A3000-to-A5000 ROM adapter

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myelin
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A3000-to-A5000 ROM adapter

Post by myelin » Mon Jan 07, 2019 6:35 am

Moving this from the in-system programmable ROM ("arcflash") project thread...

URL for this project: http://myelin.nz/acorn/a5k3krom

Here's a board that can be used as A5000-to-A3000 ROM adapter or an A3000-to-A5000 ROM adapter. I'm making it so people can use my "arcflash" boards (designed for A3000/A3xx/A4xx/A5xx systems) on A5000s too, but Joka80 is interested in the opposite (adapting an A5000 memory expansion board to work on an A3000), and I don't see why that wouldn't work too.

Here's the bottom (the A5000 side), with no space between the ROMs:
Screenshot 2019-01-06 18.30.01.png
The extra space on the board should overhang an empty part of the A5000 motherboard near the edge.

And here's the top (the A3000 side), with a 0.1" gap between each pair of chips:
Screenshot 2019-01-06 18.29.53.png
The extra space on the board here should overhang to the north; there's a tantalum capacitor (C22) up there, but I think there should be 0.2" of room to move before hitting it.

For the "normal" case (running an arcflash board in an A5000), I would solder four 32-pin chip headers on the A5KROM* spaces on the bottom and four 32-pin sockets on the A3KROM* spaces on top. For Joka80's case, I would do the same, except solder to the "wrong" footprints. (i.e. the headers on the bottom go underneath the A3KROM* footprints, and the sockets go on top, in the A5KROM* footprints.)
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Re: A3000-to-A5000 ROM adapter

Post by myelin » Mon Jan 07, 2019 8:30 pm

Here it is: https://github.com/google/myelin-acorn- ... dapter_pcb

All wired up; I need to verify it a bit more but I think this should work. I added a pin for A21 to easily connect it up to the flash board.
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Re: A3000-to-A5000 ROM adapter

Post by RobC » Wed Jan 09, 2019 4:27 pm

This is great =D> =D> :D

I have several spare A5000 8MB upgrades and I'd love to fit one to my A3000.

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Re: A3000-to-A5000 ROM adapter

Post by JonC » Wed Jan 09, 2019 5:19 pm

RobC wrote:
Wed Jan 09, 2019 4:27 pm
This is great =D> =D> :D

I have several spare A5000 8MB upgrades and I'd love to fit one to my A3000.
Umm, wouldn't that require an extra MEMC1a? or did I miss something.
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Re: A3000-to-A5000 ROM adapter

Post by steve3000 » Wed Jan 09, 2019 5:54 pm

Great work!

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Re: A3000-to-A5000 ROM adapter

Post by myelin » Wed Jan 09, 2019 5:55 pm

JonC wrote:
Wed Jan 09, 2019 5:19 pm
Umm, wouldn't that require an extra MEMC1a? or did I miss something.
Yeah, it’s not an easy upgrade! You’ll need to desolder your MEMC1A and solder on a socket, and you’ll need a complete 8MB A5000 upgrade with its second MEMC1A chip on board. Might need a longer ribbon cable too as it looked a little short in the pics Joka80 posted in the other thread.

Anyone who wants one of these boards, please PM or post here, because I’m about to do a parts order for the other project and I can get the bits for this at the same time! Looks like it’ll cost about US$20+shipping, so US$34 if you’re in the UK and a bit less if you’re in the USA. (The pin headers to attach to the ROM sockets are quite pricey.)
Last edited by myelin on Wed Jan 09, 2019 5:56 pm, edited 1 time in total.
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Re: A3000-to-A5000 ROM adapter

Post by RobC » Wed Jan 09, 2019 6:39 pm

JonC wrote:
Wed Jan 09, 2019 5:19 pm
Umm, wouldn't that require an extra MEMC1a? or did I miss something.
Yes - but the A5000 8MB upgrades I mentioned have dual MEMC boards :)

Still need to desolder the original MEMC1A from the A3000 PCB and fit a socket but I managed to remove the ARM2 and add an ARM3 upgrade.

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Re: A3000-to-A5000 ROM adapter

Post by steve3000 » Wed Jan 09, 2019 8:44 pm

RobC wrote:
Wed Jan 09, 2019 6:39 pm
Still need to desolder the original MEMC1A from the A3000 PCB and fit a socket but I managed to remove the ARM2 and add an ARM3 upgrade.
Also need to consider the clearance - you'll need to fit the A5000 expansion under the A3000 keyboard, and it'll have to avoid other components. Might just be possible, but looks tight. Alternatively could save a couple of mm by removing the A3000 ROM sockets and soldering the conversion board directly to A3000 motherboard... but that's a lot more work.

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Re: A3000-to-A5000 ROM adapter

Post by RobC » Thu Jan 10, 2019 9:08 pm

steve3000 wrote:
Wed Jan 09, 2019 8:44 pm
Also need to consider the clearance - you'll need to fit the A5000 expansion under the A3000 keyboard, and it'll have to avoid other components. Might just be possible, but looks tight. Alternatively could save a couple of mm by removing the A3000 ROM sockets and soldering the conversion board directly to A3000 motherboard... but that's a lot more work.
Ah - hadn't thought of that Steve - thanks for pointing it out.

Removing the ROM sockets is probably the way to go. It isn't too much of a hassle as I've replaced a few with battery damage in the past. Probably need an ultra-thin version of the conversion board too.

On a side note, did anyone ever produce an 8MB upgrade for the A3000? I can remember ARM3 upgrades being advertised but can't definitely recall anyone offering an 8MB upgrade.

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Re: A3000-to-A5000 ROM adapter

Post by steve3000 » Thu Jan 10, 2019 11:24 pm

Well, looking at an A3000 last night, there is actually more height clearance under the keyboard than I thought there was.

So I think you could get enough height, either but directly mounting the converter pcb to the motherboard or the 4-8mb ROM card to the converter.

However I'm now thinking it's the surrounding components and connectors on the motherboard which look more problematic to deal with - such as the keyboard connectors and battery to the left of the ROMS and possibly the 4mb RAM expansion to the right. Battery can be dealt with, but keyboard connectors are more problematic to move... (although I do have a set of spare connectors, so not impossible to relocate these if really necessary!).

There certainly was an 8mb expansion for the A3000, at least one. Unfortunately I can't find the link to the photo now, but it had a twin MEMC board with IDC cable to a custom RAM board sitting in the usual RAM upgrade slot. There was no sign of a ROM carrier board, so presumably you could get all the signals needed for the twin MEMCs from a wired connection between these two parts (unless there was additional wiring to the other side of the PCB...) Will see if I can find that photo.

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Re: A3000-to-A5000 ROM adapter

Post by myelin » Thu Jan 10, 2019 11:56 pm

steve3000 wrote:
Thu Jan 10, 2019 11:24 pm
There certainly was an 8mb expansion for the A3000, at least one. Unfortunately I can't find the link to the photo now, but it had a twin MEMC board with IDC cable to a custom RAM board sitting in the usual RAM upgrade slot. There was no sign of a ROM carrier board, so presumably you could get all the signals needed for the twin MEMCs from a wired connection between these two parts (unless there was additional wiring to the other side of the PCB...) Will see if I can find that photo.
That makes sense... along with the MEMC signals (Ras*, Cas*, Ra*), the RAM upgrade slot connects to the RAM data bus (Rd), which is connected to the ARM data bus (D) via a bunch of series resistors. I believe both upgrades just need the data bus, as they can get the ARM address bus from the MEMC socket.
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Re: A3000-to-A5000 ROM adapter

Post by RobC » Fri Jan 11, 2019 10:59 am

steve3000 wrote:
Thu Jan 10, 2019 11:24 pm
Well, looking at an A3000 last night, there is actually more height clearance under the keyboard than I thought there was.
Thanks for looking.
steve3000 wrote:
Thu Jan 10, 2019 11:24 pm
There certainly was an 8mb expansion for the A3000, at least one.
I thought I'd seen one advertised but wasn't sure. Might dig out some old AUs to see if I can find it.

I'll get my A3000 out as some point over the weekend and will see how much space the A5000 8MB boards need. Hopefully, at least one of them will fit!
steve3000 wrote:
Thu Jan 10, 2019 11:24 pm
Unfortunately I can't find the link to the photo now, but it had a twin MEMC board with IDC cable to a custom RAM board sitting in the usual RAM upgrade slot. There was no sign of a ROM carrier board...
myelin wrote:
Thu Jan 10, 2019 11:56 pm
I believe both upgrades just need the data bus, as they can get the ARM address bus from the MEMC socket.
Yes - the MEMC only deals with addresses so these RAM upgrades also need a databus connection. Guess it makes sense to take that off the ROM sockets in an A5000 and off the memory expansion in an A3000.

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Re: A3000-to-A5000 ROM adapter

Post by steve3000 » Fri Jan 11, 2019 12:48 pm

Makes sense...so there should only be a handful of signals needed between the 8mb RAM and twin MEMC board?

In which case, if you could replace the 4mb RAM in the A3000's 4mb upgrade with 8mb, then wire across the necessary signals (including the extra address line) to a twin MEMC board, you could avoid having the ROM board altogether?

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Re: A3000-to-A5000 ROM adapter

Post by RobC » Fri Jan 11, 2019 4:08 pm

steve3000 wrote:
Fri Jan 11, 2019 12:48 pm
In which case, if you could replace the 4mb RAM in the A3000's 4mb upgrade with 8mb, then wire across the necessary signals (including the extra address line) to a twin MEMC board, you could avoid having the ROM board altogether?
That's worth thinking about. I believe that the 4MB upgrade in my A3000 might well be the SIMM type so I could possibly put in an 8MB SIMM and just wire across the extra address line :idea: =D>

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Re: A3000-to-A5000 ROM adapter

Post by steve3000 » Fri Jan 11, 2019 4:17 pm

RobC wrote:
Fri Jan 11, 2019 4:08 pm
I believe that the 4MB upgrade in my A3000 might well be the SIMM type so I could possibly put in an 8MB SIMM and just wire across the extra address line :idea: =D>
I'm thinking of trying exactly the same, once can work out the required signals needed to patch across. Will keep you posted.

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Re: A3000-to-A5000 ROM adapter

Post by IanS » Fri Jan 11, 2019 4:37 pm

steve3000 wrote:
Fri Jan 11, 2019 4:17 pm
RobC wrote:
Fri Jan 11, 2019 4:08 pm
I believe that the 4MB upgrade in my A3000 might well be the SIMM type so I could possibly put in an 8MB SIMM and just wire across the extra address line :idea: =D>
I'm thinking of trying exactly the same, once can work out the required signals needed to patch across. Will keep you posted.
Would that work, I always assumed the RAM side of each MEMC would need to be talking to an independent set of RAM, the CPU side of the MEMC is largely paralleled up. Seem odd if you can link the "Ra<x>" signals from multiple MEMC's

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Re: A3000-to-A5000 ROM adapter

Post by RobC » Fri Jan 11, 2019 10:06 pm

I think the two halves of an 8MB SIMM have separate RAS signals and I think the address lines from the CPU effectively act as enable lines to the MEMCs but I'm not sure how the DRAM refresh works.

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Re: A3000-to-A5000 ROM adapter

Post by steve3000 » Fri Jan 11, 2019 11:01 pm

RobC wrote:
Fri Jan 11, 2019 10:06 pm
I think the two halves of an 8MB SIMM have separate RAS signals
Yes, that's what I understood was the case for 8mb SIMMs as they are dual-ranked...but I will be reading up a bit more thoroughly on the subject before making any plans :)

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Re: A3000-to-A5000 ROM adapter

Post by myelin » Sat Jan 12, 2019 8:36 pm

I'm pretty sure the two MEMCS will both expect to be driving the RA* lines all the time, and they'll most likely be on different refresh schedules too, so I doubt you'll be able to put them in parallel on a single 8MB SIMM.

The holy grail here (a project I'd love to take on sometime) would be a new FPGA-based MEMC, with a 512 entry CAM table and the ability to connect to 16MB of RAM. The most annoying part of this is that 68-pin PLCC plugs are expensive and hard to find, so it would be a very pricey upgrade.
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