Dallas 1-Wire Problem/question

Hello all. I have an IOM142A board from phanderson.com that I have been using for a couple of years now. I put temperature sensors in every room of the house, as well as the garage and basement. Overall it works pretty well except that I am getting a lot of read errors on certain temperature sensors.

I have three wire runs. The first run is connected to the first input on the IOM142A and has six sensors. It is a single line and ends in the attic with a single temperature sensor on the very end of the line. The second and third runs consist of a single sensor at the end of each and are connected to the second output on the IOM142A. However, I was forced to use a star configuration on these last two.

My problem is that the sensors on the end of each wire run have a lot of read errors. Occasionally some middle sensor will log a lot of errors, but generally they go weeks without a single error but the end sensors log hundres of errors in that time (my software issues a read every 15 minutes).

All the wire is CAT5E. The sensors are soldered to a three inch piece of CAT5E which is crimped to a modular plug with gold contacts. The total length of the run with six sensors is less than 100 feet. The two runs in the star configuration are approximately 15 feet and 35 feet respectively.

It's strange to me that it's only the end sensors that have problems. Should I be using some type of terminating resistor at the end of the line? Any suggestions?

Thanks. Bill

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Maybe this link will provide you some way of improvement.

See :

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To honest, I'm absolutly not familiar with one wire technology.I just discovred last night some explanation on the referenced site, including the possibility to interconnect one wire devices with homeseer .

Hope it helps


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Does the IOM142 support the Checksum data on the 1wire bus? Is the turbo mode on or off? Are you using a single pair of the cat5 cable. The Star configuration probably doesn't help much. Read errors are usually easy to detect, just throw away the data and request it again from the 1wire device.

It does sound like reflections from the remote sensor, not aware of any active termination for 1wire stuff.

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William at MyBlueRoom

First thing I'd try would be a 4.7K pullup resistor at the end of the line (4.7K from the data line to 5 volt supply). This is different from a "terminating resistor" which would be installed from data to ground. If that doesn't fix it, read on ...

This application note:

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recognizes that great care is needed in programming to achieve very high reliability with 1-wire.

But your problem seems more severe, and on the order of the severe problems addressed in this app note that is on my web site and seems to have disappeared off the Dallsemi/Maxim site..

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This app note dated March 2000 addresses what Dallsemi called "peculiarities" and "inaccuracies" and is especially relevant if you should happen to have some older DS1820 sensors that were (in my blunt assessment) defective. Unfortunately, it may not always be possible to distinguish good sensors from the 'bad' ones by their markings. As I understand it, you don't want DS1820's at all and you want DS18S20's with rev B5 or later.

What happens when you swap the misbehaving sensors with ones that are seemingly reliable?

Also: you don't say if you are using these in parasitic mode. My advice is that you don't.

Also, make sure that your supply voltage is adequate with no glitches at the end of the line.

HTH ... Marc Marc_F_Hult

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Marc F Hult

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