Re: [Emc-users] OT -- Ethernet thermometer?

[Peter C. W. <pc...@me...>]

On Sat, 23 Jul 2011, gene heskett wrote:

> Date: Sat, 23 Jul 2011 11:53:24 -0400
> From: gene heskett <ghe...@wd...>
> Reply-To: "Enhanced Machine Controller (EMC)"
>     <emc...@li...>
> To: emc...@li...
> Subject: Re: [Emc-users] OT -- Ethernet thermometer?
> 
> On Saturday, July 23, 2011 11:33:03 AM Przemek Klosowski did opine:
>
>> On Fri, Jul 22, 2011 at 3:15 PM, Igor Chudov <ic...@gm...> 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.

Yea. probably a 8 pin PIC or AVR uController with a serial say 9600 baud 
RS-422 interface, with power and comm sent on the CAT5 (say double up the 5V 
and ground wires)  local 3.3V LDO regulator so the cable 5V can droop to 3.3V 
without harm and a big cap on the remote 5V so the peak power needed to send a 
short packet into the terminated wire (30 mA or so) is supplied by the cap. 
This ought to get you to a couple hundred meters. Low average power is 
important not just for cable length but to reduce self heating if the 
temperature sensor is on board.


(a sensor on any kind of Ethernet board will be way above ambient)

>
> Cheers, gene
> -- 
> "There are four boxes to be used in defense of liberty:
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>
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Peter Wallace
Mesa Electronics

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