New project: An ACT Apricot F1 "barn find"

Blogs & guides and tales of woo by forum members.
stephen_usher
Posts: 2051
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by stephen_usher » Wed Sep 01, 2021 7:47 pm

Well, I'm getting somewhere with the RAM expansion.

I made my own footprint for the DIN socket, which will be to provide a ZX Spectrum+ 128K compatible RGBI output on the back of the machine.

Here's the first render from KiCAD. I've got both pads for directly soldering the surface mount SRAM on the board and a socket to put the daughter board into. These boards have already been built and one is going to have the component soldered on-board by PCBWay so I don't have to.

Apricot-Expansion-RAM.jpg
Apricot-Expansion-RAM.jpg (211.87 KiB) Viewed 276 times
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

stephen_usher
Posts: 2051
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by stephen_usher » Wed Sep 01, 2021 8:31 pm

The paper mock-up looks to fit correctly at least.
IMG_2364.jpg
IMG_2364.jpg (53.08 KiB) Viewed 271 times
IMG_2365.jpg
IMG_2365.jpg (56.36 KiB) Viewed 271 times
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

stephen_usher
Posts: 2051
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by stephen_usher » Wed Sep 01, 2021 9:24 pm

Bother! Just realised that I have a logic error in my bus control logic. The RAM won't work and I risk halting the whole machine in another state.

It's a good thing I'm checking, double checking and triple checking.

I need to check to see if the GALs pull the lines high or let them drift. I know that I can tri-state outputs, but that needs a control line and you can't use OR operations in the equation. Could be tricky.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

stephen_usher
Posts: 2051
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by stephen_usher » Thu Sep 02, 2021 10:44 pm

OK, it looks like my whole approach is flawed, so I'm going to have to backtrack and not have the SRAM chip accessible from the expansion card.

This simplifies things a great deal. I can still have the address latching on this board and pass the valid address to the daughter board but all the logic and memory for any expansions will have to be on there. This will cut the chip count by three (a GAL, an LS04 and an LS125) and I'll only ever use a quarter of the memory. C'est la vie.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

stephen_usher
Posts: 2051
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by stephen_usher » Sat Sep 04, 2021 1:02 pm

Well, I think that I've got the simplified version of the board completed.

Daughter boards can use the address latching and transceivers on this board but otherwise everything else is being passed through to the expansion connector. Any expansions will have to provide their own memory, address decode logic etc. but at least some functionality won't need to be duplicated.

board-schematics.JPG
board-schematics.JPG (284.1 KiB) Viewed 220 times

After checking a cut-out of the board in the Apricot F2 I discovered that I needed to change the dimensions of the board slightly and add a couple of cut-outs. I've also moved the surface mount SRAM pads to the back of the board if for nothing else to get rid of the warnings when I do a check.

Apricot-Expansion-RAM.jpg
Apricot-Expansion-RAM.jpg (181.15 KiB) Viewed 220 times
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

stephen_usher
Posts: 2051
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by stephen_usher » Sat Sep 04, 2021 8:28 pm

I had to put the 74LS04 back on the board as I'd forgotten about a couple of lines it was inverting for me. I used the opportunity to make sure that the /SELECTED line was not being actively pulled high, as a GAL will want to do, and hence push it through the 74LS04 as well. The /SELECTED line needs to be able to be pulled low by the board on the expansion connector.

During all this checking and re-checking I also discovered that I'd forgotten to replace a couple of the signals on the expansion connector, so things would not have worked well (OK, at all!).

Anyway, I think that I've now debugged the board and I'm running Freerouting. I've decided that as the main motherboard seems to be a two layer board I'll try this as a two layer board, as it will be FAR cheaper. It's a bit of a risk but the machine isn't running quickly. The CPU is running at 4MHz but the memory bus is really running at a quarter of this given the multiplexing and the enforced one wait state on every memory or I/O access.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

stephen_usher
Posts: 2051
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by stephen_usher » Sun Sep 12, 2021 4:24 pm

Well, the boards are due to be delivered tomorrow so I've been checking my stocks and I have enough of everything to build more than one up.

The pre-made SRAM daughter board is still waiting to ship from PCBWay and has been since last Wednesday. I don't know what the hold up is as they finished soldering the SRAM chip and capacitor on that board on Tuesday. Still, it just means that I'll have to try soldering a surface mount SRAM chip on the main board directly.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

User avatar
rubber_jonnie
Moderator
Moderator
Posts: 4450
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by rubber_jonnie » Sun Sep 12, 2021 4:56 pm

Sounds like you're getting on great with this!

Will it be visiting the Centre for Computing History?

I'd love to come and see it but the 9th/10th October don't work for me :(
Collector of old Atari things:
800XL + Ultimate 1MB, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k +APE Warp) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg), 1040STF long button floppy, 2x 1040 STFMs + Gotek, 2x 4160STE with 32Mhz booster, ROM switcher, Mega ST1, Mega ST4, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

stephen_usher
Posts: 2051
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by stephen_usher » Sun Sep 12, 2021 5:31 pm

rubber_jonnie wrote:
Sun Sep 12, 2021 4:56 pm
Sounds like you're getting on great with this!

Will it be visiting the Centre for Computing History?

I'd love to come and see it but the 9th/10th October don't work for me :(
Yes, I'm planning this on my Saturday set up. We'll see if the board actually works. :-)
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

User avatar
rubber_jonnie
Moderator
Moderator
Posts: 4450
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex
Contact:

Re: New project: An ACT Apricot F1 "barn find"

Post by rubber_jonnie » Sun Sep 12, 2021 6:32 pm

stephen_usher wrote:
Sun Sep 12, 2021 5:31 pm
rubber_jonnie wrote:
Sun Sep 12, 2021 4:56 pm
Sounds like you're getting on great with this!

Will it be visiting the Centre for Computing History?

I'd love to come and see it but the 9th/10th October don't work for me :(
Yes, I'm planning this on my Saturday set up. We'll see if the board actually works. :-)
Hmm, talking to my other half, might be able to make the 9th work :)
Collector of old Atari things:
800XL + Ultimate 1MB, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k +APE Warp) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg), 1040STF long button floppy, 2x 1040 STFMs + Gotek, 2x 4160STE with 32Mhz booster, ROM switcher, Mega ST1, Mega ST4, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

Post Reply

Return to “MEMBER BLOGS”