[time-nuts] Form factor

[Magnus Danielson]

On 12/23/2010 03:05 AM, Chris Albertson 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.

Using a serial interface is nice, but I2C is not the one of my 
choosings. I'd go for plain serial interface, RS-232 like, but not 
necesserilly with RS-232 levels. Possibly using RS-485. A party line has 
several issues with it, but reduces the cost. It's more suitable for 
control than pumping data out of the device.

> 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.

Within such a situation, RF-chokes and transoformers would suffice for 
isolation. Optical isolation using normal opto-couplers is cheap and 
space-saving considering that for most uses there just isn't much of 
data-speed except for a few things.

Cheers,
Magnus