DS1820 1-wire Temp Sensor
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I am having trouble communicating with the ds1820 1-wire temp sensor
over a long distance. Can anyone help me out. I've been away from the
electronics field for a while and don't really remember how a lot of
these devices work as far as for instance the slew rate being
affected and such.
over a long distance. Can anyone help me out. I've been away from the
electronics field for a while and don't really remember how a lot of
these devices work as far as for instance the slew rate being
affected and such.
Comments
On Fri, 16 Nov 2001 04:12:53 -0000 xlir@m... writes:
> I am having trouble communicating with the ds1820 1-wire temp sensor
> over a long distance. Can anyone help me out. I've been away from
> the
> electronics field for a while and don't really remember how a lot of
>
> these devices work as far as for instance the slew rate being
> affected and such.
>
>
>
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catagory 5 tel cable with 3 pairs connecting one pair to the data
line and ground and another wire for 5v supply. I think the wire is
probably 26awg
--- In basicstamps@y..., kelvin l schroeder <kls48@j...> wrote:
> how long is a long distance and what size wire
>
> On Fri, 16 Nov 2001 04:12:53 -0000 xlir@m... writes:
> > I am having trouble communicating with the ds1820 1-wire temp
sensor
> > over a long distance.
>over a long distance. Can anyone help me out. I've been away from the
>electronics field for a while and don't really remember how a lot of
>these devices work as far as for instance the slew rate being
>affected and such.
The chip requires a pull-up resistor. The slew rate is determined by
the resistor in conjunction with the capacitance of your cable. The
RC time constant has to be less than about 10 microseconds, or else,
it won't work.
In order to get better results on a long cable, use a lower value for
the pullup resistor. The nominal recommended value is 4.7 k, but you
can go down to 2k or even 1k in a pinch. The rated sink current into
the output pin is 4ma for 0.4 volt Vlow.
-- regards,
Tracy Allen
electronically monitored ecosystems
mailto:tracy@e...
http://www.emesystems.com
in between. An app note from dallas semiconductor mentions and
active pull up. Can you give me some specifics on this. Like I say
it's been a whalie since I've done this sort of thing.
--- In basicstamps@y..., Tracy Allen <tracy@e...> wrote:
> >I am having trouble communicating with the ds1820 1-wire temp
sensor
> >over a long distance. Can anyone help me out. I've been away from
the
> >electronics field for a while and don't really remember how a lot
of
> >these devices work as far as for instance the slew rate being
> >affected and such.
>
> The chip requires a pull-up resistor. The slew rate is determined
by
> the resistor in conjunction with the capacitance of your cable.
The
> RC time constant has to be less than about 10 microseconds, or
else,
> it won't work.
>
> In order to get better results on a long cable, use a lower value
for
> the pullup resistor. The nominal recommended value is 4.7 k, but
you
> can go down to 2k or even 1k in a pinch. The rated sink current
into
> the output pin is 4ma for 0.4 volt Vlow.
>
> -- regards,
> Tracy Allen
> electronically monitored ecosystems
> mailto:tracy@e...
> http://www.emesystems.com
>I going about 1000'. The cable I was using to test it with was
>catagory 5 tel cable with 3 pairs connecting one pair to the data
>line and ground and another wire for 5v supply. I think the wire is
>probably 26awg
....
>I have tried changing the pullup resistor all the way down to 1k and
>in between. An app note from dallas semiconductor mentions and
>active pull up. Can you give me some specifics on this. Like I say
>it's been a whalie since I've done this sort of thing.
First thing: It does work when you have your DS1820 right next to the
Stamp or on a very short cable, right?
1000 feet of cat5 may be out of the question. I don't have the
capacitance per foot at hand, but it may be just too much for a
simple pullup resistor. The basic speed of the one-wire system is
similar to 9600 baud RS232, and one would never consider running 9600
baud open drain mode over 1000' of cable. I have heard that there
are line amplifiers available for the one-wire stuff, but I don't
have a reference for you. Maybe Jon or someone else here will know,
or you can poke around on the Dallas or One-wire sites for
suggestions.
The stuff about active pullup in the data sheet addresses a different
problem (parasite power), which does not apply to your setup, because
you are supplying the 5 volts separately over a wire in your cable.
Be sure you have a 1uf capacitor across the supply from Vss to Vdd at
the DS1820, but I don't think that will resolve the problem.
-- Tracy
>
>--- In basicstamps@y..., Tracy Allen <tracy@e...> wrote:
>> >I am having trouble communicating with the ds1820 1-wire temp
>sensor
>> >over a long distance. Can anyone help me out. I've been away from
>the
>> >electronics field for a while and don't really remember how a lot
>of
>> >these devices work as far as for instance the slew rate being
>> >affected and such.
>>
>> The chip requires a pull-up resistor. The slew rate is determined
>by
>> the resistor in conjunction with the capacitance of your cable.
>The
>> RC time constant has to be less than about 10 microseconds, or
>else,
>> it won't work.
>>
>> In order to get better results on a long cable, use a lower value
>for
>> the pullup resistor. The nominal recommended value is 4.7 k, but
>you
>> can go down to 2k or even 1k in a pinch. The rated sink current
>into
>> the output pin is 4ma for 0.4 volt Vlow.
>>
>> -- regards,
>> Tracy Allen
>> electronically monitored ecosystems
>> mailto:tracy@e...
>> http://www.emesystems.com
>
>
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