Acorn Electron cartridge board thickness

[paulb]

I've been looking into doing a prototyping/breakout board for the Acorn Electron Plus 1 cartridge slot, and I think I have most of the basic dimensions in order, thanks to Dave H's assistance with the cartridge case work, but the one thing that concerns me is the actual board thickness. The Acorn documentation (AppNote 014 "Acorn Electron Cartridge Interface Specification") specifies a "0.05 inch slot" but what I'm looking for is the actual thickness people have used when making their own cartridge boards.

I don't have precision measuring equipment here, but looking at Dave H's Advanced Dual ROM Adapter, it looks pretty close to 1.2mm which is what seems to be offered amongst the choices at various PCB manufacturers (0.5mm or 0.6mm, 0.8mm, 1mm, 1.2mm, 1.6mm, 2mm), with some being premium options. Some prototyping boards may be 1.5mm, apparently, which supposedly work with the Electron's cartridge slot but not with the Master's. Although I don't personally care about the Master, I'd aim to make this board compatible.

Does anyone have any experiences they want to share on the topic?

[CMcDougall]

The M128 cart for 2 roms to be fitted works also in Plus1, so if you have that at hand then measure that thickness.

[Prime]

The boards I have made in the past are standard 1.6mm thick and fit in both the Master and the Plus 1 slots, and a load of others on other retro machines, PC ISA, Dragon / CoCo cart port for example.

Cheers.

Phill.

[paulb]

The boards I have made in the past are standard 1.6mm thick and fit in both the Master and the Plus 1 slots, and a load of others on other retro machines, PC ISA, Dragon / CoCo cart port for example.

This is very useful to know, thanks! I may well get some made at that thickness from OSHPark and see how it goes.

I did stack the Advanced Dual ROM Adapter and Mega Games Cartridge and measure the combined thickness, and this gave a result of around 3mm, so my estimate of 1.2mm was probably more of a guess than anything else.

The aim is to make the files available for this, too. There has been talk of prototyping cartridges, but the people who are most interested are the least experienced, and the people with the most experience don't really have much of a need for that kind of thing.

[1024MAK]

There was some discussion of suitable PCB's when MartinB was developing his EUP & ROM/RAM board (link to the thread). I'm sure thickness was also talked about, but I can't remember if it was in this thread, or another one

If I can find by digital calapers at the weekend, I'll measure some Elk cartridge PCB's and post the results.

Mark

[paulb]

There was some discussion of suitable PCB's when MartinB was developing his EUP & ROM/RAM board (link to the thread). I'm sure thickness was also talked about, but I can't remember if it was in this thread, or another one

If I can find by digital calapers at the weekend, I'll measure some Elk cartridge PCB's and post the results.

Thanks for the offer! I'm inclined to just order some 1.6mm boards, anyway. I've skimmed the EUP thread a few times and I didn't notice any board thickness details, hence my query here. Forums just aren't the best way of efficiently archiving information like this, I guess.

This is the kind of thing I have in mind, by the way:

PCB 3D preview

I saw that the DRA and MGC boards have slightly tapered connectors and also shortened pads (A1, B1, I think) for polarity purposes. Any thoughts on either of those topics?

[1024MAK]

The +5V power connection has a shorter "pin" so it connects last if the user is being daft enough to plug it in with the power on...

Mark

[paulb]

The +5V power connection has a shorter "pin" so it connects last if the user is being daft enough to plug it in with the power on...

I didn't see this in the AppNote, but maybe it's in there somewhere. It looks like various designs shorten it by about 2mm, so I'll adjust the design appropriately. That was very useful to know, thanks!

[daveejhitchins]

Delayed response . . . 1.6mm is correct. Also double check the width of the section that fits into the connector. The measurement I used is listed in MartinBs thread.

Dave H

[paulb]

Delayed response . . . 1.6mm is correct. Also double check the width of the section that fits into the connector. The measurement I used is listed in MartinBs thread.

The thing I see with your boards is that they are "tapered" to an extent on the lower edge. In other words, the boards get slightly thinner at the very edge, maybe to fit more easily into the socket. I don't know the right terminology here, so searching for guidelines has proven to be difficult. I also don't know if this really matters or not.

I did find this post about measurements in that thread, and the matter of the width does come up prominently, but I started from your measurements provided for the cartridge cases with an edge connector width of 56.5mm, and I think that's what you actually use yourself.

Thanks for interrupting your holiday to reply to this topic!

Updated 3D PCB preview

[MartinB]

I've just done a spot of digital calipering on EUP and the board thickness is 1.5mm with a tongue width, including the epoxy shim I added (see below), of 57mm.

I always felt that without a case, the PCB could/should have been slightly thicker, say 1.8mm, to make it more stable - I generally use a couple of foam wedges to hold it securely upright.

[paulb]

I've just done a spot of digital calipering on EUP and the board thickness is 1.5mm with a tongue width, including the epoxy shim I added (see below), of 57mm.

I always felt that without a case, the PCB could/should have been slightly thicker, say 1.8mm, to make it more stable - I generally use a couple of foam wedges to hold it securely upright.

Thanks for taking the time to measure the board!

I'm inclined to go along with Dave H here and assume that the 0.4 - 0.8mm discrepancy (maybe 0.2 - 0.4mm at each end if divided equally) won't cause alignment problems. I guess that the worst case between Dave H's measured maximum socket size of 57.3mm and your 56mm was just enough to exceed the 1.27mm pad width or at least make things very marginal.

I don't anticipate making a case for this board, although I guess it might be possible to use the rear half of a case for stability if necessary.

[MartinB]

Oh yeah, definitely go with Dave's numbers - I'm certainly no expert in PCB design and procurement. Indeed, apart from one two very minor generic electronic projects, BeebSID and EUP are the only boards I've ever 'produced'....

[paulb]

Oh yeah, definitely go with Dave's numbers - I'm certainly no expert in PCB design and procurement. Indeed, apart from one two very minor generic electronic projects, BeebSID and EUP are the only boards I've ever 'produced'....

Well, they are far more sophisticated than this board, so it has been useful to hear what your experiences were. We'll see if there are any other comments for a while, but then I'll be placing an order and seeing what I end up with.

Thanks once again!

[MartinB]

btw Paul - are you selling any on after you get a few made? I'd take one....

Edit :

The aim is to make the files available for this, too.

Scratch that question - just noticed that you'd already said about eventually making the files available so anyone can get their own made of course...

[daveejhitchins]

Delay again ('tinternet is appalling, here - just have to suffer )

1.6mm is a standard thickness. An edge connecter should always be tapered - you can specify it, however, as some suppliers don't do this as a matter of course! It won't make that much difference! In reality.

Dave H

[paulb]

Delay again ('tinternet is appalling, here - just have to suffer )

1.6mm is a standard thickness. An edge connecter should always be tapered - you can specify it, however, as some suppliers don't do this as a matter of course! It won't make that much difference! In reality.

Thanks once again! I had a look to see if this was possible to specify in KiCad, but the wisdom on the Internet seemed to indicate that it might be more of an "ask your board house" kind of request. I think the term I found was "scoring", but that does overlap with people trying to get boards made that can be broken apart, so I'm not completely sure that my search was well-targeted.

[daveejhitchins]

There's two ways the board house do this: 1) as you've mentioned, score the board, on that edge or 2) run the edge through a machine to add the bevel.

And yes, you can just specify the detail on your dimensioning drawing.

Dave H

[paulb]

There's two ways the board house do this: 1) as you've mentioned, score the board, on that edge or 2) run the edge through a machine to add the bevel.

And yes, you can just specify the detail on your dimensioning drawing.

I've sent OSHPark a mail. Let's see if there is anything they might have to say about the matter. It might not be possible with the service I'll be using, of course.

[paulb]

So, OSHPark's representative told me that v-scoring, which is what would be needed to achieve the tapering, isn't supported on the service I want to use. There were some reasons related to accuracy and that it would be an additional step that would complicate the workflow.

I'm still inclined to get these boards made, especially if a lack of tapering isn't likely to make a real difference, so I'll probably place the order today and set the wheels in motion.

[MartinB]

I'm not aware that I had any tapering on EUP (hadn't even heard of it!) and tbh, given the nicely curved contacts in the receiving cartridge socket, I don't see any need for a board chamfer.

[paulb]

I'm not aware that I had any tapering on EUP (hadn't even heard of it!) and tbh, given the nicely curved contacts in the receiving cartridge socket, I don't see any need for a board chamfer.

I noticed that your EEPROM cartridge experiment used a cartridge that didn't seem to have a tapered edge connector. If that was good enough for Acorn, then I guess it'll have to do for me.

[paulb]

I've now updated the status of the prototyping cartridge in a new thread.