Open source A5000 RAM

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cmorley
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Open source A5000 RAM

Post by cmorley » Tue Sep 03, 2019 5:31 pm

As per the A3020 RAM I've drawn an open source A5000 RAM module. Again Zarchos is funding the prototypes then I will release all the files once it is working.

There is a small blunder with the prototype PCB - it turns out that 2x 0.1" isn't 0.3"... who knew? I can get the connector on well enough to prove them electrically.
Photo0792.jpg
Since the A5000 uses a separate MEMC like the A3000 does I put provision in for the buffers I had to add to the A3000 RAM boards to cover over the poor signal integrity on the A3000. This might or might not be needed on the A5000 so they are optional on the prototype.

Once tested assuming it works I will draw the release PCB with the correct connector footprint and buffers/no buffers as required.

I don't have an A5000... so a volunteer for testing would be appreciated. Someone who can perform IanS' file copy test.

It'd be nice to get some testing in soon so this and the A3020 RAM can be demoed at the next ABUG.

Chris

steve3000
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Re: Open source A5000 RAM

Post by steve3000 » Tue Sep 03, 2019 6:04 pm

Oh, exciting!

I'd happy to volunteer and have an 8mb A5000 so can test with/without the 8mb upgrade fitted.

I also have an old 'prototype' 1mb A5000 in the loft, but suspect your design requires 2mb on-board...or does it replace the on-board RAM completely?

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IanS
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Re: Open source A5000 RAM

Post by IanS » Tue Sep 03, 2019 6:07 pm

cmorley wrote:
Tue Sep 03, 2019 5:31 pm
I don't have an A5000... so a volunteer for testing would be appreciated. Someone who can perform IanS' file copy test.
Nor do I, or I'd be happy to help. (I don't own an A3020 either)

Is this and the A3020 boards a full 4MB, completely disabling the on-board RAM?
Last edited by IanS on Tue Sep 03, 2019 6:42 pm, edited 1 time in total.

cmorley
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Re: Open source A5000 RAM

Post by cmorley » Tue Sep 03, 2019 6:39 pm

steve3000 wrote:
Tue Sep 03, 2019 6:04 pm
Oh, exciting!

I'd happy to volunteer and have an 8mb A5000 so can test with/without the 8mb upgrade fitted.

I also have an old 'prototype' 1mb A5000 in the loft, but suspect your design requires 2mb on-board...or does it replace the on-board RAM completely?
Splendid thank you. It has 4MB on card and you disable the motherboard RAM with LK23 & LK24 so it is supposed to work with a 1 or 2MB A5000.

I will PM you and sort out getting the cards to you.

cmorley
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Re: Open source A5000 RAM

Post by cmorley » Fri Sep 06, 2019 7:26 am

One board built up for now. No buffers fitted. If they aren't required I will delete them from the PCB when I fix the footprint of the DIN connector.
Photo0794.jpg
Photo0795.jpg

Kazzie
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Re: Open source A5000 RAM

Post by Kazzie » Fri Sep 06, 2019 3:23 pm

I may be having a dumb afternoon, but I can't see where the buffers would have been placed, based on the tracks seen on the boards. :?
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myelin
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Re: Open source A5000 RAM

Post by myelin » Fri Sep 06, 2019 5:23 pm

Zoom in on the photo of the back of the board above... the buffers go on those 14 SOT-23-5 footprints (which have solder blobs bridging the input and output right now). Plus one for Ra9 on the front of the board, by the looks of things.
Last edited by myelin on Fri Sep 06, 2019 5:24 pm, edited 1 time in total.
SW/EE from New Zealand, now in Mountain View, CA, making BBC/Electron hardware projects for fun.
Most interesting: Arcflash, FX2+PiTubeDirect Tube/Cartridge adapter, USB cart interface.

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Re: Open source A5000 RAM

Post by Kazzie » Fri Sep 06, 2019 7:00 pm

myelin wrote:
Fri Sep 06, 2019 5:23 pm
Zoom in on the photo of the back of the board above... the buffers go on those 14 SOT-23-5 footprints (which have solder blobs bridging the input and output right now). Plus one for Ra9 on the front of the board, by the looks of things.
Ah, thanks. I hadn't zoomed it, and presumed that they were surface-mounted passives.

I'm still soldering things at 1970s scale, when I get the choice! :P
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

cmorley
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Re: Open source A5000 RAM

Post by cmorley » Tue Sep 17, 2019 5:53 pm

Testing by Steve has shown it doesn't work without buffers. I am not surprised because the A5000 has a discrete MEMC just like the A3000 (which didn't work either). So solder the buffers on next for a retest on the spaces I put on the the board for this very eventuality!

I had hoped the A3000 problems were PCB layout but I think they are MEMC. I suspect it doesn't actually have enough ground pins. You get away with it with slow RAM from the 90s but not with 2018 tech. I wonder if Acorn knew about this BITD? (rhetorical)

cmorley
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Re: Open source A5000 RAM

Post by cmorley » Thu Sep 19, 2019 11:01 am

Steve sent the boards back so I soldered the buffers on this morning (which arrived from RS yesterday). Flipping things are twice the price they were last year - but amusingly buying 150 is cheaper than buying 75 :lol:

I'll bring it to ABUG at the weekend then post it to steve3000 again for further testing. This fix worked on the A3000 so I am hopeful it will fix the problems.
Photo0802.jpg
Last edited by cmorley on Thu Sep 19, 2019 11:03 am, edited 1 time in total.

cmorley
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Re: Open source A5000 RAM

Post by cmorley » Sat Sep 21, 2019 5:56 pm

Testing at ABUG today the RAM with buffers on works flawlessly so far. I will post it to steve3000 next week to see if it is fixed in his machine too. If so I will tidy up the PCB layout and release the Gerbers :D

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Re: Open source A5000 RAM

Post by cmorley » Tue Sep 24, 2019 8:32 pm

I redrew the board today to fix the connector pitch :oops:. I had to move all the buffers because they didn't fit where I'd put them. I've moved them all onto the component side leaving an almost intact ground plane 8)

Top:
a5000_4mbfront.png
Bottom:
a5000_4mbback.png
Assuming the buffers fix everything for steve3000 I'll order up some of these boards. Might jig the silkscreen around a little but otherwise I think we have success :D

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Re: Open source A5000 RAM

Post by dp11 » Tue Sep 24, 2019 8:35 pm

might as well put all the smt on one side. Looks good.

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Re: Open source A5000 RAM

Post by dp11 » Tue Sep 24, 2019 8:40 pm

I'd also add a little decoupling for the buffers easy not to fit if it isn't required.

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