morning coffee musing: could you make a discrete computer (7400 chips, but the modern versions with fast slew rates and all that) that can run at triple digit MHz?
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Dave Andersonreplied to Extreme Electronics last edited by
@Extelec yeah, glancing at the datasheets for fast logic chips, the quoted fast numbers are caveated quite heavily by how much load capacitance there is. Impedance management in general is where I suspect I don't have enough knowledge to know exactly where the cliff edge is.
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Extreme Electronicsreplied to Dave Anderson last edited by
@danderson My thought was a "retro" bus, but with differential signals for all the reasons we've stated. (plus x-talk) Terminated at each "end" SCSI style.
complexity is the down side.
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Dave Andersonreplied to Extreme Electronics last edited by
@Extelec yeah a classic system bus would be the dream, I just straight up don't know enough about high-speed design to know exactly how, I'd have to go learn more.
It's one of those things that's definitely possible since more integrated chips have I/Os like this effortlessly these days. Big question is whether the building blocks are still available as discrete chips, and if the physics tolerate the looser numbers on everything once you de-integrate.
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@Extelec Well no correction, I know enough to know that multi-device busses on a high speed line where each one grabs the signal partway down the transmission line is Harder(tm) than a simple pair with devices at the ends, but not any of the details of exactly what goes wrong and how it needs to be mitigated.
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Extreme Electronicsreplied to Dave Anderson last edited by
@danderson Multiple reflections & driving two lines of different lengths & impedances simultaneously, a nightmare of unknowns and variables.
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Extreme Electronicsreplied to Extreme Electronics last edited by
@danderson And return paths from different parts of the board. ?
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Dave Andersonreplied to Extreme Electronics last edited by
@Extelec In theory, if the switching rate is fast enough, the return paths will hug the forward signals, so that part at least should be the same design rules as everything else. Reflections off stubs and impedance discontinuities is likely the big killer. I know people design stuff like that all the time, so there must be known techniques for managing it. Or maybe everyone just reaches for a field solver and iterate based on simulation data.
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@danderson there's always Potato Semi....
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@mxshift Is... is this for real? I'm genuinely having trouble telling if these are real things I can buy
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@danderson I bought a few chips from them a few years ago. No idea what the state of their business is now
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@mxshift Well the ebay store seems to be no more, and half the distributors no longer exist and the other half don't advertise their products. It doesn't look great... otoh that could just be small company vibes.