This is the first of a series of specifications for this board.
As the specification becomes more detailed and complete, it will
be re-mailed to the list. Occasionally it will be summarized and
sent to comp.os.linux.
* Mechanical details
The finished board must fit in a standard IBM compatible case,
and should if possible fit in a mini-case. The CPU may need
one of those 486 "chip fans" or something like that in this
case; this needs investigation.
Because of the cost of assembly, the board should not have any
Surface Mount Devices at all (J-lead components that can be
socketed are ok). Hand assembly of SMD boards is *very*
difficult in all cases except maybe prototyping. The cost
of setting up and having boards assembled automatically is
not cost effective until you get into the thousands. I expect
to be hand assembling the first few production runs of this
board to save costs.
IDT 79R3051E or IDT 79R3081E running at 20, 25, 33 or 40 MHz
The choice of CPU is based on price, flexibility (hardware wise)
suitability for the project, and of course, availability.
My preference is for the 3051E-40. I have a price from one
distributor for us$155 in units of 100.
There is some difficulty in designing hardware to run at 40MHz,
however, this will be mitigated by the use of the IDT 79R3730
* Peripheral controller
IDT is just about to release the IDT 79R3730. This is titled
"Raster Image Processor". It features:
* DRAM controller:
- can drive some reasonable numbers of RAM chips;
- can interleave banks;
- can generate burst mode accesses;
- can handle different sized DRAM dynamically;
- can check parity;
* Has two separate buses (aside from the main bus) that
can handle talking to slower devices without holding
up the main bus. This happens by use of a DMA controller,
bus sizer and FIFO's. Each bus can have several DMA
running concurrently and still allow access to other
peripherals on the same bus;
* DMA logic to read and then rasterize data. This is
intended for printer engines, but could easily be
adapted to a CRT;
* Individually programmable I/O pins (some of which
get used for parity checking);
* PGA, PLCC or MQUAD packages;
* Handles reading 8-bit EPROMS 4 bytes at a time
to allow the use of a single boot ROM.
The CPU needs only to talk to the 3730 and one set of buffers
for the RAM. This will make a big difference to how hard it is
to design the high speed stuff, making 40MHz probable.
The problem is that IDT aren't going to release it until some
time around September. I'm working on them to get some advance
samples and a more complete data sheet. Until then the latest
IDT CPU data book contains advance information.
* 32 SIMMS sockets
The IDT 79R3730 allows the use of DRAM up to 4MB in size.
32 SIMM sockets allows 128MBytes of RAM. If more than one
bank is installed, then they can be interleaved giving almost
double the bandwidth to the memory (it reduces every 2nd
cycle to a single cycle). I'm guessing that using 70ns
SIMMs that we would need 3 wait states per access.
The SIMM sockets should handle the standard 30 pin IBM
compatible SIMMs, for price reasons.
* Boot EPROM
Because of the 3730, we can get away with just one EPROM.
Maybe two just for kicks. Access to this is slow, so if
speed were are problem, then code could be copied to RAM.
* Feature selector
Some of the I/O pins on the IDT 79R3730 should be used for
a "feature selector". This could allow setting things like:
- use of parity;
- big endian/little endian;
- use of video;
- diagnostic mode;
This part of the design is trivial. I expect it to just
fall into place when the time is right.
* Keyboard controller
This should certainly handle IBM compatible style keyboards.
The only way I know to do this is to use the 8051(?) micro-
controller to drive the keyboard exactly the same way as
every mother-board I've seen does. This circuit should
connect easily to the 8-bit bus of the 3730.
* Non-Volatile-RAM and Real-Time-Clock
This should be looked into. The clock is almost a requirement
but the NV-RAM could be replaced with the feature selector.
* Serial port
Because of the ISA bus, serial is always possible. It might
be a good idea, if just for debugging to have some serial
ports on the mother-board. If this is the case, they should
be at least as good as 16550's.
* ISA bus
This is probably the hardest part, but necessary. There are
plenty of ASIC's out there that can handle the whole ISA spec,
however, they are designed to interface to x86 CPU's and are
usually surface mount. With the possible exception of the
-MASTER signal, the ISA spec could be implemented using not
too much glue logic. Remember that in the quantities that
we would be buying in it may be cheaper to buy a handful
of PALs than one ASIC and a few PALs to interface to the
If the -MASTER signal was not implemented, no ISA device that
does first party DMA would work (read Adaptec SCSI controllers).
Other than Adaptec, I know of no other devices that use
-MASTER. Anyhow, we have a SCSI controller on the mother-board
that can replace the Adaptec.
* SCSI port
It has been suggested that we use a NCR 53C94 SCSI controller
chip. I know nothing about this chip. I expect that it could
be connected to the 8-bit or 8/16-bit bus of the 3730.
* Ethernet port
It has been suggested that we use an Am7990 (Lance) Ethernet
controller. I know nothing about this chip. I expect that it
could be connected to the 8-bit or 8/16-bit bus of the 3730.
The IDT 79R3730 handles the DMA of rasters from DRAM and the
serialization into 1, 2 or 4 bits per pixel. This could be
buffered to form 8-bits per pixel then fed into a suitable
colour palette. Then all that remains is to generate the
pixel clock and the appropriate sync signals. The sync stuff
could come from a motorola 6845 or the like. Because of the
3730, this section should be real cheap. If it doesn't turn out
that way, then we should abandon this for some video controller
on the ISA bus. Resolutions up to 1280x1024x256 should be
Floppy disks are not high bandwidth items. They can and should
be supported by some floppy only controller on the ISA bus.
Using some cheap IDE controller with floppy connectors might
be an even cheap solution.
Note that this machine doesn't do DOS, so a floppy is probably
optional in lue of a SCSI tape drive.
Neil Russell (The wizard from OZ)
Pyramid Technology Email: firstname.lastname@example.org
3860 N. First Street Voice: (408) 428-7302
San Jose, CA 95134-1702 FAX: (408) 428-8845