"Minus One" - 3 x cartridge Electron expansion

[myelin]

UPDATE: Circuit, VHDL, and latest docs are on GitHub.

I've been looking forward to these PCBs showing up

I haven't had any luck getting my Elk to work with its Plus 1 attached, and haven't had any luck finding another affordable one on eBay, so I went ahead and designed this thing. It's basically just the cartridge circuitry of the Plus 1, except with a third cartridge socket, and a CPLD instead of the discrete logic.

I'm not particularly proud of the wiring; despite spending ages trying to get all the signals cleanly out of the expansion connector and to the cartridge slots, what I ended up with was kinda messy and loopy, but with any luck it'll work.

Also pictured: a couple more variants on my 32kB flash cart.

[paulb]

I've been looking forward to these PCBs showing up

I haven't had any luck getting my Elk to work with its Plus 1 attached, and haven't had any luck finding another affordable one on eBay, so I went ahead and designed this thing. It's basically just the cartridge circuitry of the Plus 1, except with a third cartridge socket, and a CPLD instead of the discrete logic.

I'm not particularly proud of the wiring; despite spending ages trying to get all the signals cleanly out of the expansion connector and to the cartridge slots, what I ended up with was kinda messy and loopy, but with any luck it'll work.

Also pictured: a couple more variants on my 32kB flash cart.

Very nice to see new circuit boards! I hope they meet your expectations.

[roland]

Why are your boards copyright by Google Inc?

[ThomasHarte]

Why are your boards copyright by Google Inc?

This might help to answer that.

[myelin]

Why are your boards copyright by Google Inc?

This might help to answer that.

Exactly Pretty much every employment contract in California gives the employer rights to anything you do on company time, using company equipment, or (and this is the kicker) relating to the company's business... and Google has business in just about every conceivable field. If you want to do a personal project while employed at Google, there's a process to get the company to formally disclaim any interest in the project, but if you're releasing it as open source anyway, it's easier to just release it as an unofficial Google project, which means that browsing Stardot at work, soldering up boards using the EE lab at the office, etc, are all fine. I'd rather be able to use my company laptop and occasionally spend company time on this than have my own name on the copyright.

[paulb]

Why are your boards copyright by Google Inc?

This might help to answer that.

Don't believe anything they say: Google are looking to corner the Electron peripherals market!

[myelin]

Don't believe anything they say: Google are looking to corner the Electron peripherals market!

Resistance is futile!

[myelin]

An update here: the board works really well! I only had one socket available to solder in, so I've only been able to test it out with a single cartridge (the center slot, occupying ROM banks 2-3), but it seems to work perfectly. I've verified with the following, with no errors:

- An original Acornsoft Countdown to Doom cartridge
- Dave's Mega Games Cartridge
- Two of my own flash cartridges (the original one based on Paul's breakout board, and a new one that matches the form factor of the small Acornsoft PCBs).

I'm also working on getting my Elk PiTubeDirect cartridge working. It seems promising (all the debug code is working properly, and I can read out the Tube startup banner from BASIC), but the OS isn't talking to the Tube yet, because I don't have the Tube Host code installed anywhere.

[Elminster]

It must be the drugs but looks like you glued your Electron to the ceiling!

So is the issue that you current Plus 1 is busted so you have decided to reengineer the cartridge bit?

Edit: Have you thought about taking on this project? Sounds like you are half way there already and in your interest as you have no plus 1

[algenon_iii]

Nice idea, considerably more compact than a Plus 1 as well.

As an aside, has anyone ever considered an Electron Plus+? Basically a single expansion that gives all of the BBC B ports (and ROM sockets) in one handy unit that plugs into the back of the electron (perhaps retrofitted into a Plus 1 case).

[Elminster]

Nice idea, considerably more compact than a Plus 1 as well.

As an aside, has anyone ever considered an Electron Plus+? Basically a single expansion that gives all of the BBC B ports (and ROM sockets) in one handy unit that plugs into the back of the electron (perhaps retrofitted into a Plus 1 case).

Did you click on the link I posted above? That is what the thread covered but dont think it moved much. Would be good if Myelin picked up the mantle.

[myelin]

It must be the drugs but looks like you glued your Electron to the ceiling!

So is the issue that you current Plus 1 is busted so you have decided to reengineer the cartridge bit?

Edit: Have you thought about taking on this project? Sounds like you are half way there already and in your interest as you have no plus 1

That's pretty much it. The overall plan here is just to make interesting things for the Electron, with a preference toward making stuff that (a) nobody else is working on already and (b) I can actually test. So things like Tube and 1MHz Bus interfaces are off the table for now because (a) Dave Hitchins already has that taken care of with the New AP5 and (b) I don't have any Tube/1MHz hardware here so I'll have no way to know if I've gotten it right

If I were to make something like a Plus 1, I think my wishlist would look like:

• As many cartridge slots as possible (3 or 4)
• CH375B for USB
• SD or micro-SD slot
• Serial port or USB serial (I'd probably include an ATMEGA32U4 like
• Flash and SRAM
• A PiTubeDirect socket for a Raspberry Pi

The problem with making one big project that does absolutely everything is that you need to redo the whole thing after realizing you missed that one critical feature. I think I'd rather make a "Minus Six" with just six cartridge slots and nothing else, and do the rest as cartridges. Or possibly use a big CPLD and expose a "super user port" that can be programmed to add in stuff like Tube/1MHz/serial later on

Looks like we've actually got two "New Plus 1" threads btw: Plus 1, TNG and Redesign Electron Plus1.

Before all that... next on the list is to finish off my dual ported ram project: a cartridge with SRAM and a USB interface, that'll give you 32kB of sideways ram, that can also be read and written from an external computer. So you'll be able to switch out ROM images without turning the computer off, and have a nice fast way to get a whole lot of data in and out.

[Elminster]

Looks like we've actually got two "New Plus 1" threads btw: Plus 1, TNG and Redesign Electron Plus1.

Think there are several more kicking about and many discussion in the pub/inn/tavern over the years. Just need to some one to build it. And I think you probably now the closest as most people have added to or moded exiting plus1. You have built a semi replacement. I'll give you a few weeks to knock up the everything design, I'll expect my prototype bored in a few months ?

[fordp]

I'm also working on getting my Elk PiTubeDirect cartridge working. It seems promising (all the debug code is working properly, and I can read out the Tube startup banner from BASIC), but the OS isn't talking to the Tube yet, because I don't have the Tube Host code installed anywhere.

minus_one and elk_pi_tube_direct.jpeg

Looks like this is the NZ version with the photo upside down!

[paulb]

The problem with making one big project that does absolutely everything is that you need to redo the whole thing after realizing you missed that one critical feature. I think I'd rather make a "Minus Six" with just six cartridge slots and nothing else, and do the rest as cartridges.

I think this is the best strategy, too. It would be nice to have some enhancements to the cartridge connections, though, adding the missing address pins, for instance. Maybe there would need to be a separate connector for these extra pins to prevent compatibility issues. You wouldn't want to mess around with different connector widths, certainly.

[myelin]

I think this is the best strategy, too. It would be nice to have some enhancements to the cartridge connections, though, adding the missing address pins, for instance. Maybe there would need to be a separate connector for these extra pins to prevent compatibility issues. You wouldn't want to mess around with different connector widths, certainly.

Ahh... this is such a can of worms! Whenever I start thinking about this, I usually end up concluding that the current cartridge spec, annoying as it can be, is good enough -- it's standard, supported by a ton of hardware, gives access to &FCxx and &FDxx, and gives each cartridge two sideways bank IDs that are guaranteed to not conflict with anything.

Having access to the rest of the address pins would let you do clever things like bus monitors, but unless you want to disconnect the ULA (and lose the keyboard, cassette port, RGB output, etc) or desolder the OS ROM, you won't be able to respond to any memory accesses outside the cartridge space anyway, which makes that less appealing. I have a PCB here that will let me hook an FPGA up to the expansion port and mess around with that sort of stuff, but despite all my brainstorming, I haven't come up with anything else that would really take advantage of the extra pins...

(Having more than two cartridge slots, and maybe remapping nOE2 to point to a different bank on each cartridge, on the other hand, would be really useful...)

[algenon_iii]

Nice idea, considerably more compact than a Plus 1 as well.

As an aside, has anyone ever considered an Electron Plus+? Basically a single expansion that gives all of the BBC B ports (and ROM sockets) in one handy unit that plugs into the back of the electron (perhaps retrofitted into a Plus 1 case).

Did you click on the link I posted above? That is what the thread covered but dont think it moved much. Would be good if Myelin picked up the mantle.

I have looked at the thread now, but hadn't when I first wrote my message.

[paulb]

Having access to the rest of the address pins would let you do clever things like bus monitors, but unless you want to disconnect the ULA (and lose the keyboard, cassette port, RGB output, etc) or desolder the OS ROM, you won't be able to respond to any memory accesses outside the cartridge space anyway, which makes that less appealing. I have a PCB here that will let me hook an FPGA up to the expansion port and mess around with that sort of stuff, but despite all my brainstorming, I haven't come up with anything else that would really take advantage of the extra pins...

I think the Jafa Mode 7 Adapter intercepts (but does not override) address accesses in the complete memory space in order to know what the ULA is transferring to the screen. Certainly, it appears to track what is being written to the ULA registers. Without knowing the upper address bits, cartridges cannot know what is really going on in general on the address bus.

Also, I think you could transfer data directly to RAM if you had access to all the address pins, but without them you would have the upper address bits with unknown values, which isn't likely to be good enough if a peripheral wants to bring RDY# low and access the bus itself, which I believe should be possible.

[myelin]

Ahh, MODE 7, of course! Or making an alternate display that outputs HDMI, etc. These are good applications I wonder if we could make an expansion that keeps NMI low all the time to disable the ULA, for a fast MODE 2 etc.

I looked into the feasibility of making bus master devices (that drive the A pins) but the version of the 6502 in the Electron doesn't have the BE pin that tells it to let go of the A lines, so that's a no go unless we want to implement that using buffers on the expansion port so cartridges can talk to each other.

[1024MAK]

The full address bus is useful for sniffing / monitoring the CPU and ULA.
It is also occasionally useful for write only access where a write will not upset the circuitry on the main board.

At least one pin, maybe two on the cartridge slot are not used by 99% of cards. There may be one or two other pins where the signal is hardly used by cards.

Mark

[myelin]

At least one pin, maybe two on the cartridge slot are not used by 99% of cards. There may be one or two other pins where the signal is hardly used by cards.

Ooh, I didn't think of that. With the CPLD I could make nOE, nOE2, nINF*, MADET, and maybe some other Elk/Master signals software selectable, which would allow for this sort of thing.

[fordp]

I would add two things to this project to make it MUCH better.
1) SD Card Slot to use David Bank's Printer Port MMFS.
2) A built in Flash Chip that emulates at least one cartridge (May as well be able to emulate 3 cartridges in one chip) to allow MMFS with just this board.

[myelin]

I would add two things to this project to make it MUCH better.
1) SD Card Slot to use David Bank's Printer Port MMFS.
2) A built in Flash Chip that emulates at least one cartridge (May as well be able to emulate 3 cartridges in one chip) to allow MMFS with just this board.

Indeed It would be pretty feasible to add four banks of sideways RAM and four banks of flash, which would result in a memory layout kind of like on the Master.

I haven't touched the Electron in a few months, TBH, after getting a Beeb and (more recently) a Master 128 motherboard to play with. My latest project is a mezzanine board that breaks out all the CPU pins and converts the signals to 3.3V, which makes *internal* expansions possible -- and the first one of those that I make will have SD, flash, RAM, and a USB serial port, which I think covers most of the things I'm interested in.

[DBCohen]

Well, came across my original Electron from 1983 in the garage the other day (forgot I still had it - still boxed with writing by 13-year-old me on outside!), and it works fine. Keen to get this set up and working again, but after spending ten minutes loading software from my iPhone over the tape interface I reckon teenage me was a lot more patient!

I had a Plus 3 back then, but I know that is sold long ago. Seems a Plus One should be the way to go to allow some Roms or disk access via the slots, but eBay has none and the pricing on their seems to have gone stupid! I have seen the GoSDC thing, but that seems a tad pricey for a single function board. Cartridge slots would allow me to do a lot more.

Is there somewhere I can buy these Minus One boards (made up, that is; I can't solder for toffee)?

Thanks,

David.

[myelin]

I had a Plus 3 back then, but I know that is sold long ago. Seems a Plus One should be the way to go to allow some Roms or disk access via the slots, but eBay has none and the pricing on their seems to have gone stupid! I have seen the GoSDC thing, but that seems a tad pricey for a single function board. Cartridge slots would allow me to do a lot more.

Is there somewhere I can buy these Minus One boards (made up, that is; I can't solder for toffee)?

Sort of! I should warn you that the Minus One isn't exactly an "end user" product like the Plus One; it's a lot cheaper and lets you have three cartridges rather than two, but:

- I couldn't find a socket that perfectly fits Electron cartridges, so you have to be a bit careful when plugging them in; you can't just jam them in or you might get a short circuit. (You have to carefully center them when plugging them in.)

- The Minus One is a little 'bare' looking, if you care about aesthetics... it's a big 10cmx10cm circuit board that hangs off the back of the Electron. I wonder if someone could be convinced to work up a case model for a Plus One, modify it to fit the Minus One, and 3D print a few There have been various case fabrication projects for cartridges on here, but nothing for Electron expansions yet.

If you're still interested, I can send you a bare board for free, or I can post it to someone near you who can do the soldering If you're in the USA, I can solder one for you, and if you're in the UK I believe there's a couple of people on here that have offered to help solder. The components on the board cost around US$15-16.

Cheers,
Phil

[daveejhitchins]

@DBCohen . . .

Check your PMs

Dave H

[DBCohen]

Thanks all, very helpful offers. Will let you know how I plan to proceed.

David.