[time-nuts] Form factor

Chris Albertson albertson.chris at gmail.com
Thu Dec 23 00:07:00 EST 2010


I don't see how a backplane bus with modules you plug in is simpler
then having only the modules with no backplane bus.   The problem with
a hard backplane is that you have to know in advance what you are
going to plug into it.   Another (seemingly unsolvable) problem is the
mechanical design.  No one has yet come up with one that can be build
at home with normal tools or a low-cost of the shelf.

I think what we are talking about is the same think but only the "back
plane" is cable that you daisy chain from one module to the next.
Call it a "flexible backplane.  It might be a small 10 pin ribbon
cable, a coax or a fiber cable.

I don't know if opto-isolation is required but in any case that is a
feature of a module that a module designer may or may not need.  At
the system level we don't care what is inside each module.  So I
agree, any talk about isolation is premature.  However we do need to
pick a bus

The problem with picking a size for a PCB is that we can't know what
the final design will be.  With luck people will be building modules
for a long time and building modules to do functions we have not
thought of.  Without knowing what these funtions might be we can't
know the design.

I do agree with "KISS".  The best thing is to simply eliminate
everything you can.  People could not agree on a chassis design so,
out that went, no chassis execpt as an option built by the end user.
But the PCB size is selected to give the end user a wide selection of
options.  Same with how to interface this to a computer of if we even
want a physical front panel.  It appeared that argument would never
end, so out with that too.  The system just has a bus or 2 or 4 wires
(TBD) if if yu like some kind of computer interface then build an
"interface module".  So what is left?  Just an interface spec.  When
you remove the last module yu have just air in the box.  Seem to me
that is KISS is extreme.

The simplest Open Counter, I can think of is just two modules, (1) A
basic low performance frequency counts like the FCC1 and (2) and
interface module to translat the system bus to USB.  That could be
build with 1/2 dozen chips for under $50.  A more complex system would
have better modules, cost more and I think is what people want but  I
think a good measure of our quality of system engineering will be the
size and cost of a minimum system.


On Wed, Dec 22, 2010 at 7:56 PM, Bob Bownes <bownes at gmail.com> wrote:
> 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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-- 
=====
Chris Albertson
Redondo Beach, California




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