On Saturday, July 23, 2011 11:33:03 AM Przemek Klosowski did opine:
> On Fri, Jul 22, 2011 at 3:15 PM, Igor Chudov <[email protected]> wrote:
> > El cheapo ethernet routers cost $9.99:
> >
> > http://www.newegg.com/Product/Product.aspx?Item=N82E16833166034
> >
> > I believe that a cheap Ethernet thermometer would cost roughly as
> > much, if someone wanted to produce it.
>
> Yeah but this Rosewill box is really a plastic box with five
> connectors surrounding a dedicated ethernet switch chip---a mass
> market item that's cheap because they make zillions. Anything else has
> volumes that are multiple orders of magnitude smaller.
>
> To get a good price, the only way is to judo the strength of the mass
> market---repurpose an OpenWRT router or a cast-off PC. Even then,
> however, you need the Ethernet cable AND the power line---unless you
> jigged the power (low-voltage DC, of course) over the unused pairs in
> the Ethernet.
>
> This gives me an idea--I think it might work to hook one of those
> two-wire or I2C Dallas/Maxim temp sensors over a long 'ethernet'
> cable, and bitbang them. Does anyone know what are the practical
> limitations on I2C/two-wire? A twisted-pair ethernet cable should
> help...
Unforch, I2C is not really suited for long distances, the cables
capacitance limit is 400 pf, it is not a "terminated" transmission line
design by any stretch. No cat5 type cable has any advantage in its twisted
pair, differential receiver design. Neither do the std 4 wire phone
cables. 40 feet and I2C is Dead in the Water.
That isn't saying that a 3 wire circuit, with active pullups that also
serve as terminators for echo & ringing control, couldn't be made to work
at 100's of yards, but the thing is going to need about 5 watts of power
available at both ends of the cable for driver and term power. If flat
ribbon cable was used, which has an impedance of about 110 ohms, then the
chips designed for active terms on a scsi bus could be 'borrowed', but for
other cable types, like 4 wire round or flat telco and cat5 twisted pair
styles, the term match will not be as close as they will range down to the
60 ohms area. One might be reduced to looking at first one end, then the
other of the circuit and adjusting the terminating R for minimum ringing
and echo's as seen on a 100 mhz scope. Not practical for a just plug it in
and its supposed to work, even for folks who have no clue what 'VSWR'
stands for.
The lesson is to stick with properly terminated twisted pair cabling if any
distance is involved.
Cheers, gene
--
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soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Now is the time for all good men to come to.
-- Walt Kelly
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