In development: Plus 1 Mini

[Ramtop]

Last week I was fortunate enough to have the opportunity to show the first prototype of my new design at this month’s ABUG Scotland meeting. For everyone who couldn’t make the trek to sunny Falkirk I thought I’d post some details.

This is what I’m tentatively calling the Plus 1 Mini (P1M). It’s a reduced form factor alternative to the original Plus 1. I say alternative rather than replacement because the P1M’s design goals go somewhat beyond providing some cartridge slots, toward being an all-in-one expansion that provides as many useful features as possible as standard, leaving the cartridge slots free for other uses.

The two cartridge slots are fully Plus 1 compatible (as far as I can tell) and all signals are implemented. In addition, there are also four banks (64K) of sideways RAM available by default – there’s actually 128K on board and the final hardware will possibly have an option to disable the cartridge slots for more RAM.

An SD card slot is on the rear of the unit, which works with MMFS.

On the left side is a 9-pin joystick port compatible with Atari/Commodore type joysticks and Amiga mice. Compatibility with Atari ST mice should be added in the next revision.

Finally, there is an internal header for a Raspberry Pi Zero/Zero2 for use as a PiTube co-processor. This header may also be usable for things like a sound card, WiFi, RAM disk, etc. This is the one feature not currently working in the prototype as I used the wrong footprint for the header

The case pictured isn’t final, although the finished one should look quite similar and will be in a colour more in keeping with the Electron. It’s a nicely robust case 3D printed in just three parts.

I’m hoping to have this finished and available to buy sometime around June-ish timeframe, lots of testing still to do!
Unfortunately, the bad news is the P1M will effectively be a limited edition. It’s designed around an AMD/Xilinx XC95144XL CPLD chip and those are going out of production about the time I expect to finish the P1M. I have a stock of chips, but right now I’m expecting this to be a 2024 only product.

[rmbrowngr]

Fantastic work! Well done.

A flood of Plus 1’s at the moment!

[Multiwizard]

[DaveLecky]

Looks and sounds great Gary,

I’m sorry I had to miss the meeting in Falkirk but I’m sure I’ll see it in due course.

See you at the next one……

Dave

[maniacminer]

Looks good I've got a few Electrons that need expanding! I've ordered one of Dave Hitchins' new Plus-1 expansions. What's the cost likely to be? Are you taking orders?

[Ramtop]

A bit too early to take orders, possibly next month when I have a second prototype with all the features working.

I'd like to be able to keep the price for Stardot members under £100, but that really depends on component prices at the point I start building them.

[0xC0DE]

What a nice surprise, great stuff!

[Ramtop]

Making steady progress with this. The PiTube port is now working on the first prototype, and the second prototype PCB design should be on its way to China for fabbing next week.

Alas, there's an annoying issue. I can't fit both PiTube and mouse support into the CPLD. The mouse interface is Amiga-esq, with a pair of 8-bit counters that keep track of movement. This is good because it lowers the CPU overhead of dealing with the mouse, but it's very costly in terms of macrocells in the CPLD. So I may have to remove this and reserve mouse support for a future 'Plus 1 XL' or something.

The second prototype will add some additional signals to the PiTube port enabling it to support interrupts and some more address bits, which will be useful for non-Pi devices. Currently the plan is to have a RAM disk and sound card ready not long after the P1M is released.

[1024MAK]

Sounds interesting

You say that the joystick port is Atari compatible which is fantastic. Does it support two fire/trigger buttons? Does it include a +5V supply pin. Is it compatible with Sega Genesis / MegaDrive controllers/pads? Does it have some protection (in line resistors) against whatever it is that users do that kill joystick ports / interfaces.

And how is it read by software? Does it pretend to be the analogue joystick like on a Acorn Plus 1 or does it use one of the First Byte joystick I/O addresses. Or did you do something different again?

How many, if any, Electron software packages or games had mouse support? I suspect not many, if any. Hence although mouse support is a nice to have, if you have a lack of resources, best drop this. The other way would be to use an inexpensive microcontroller to provide the counters.

Mark

[Ramtop]

The joystick port is mostly a cut-and-paste from the ElkSD128, so the feature set is the same. Two buttons are supported, there's a +5v line on the port, Plus1, Slogger and First Byte interfaces are emulated at hardware level so no special software is necessary. I don't have any console controllers to test with, but just looking at the pinout the MegaDrive controller seems to be a no-go.

Every line on the joystick port has chunky current limiting resistors, to ensure only a very determined idiot can fry the port

And yes, the lack of mouse aware software on the Elk is a major strike against including mouse support in the hardware. It's a pet feature of mine, but difficult to justify. It may be smarter to implement it on a cartridge using an MCU to provide PS/2 and USB mouse support.

[1024MAK]

Excellent reply Gary

Thank you

Mark

[ElectronTubs]

This looks really interesting - does it pretty much replace the elksd128, but also gives two cartridge slots?

[Ramtop]

Pretty much, yes. The P1M is a big ElkSD128 with cartridge slots and a PiTube header, it was designed to have as much commonality between the two as possible to save time and money.

Still making steady progress with the design, I've added the ability to disable the cartridge slots to get an additional 64K of sideways RAM and also added a 'dumb mode' command that turns off everything except the cartridge slots, for best compatibility with the original Plus 1. I'm considering adding headers and mounting points on the case for a couple of activity LEDs - SD card and PiTube - but they probably wouldn't be fitted as standard.