[rescue] Cost of custom MBus/SBus (was: Re: EIO card in HP 4M)

[Romain Dolbeau]

Le mar. 13 avr. 2021 C  16:59, Mouse <mouse at rodents-montreal.org> a C)crit :
> Or enough resources can be got together to have a new run of connectors
> made.  I assume that's impractically expen$ive?

Probably... Unit cost could be reasonable (read: less than $25-$30
apiece, or 3-4 times the cost of a similar mass-produced connector),
but there's probably not a market for thousands of new MBus modules
(I'd want 4 to 8, if they can work in pairs :-) ).

For a point of reference, the FPGA board I used for my SBus device
retails from $160 to $350 depending on the FPGA size (I've got the
smallest one, quite a lot of space already).
The actual SBus adapter board is a fairly simple 4-layer PCBs with not
that many components, and having a couple of them made and populated
cost me $300 (along with 3 unpopulated PCBs, minimum order is 5).
At >$300 ($160+$300/2) apiece I don't think anyone else is going to
give that design a try - way too expensive even if I get the micro-sd
card to work properly (still on it). Haven't tried using the 256 MiB
of SDRAM yet (thinking of a 'fast' swap area for NetBSD), but even
with that feature it would be a tough sell. Instead you can get a
full-featured Rpi4 8GB that will run circles around the SS20 and have
more than enough left for a nice dinner out when Covid is over...

I suspect doing a larger run (50-100) of a full-custom PCB design with
a similar FPGA would require at least 6 layers, and could be $150-$200
per unit - a bit more than a comparable development board but not a
lot (by ignoring the engineering cost i.e. assuming volunteer work for
the design itself). It could offer a fuller set of 'modern' connectors
than my prototype (micro-sd, hdmi, 64-256 MiB SDRAM for a framebuffer
or other use, perhaps ethernet or fast serial ports). I still don"t
think there's enough interest to make it happen, and that's with an
already existing proof-of-concept... (if I had the skill to do it, it
would definitely happen, but that's too complex a PCB for me, so I
stick with my current toy :-) ).

For a custom FPGA/MBus module that would probably also requires 6+
layers, not only one would need to implement the core (Leon or temlib
might help), but also the MBus protocol Level 2 (who doesn't want SMP
?), make the hardware talk to the motherboard at 50 MHz (despite the
fact that high-frequency MBus is known to be an issue... but then in
2021 it might be easier to get within specs, and 40 MHz can be an
acceptable fall-back solution), and then figure out how to convince
the PROM to work with the new module. With a decently fast FPGA, maybe
the module could be made for less than $250-$300 *if* you can get the
connector somehow. Yes, I've actually thought about trying, but I
don't have the required skillest, too much of a software guy...

Cordially,

--
Romain Dolbeau