RE: New project...PCI based servo control board

[jekasunich-at-worldnet.att.net]

Craig wrote:

>>> One closing thought on board #2, I'd like to consider a SIMPLE bus
>>> expander be included on it

>> Excellent idea!  Why waste FPGA gates on simple input and output
>> ports. Let the FPGA implement an expandable bus.  How about 8 
>> data lines, 4 address lines, and read and write strobes?  In a
>> minimal system, the strobes, go directly to a '374 and a '244 
>> for 8 bits of output and 8 bits of input.

>> Next step up, the >strobes go to a single '139, which can decode
>> strobes for up to 4 '374s and 4'244s, for 32 outputs and 32
>> inputs.

>> Finally, a maximum system could use 2 to 4 '138s to decode up to
>> 16 ports, or 128 outputs and 128 inputs.  All with only 14 wires
>> in the cable.  The interface board has '138s, '139s, '244s, and 
>>'374s, all commodity items that anybody will be able to buy for 
>> years to come.

> I'm fine with a bi-directional bus, but need a mechanism to
> handle who is driving the bus...just a little more complexity
> I think.

I've posted a drawing at
http://home.att.net/~jekasunich/Bidirectional_IO.gif
that shows the FPGA logic to handle bus reversals, and
three configurations for the interface board - minimim
I/O (8 in, 8 out), medium I/O (32 in, 32 out), and maximum
I/O (128 in, 128 out).  The medium and maximum versions
can be built in chunks, so if you need 24 in and 16 out, 
no problem.

> I'm traveling for the next 3 days or so, but I'll 
> try to start a spreadsheet where we can capture things
> like I/O , functional blocks, and estimated gate counts.
> Is there a place to conveniently post such a document? 

I've been using my personal web page space, but I'm the 
only one that can use it.  A dropbox of sorts would be 
great.  The CCED forum on Yahoo has a files area they 
use for that kind of stuff.  Many folks here also visit
CCED, maybe we could leave stuff there?

> Actually, I was thinking of putting up a web page (as I
> have a hosting service that I'm already paying for...), 
> but just haven't taken the time to do so.  Any thoughts
> or preferences?

Unfortunately no.  You have hit on a major problem.
We are quickly going beyond what can easily be communicated
in a text environment.  I'm really looking forward to EMC 
Monday at NAMES, but even that will mostly cover broad 
subjects.  We need a way to exchange schematics, board 
layouts, VHDL files, and other highly detailed stuff.

John Kasunich