[time-nuts] Form factor

Bob Bownes bownes at gmail.com
Thu Dec 23 03:56:30 UTC 2010


Perhaps I'm old school, but this sounds overly complex to me.

Probably the most important thing I learned in engineering school
(besides where the beer and amiable consorts were) was KISS. The more
complex, the less likely the project is to complete.

So I pose the question: Do we need a bus like this at all to meet the
basic goals?

The suggestion of a main board with input/output modules that plug
into the main board sounds much simpler. If the IO needs to be
modified, you build a separate module. If you think there are going to
be parameters that will be switched on/off/adjusted/whatever on that
module, put a few io pins from the main controller on the connector to
the module. May never get used, but if someone wants to rework the i/o
module, you're all set. Many of the popular controllers can put an I2C
bus or a serial port on any pin pair. Libraries for both are common
for the controllers I'm familiar with.

Example: How much complexity do you need in an input module? What are
the features you might like to manipulate there? Would it really take
more than, say, 8 pins?

The discussion about opto isolation and/or differential pairs and the
like seems like overkill as well. My vision is that this thing fits
into a single box. If you need that much isolation or are worried
about that much noise on a bus inside a box, either you are pushing
some serious speed or operating in an environment that's way more
hostile than I would like to be sitting in and observing the front
panel.

With that said, I also concur that any module should be easy to test
in a standalone mode. Not necessarily useful to many folks outside of
the opencontroller project, but easy to test. That's why, for example,
I suggested that the core counter module be usable with nothing more
than TTL inputs/outputs. An input module then might be nothing more
than appropriate signal conditioning.

Final design, will, of course, dictate pcb sizes.

Bob


On Wed, Dec 22, 2010 at 9:05 PM, Chris Albertson
<albertson.chris at gmail.com> wrote:
> On Wed, Dec 22, 2010 at 3:32 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
>
>> I used a shared opto-isolated async bus.  You need two optocouplers
>> per microcontroller, and one place you power the shared bus, and
>> you're all set.
>>
>> I have yet to see an microcontroller without an async port.
>
> Opto-isolater?   Why not just use fiber cable between cards.    I know
> it sounds exotic but also seems to have half the parts count.  those
> s/pdif jacks are so cheap and I bet you can use them as pretty much
> drop in replacements for opto-isolators.  Would s/pdif jacks work as a
> physical layer?
>
> Really I just used I2C in my write-up as a place holder.  I you say
> nothing no one ever says a blank paper is wrong and suggests something
> better.
>
> s optical isolation required when all the modules are sharing a common
> power supply?  Does this means all the coax connectors need to be
> isolated.  I guess some one better step up and propose a grounding
> scheme.  That's not going to be me.
> =====
> Chris Albertson
> Redondo Beach, California
>
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