The original thinking behind the project was to leverage the 1088XEL design (which is a mini-ITX-compatible motherboard) and provide the expansion slots as an integral part of the case, just as in a PC.
However after some investigation, it turns out that if a sufficent quantity are ordered (Q100 or at least Q50), I can get a custom-designed plastic case for a "reasonable" cost - somewhere in the order of $600 for the non-refundable CAD engineering, and then $20 per unit, including printing/logos etc. This makes the decision far harder, if there is sufficent demand for the project.
Until I have orders-in-hand, the ITX-compatible case is the go-to design.
The Mini-ITX enclosure
Two slots isn't very many - especially when expansion is the whole point, and the native 8-bit OS supports 8 devices, so we're going to support at least that many.
If you look at the third image in the slide-deck, there are two main gaping holes in the back of the case. The lower is obviously the back-panel for the mini-ITX motherboard, but above that is where the PSU would normally go.
My proposal is that we purloin that hole (since the 1088XEL doesn't need it, and if we're connecting to an XE or XL, they don't need it either) and repurpose it as expansion slots.
Worth noting is this particular enclosure actually makes this more appealing because it provides a larger hole than the ATX PSU and a removable PSU-dock that would normally house the PSU. Look carefully at the differences in the PSU space between images 2 and 3.
So then the questions come down to: "How do we turn a gaping hole into slots" and "How do we attach our cards to these new slots". To answer in reverse order:
- If you look closely at the 3rd image, you can see that the screws that fasten the ATX PSU-dock to the back of the case affix to threaded holes in the case. That means we can use those threaded holes to support our slot-carrier.
- The plan is to design a backplate out of Aluminium that has holes that match the screw-placement on the back of the enclosure, and that is "scored" to fold inwards to provide support for the PCB that provides the slots, and also to provide the hole at the top for the card to screw into. Once the Aluminium is folded, good old cyanoacrylate (or Superglue) will keep the fold strong, and the glue sticks well to Aluminium.
- We'll machine cut-outs for the slots themselves, and make them match the profile for low-profile PCIe expansion socket blanks.
- I have a Carvey which is allegedly capable of machining soft metals like Aluminium - something I intend to put to the test, but even if it turns out I have to get the panels machined professionally, this wouldn't be too expensive.
It's clear from the pictures that there are in fact two full-height slots already in the case. My plan is to allow both of these to be used using IDC cables to connect to the device-interface PCB, given that the ITX motherboard will be in the way if it's mounted. Another thought I had for one of these would be to allow cartridges to plug in, it'd be around the back, but it's still the most accessible place for a cartridge without removing the front.
From The Front
Another plan for the device-interface board is to support the motherboard front-panel cables. We have a few ports that could be directly useful to the 8-bit host:
- The power-switch. It would be nice if the entire computer and expansion bus could all be turned on/off via the front-panel switch. This could also facilitate the power-sequencing, so that the 8-bit board is only powered once the expansion bus is up and running, thus ensuring everything is ready. It's (very) slightly complicated by the fact that it's a momentary switch, not a on-for-power/off-for-no-power, which means something will have to be permanently powered, monitoring the switch, but that doesn't seem too complicated.
- There are 2 USB-3.1 ports on the side, which are both connected via the standard 20-way IDC socket that would normally plug into the mini-ITX motherboard. The thing about USB-3.1 is that each of the ports has 9 wires, including the shield as GND. That maps pretty well to an Atari joystick interface, and all it would need is a way to mount the 20-way IDC connector such that it can talk to the 2 10-way IDC connectors on the 1088XEL, and a small PCB that has a USB-3.1 connector on one side and a 9-way D connector on the other. After that it's a matter of getting the wires the right way around. As a bonus, it would extend the joystick wire length, because you'd have [enclosure]<—>[USB 3.1 cable]<—>[tiny PCB in a case]<—>[Atari joystick cable and attached joystick].
- There's a Hard-disk LED that would be nice to link up to some sort of disk-i/o or bus-i/o indication - even better if it were under software control.
- Finally, there are audio connectors for headphones and line-out or mic-in (not sure which). It'd be exceptionally cool to link up the 1088XEL's audio circuitry to this so either headphones or stereo speakers could be linked up without any problems.
The only downside to this case, in fact, is that it doesn't have a 5.25" optical-disk panel. I was hoping to get an LCD/Plasma/VFD display in there. To get that, you'd have to go to the '120 or '130 model, not the '110, and that makes the case volume (by extending the front-to-back distance) quite considerably larger. I don't think it's worth it. If we do go down the custom-enclosure route, though, there will be an LCD panel for information purposes.