Ok, there's been lots of talking and I think we're getting closer to what
is going to be practical for the final system. Although this isn't in any
way final, what I think looks best at the moment is the following.
* IDT R3051/52 CPU at 40MHz: I think we're pretty much hooked on the CPU
_type_ now, there are good reasons for using this one, and none of the
other potential devices are really cheap enough when you include the cost
of support chips. Obviously, we could use the '81 if the whole board cost
ends up ok.
* IDT 3730 I/O controller: I like the sound of this chip, even if we're not
going to make use of it's fancy "video DMA" hardware. Perhaps this feature
could be used to drive a DAC?
* Dual or quad UART: gives us important motherboard-based serial ports for
* Keyboard controller: I haven't actually been able to locate much info on
these even after looking quite hard. The chip used in the PC is a simple
8051 while the AT used an i8041/2 microcontroller. Could someone in the US
give AMI a call about these? I know they manufacture them, but have no data
or pricing info on them. I suspect they'd be pretty easy to interface to
the 3730 I/O bus.
* Video controller: I'm still for a TMS34010-based solution. Here, the
problem is mainly software. I think using this chip would help to cut total
system costs quite remarkably. Steve's probably going to flame me about the
NSC chip, but I think I prefer this one.
* Ethernet controller: Providing it's cheap enough, it looks like the NSC
8390 and it's interface chip might be the right way to go here. If the
devices are expensive, I'm in favour of leaving this section of the board
* SCSI controller: I think we've pretty much settled on the 53c94? Again, I
don't have pricing info, but I think it's reasonable. This chip is pretty
vital to the whole system, and I personally think the 53c94 is the best
chip for the system.
* Boot EPROM: This is pretty straightforward. How much are we shooting for?
I'd suggest 64k or 128k - I don't think we can really use much more than
that. I'd like to have a complete system configuration/testing program in
this along with the code necessary to read a boot image from SCSI, Ethernet
and serial interfaces. I am willing to spend quite a lot of time on this to
make it right <grin>.
* RTC + NVRAM: Don't know much about these. Can someone call Dallas Semi.
and find out what they have available? I guess we're looking for about 1 or
2k of NVRAM and some reasonable RTC.
* I/O bus: I have personally given up on the idea of an ISA bus. No one
seems to have come up with an easy and cheap solution to implement one. I
do like Steve's simple/proprietry 8 bit I/O bus, but I'd prefer not to have
to have the 3041. I think this interface could be stuck onto the 3730
relatively easily, and think it would serve all our needs as far as simple
expansion goes. I really can't think of an ISA-based I/O device that anyone
would want to add to this system that couldn't also be done with the
proprietry interface. Again, I'd vote for leaving the I/O connectors
"unpopulated" if they are a significant cost.
Does this sum up the current thinking ok? I'll listen to arguments about
the video system, but I think the 34010 is really the best option at this
stage, having the necessary attributes: low cost and low complexity. I
think having our own 8 bit I/O bus is the best option, but I think it would
be better to drive it with the 3730 than the 3041, reducing the complexity
in the memory mapping and bus structure. The other parts are pretty much
fixed, as far as I can see...
For the moment, the devices I can see being put on the I/O bus are: more
serial ports, a DAC for sound, a parallel port and a floppy drive
controller. I think that would pretty much cover everything, and having
seen Steve's design before, I'd have to say it would be pretty easy to
design and build the necessary cards. I really can't see the point of an
ISA bus at this stage...
Pat -- "There's only one thing left to do Mama, I got to ding a ding dang
my dang a long ling long" (Jesus Built My Hotrod -- Ministry)
GCS d* -p+ c++ l++ m--- s+/- !g w- t- r