A series of computer cards using the VME bus as a common backplane, in the style of modular computer equipment, laboratory test equipment, and industrial controls systems.
I've always liked the VME bus for its adaptability. It's an asynchronous bus with dynamic sizing, so it can support both controllers and peripherals that require different signaling speeds and bus widths to communicate, without requiring a specialized bus interface chip. It supports multiple controllers without the need for a special slot on the backplane, because it uses daisy chained priority bus arbitration. It uses standard and readily available DIN41612 connectors, comes in a 3U variant, and it's possible to still buy equipment that conforms to the standard, namely card cases and power supplies. The 3U Eurocard standard, which uses 160x100mm boards, is also fairly affordable for PCB printing, and it's possible to make 100x100mm cards, which can be printed for as low as $2. The bus is perhaps slow by modern standards, but for CPUs that are below 100MHz, it's the perfect bus to build 16-bit and 32-bit retro computers around.
My intention is to build a backplane, dedicated bus arbitration and interrupt controller card, and a generic 68030-based CPU card to start. I have ordered an off-the-shelf desktop card case from Schroff via Digikey, on backorder, and I plan to adapt an affordable MeanWell power supply. I'm hoping to be able to install multiple CPU cards into the same system to make a simple multiprocessor machine. I've already made a generic card using a microcontroller wired to the bus, that I intended to use as a bus analyzer called BigBoy, but I haven't written software for it yet.
I also intend to eventually make a 68010 version using only DIP chips and hopefully all hardwired logic, so that a specialized PLD programmer won't be needed to build one. I might also make cards with other CPUs. It should be possible for the system to support different CPUs in the same system at the same time. My larger hope is for the VME bus to be used more broadly in the hobbyist computer community as a common means of interoperability.
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it uses the eurocard form factor, and equipment such as cases can still be bought new or used
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it's asynchronous, so it can support devices operating at different speeds
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it supports dynamic bus sizing for 8-bit, 16-bit, and multiplexed 32-bit operations in 3U
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in the 6U form factor, it can also support full 32-bit or multiplexed 64-bit operations
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the DIN41612 connectors it uses are still widely available with solder tails
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supports cheap 3U board sizes of 100mm x 160mm or 100mm x 100mm
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also supports 6U board sizes for bigger and more complex devices
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it can be a bit more complicated to interface to for simple 8-bit systems
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it's slower than newer buses like CompactPCI
- https://hackaday.io/project/192539-comet68k
- https://github.com/tomstorey/COMET
- computer card is missing bus arbitration, but the backplane supports it
I bought a brand new Schroff PropacPro 3U 63HP 266mm deep Unshielded Complete Desktop case. I initially requested a quote from Schroff, but unsurprisingly they sent me back a list of authorized redistributors instead. It turned out that Digikey did actually have the cases available for backorder but the listing was uncategorized, so you had to search specifically for the part number to find it.