post by 1024MAK » Mon Sep 29, 2014 9:31 am

There is indeed a difference between the Election cartridge interface and the Master 128 cartridge interface. The R/W line is different for some reason.

But I forget the details :(

I think a different modification will be needed for a cartridge that is going to be used in a Master.

Mark

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Re: Elk Cartridge EEPROM

Post by MartinB » Mon Sep 29, 2014 9:40 am

Was typing whilst Mark replied so apologies if repeating any answer...

For the Master, you'd still aim at the same final schematic but from memory (I'm at work), the only significant difference is that RnW is on a different edge connector number than the Elk. I'd have to see if the the relevant two pins are a 'not connected' on the opposing machine and if so, you could common the connection and have a dual-use cart but if not, it would either be a case of bespoke Elk and Master versions or possibly some more logic to OR the two possible sources of RnW. The only other item to check is that the Master equivalent of Elk ROMQA, the LSB of the slot pair decode, is on the same edge connector pin in a Master - if not, more of the above blah :wink:

Mark K - I think the logic chip is often an HCT139 rather than an LS. I've seen both used in Acorn carts but I think the HCT was used for the heavily CMOS Master.

EDIT : Just checked, RnW is A4 on the Elk and A11 on the Master and they both have alternative functions for their opposites so they couldn't simply be commoned to give a dual-compatible eeprom cartridge. ROMQA however, is the same pin (A16) and function on both. In summary then, virtually identical modifications for both apart from one connection which could I suppose be managed by a third (changeover) switch but I think I'll just make one of each.

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