Sensing and Control for hobbyists and schools: Connecting using 1-Wire (aka MicroLan)

    Cat 5 is best.
    Magic Resistor Cure for Unreliable 1-Wire LANs.
    iButtonLink- cure all?
    Wiring conventions- which wires carry what signal?
    How big can 1-Wire LAN be? What topologies?
    Special notes applying to Dallas windspeed / direction sensor.

This topic is making trouble for me in trying to organize this site nicely for you.

The MicroLan is a good way to connect sensors and actuators to a computer... but the MicroLan itself is connected to the computer through an adapter. There are adapters to connect via the serial port, the parallel port, and via USB. If that isn't enough choices, there are also devices which do more than just connect the MicroLan and the PC... they have some internal intelligence themselves, and "interpret" between reasonably "English-like" dialog on the PC side, and the more esoteric needs of the devices on the MicorLan side. These are discussed in my pages devoted to serial, parallel and USB connections to sensor/ actuator arrays.
One set of tools you may want to use in sensing and control projects is the 1-Wire or MicroLan local area network, and the chips that are made for it. Both terms are trademarks of Dallas Semiconductor, now merged with Maxim. There's lots more about this at my Dalsemi-promoting page. You may also be interested in the relevant discussion groups and forum archives.

For 1-Wire LANs, Cat-5 cable seems to be the way to go. There's more about Cat-5 in the page "above" this one. There are a number of suggestions here about increasing the reliability of 1-Wire LANs. Please don't take them as implying that 1-Wire is UNreliable.... they're just ways around the problems that can (who knows how often) arise..

If you are having trouble getting reliable behavior from a 1-Wire net, and you have looked carefully at the usual suspects, try the following. I've yet to see a full explanation of why it should work explained... but for several people it DID work: Try adding a low value resistor (I've seen 100 ohms mentioned; I've seen 20 ohms) in series with the data line and/ or the ground line. Try the one on the data line first. Mount them about a foot from the DS9097. They are said to help avoid ringing and overshoot, and to reduce the chance of ground loops.

If you bought Dallas's excellent 1-Wire evaluation kit, you will have two "curly" leads, two leads like a self coiling telephone handset lead. If they work, great. They are handy for desktop experiments. Some users have reported that they were the source of problems when used as part of larger LANs.

Another possible reliability-enhancing step: Replace your DS9097 1-Wire to PC interface with an "iButtonLINK", at least I think that's the name for it. Click here to visit their site, at the iButtonLINK domain. (I haven't, by the way, seen anything to suggest that their use of the term "iButton" in their domain name has the blessing of Dallas (trademark owner), but that doesn't stop the fact that for about $45 you can have a spare, possibly better 9097!) I haven't tried one myself, but I don't think your software is going to "see" any difference, unless you've used a silicon serial number which is part of some 9097s. In that case, you'd only have to tell the software about the new silicon serial number, which I suspect will be present in an iButtonLINK.

N.B.: The iButtonLink puts 5v on one of the outer pins of the connector, unlike the DS9097. Be sure that does not conflict with uses any of your 1-Wire modules may have made of that connector.... there are some that would. (It is easy enough to not connect to that pin of the connector, if need be.)

How big? What Topology?

In THEORY, you can build quite big networks using 1-Wire. I haven't seen any definitive statement on what limits exist regarding topology. (I.e. the layout and connections of the wiring.)

People do report happy experiences with LANs extending over scores of meters.

Coming back to topology: Suppose you want a temperature sensor in each room of a small house. Do you have to snake a single wire in one "line" from A, to B, to C...? No. You can have several on one wire, several on another, others on their own wires. By this I mean that you may have several "strings" of sensors leading into the PC from different directions... in LAN terms, a star topology is allowed. In electronic terms, each 1-Wire device is connected in parallel with all of the other 1-Wire devices.....

...but. (Yes, 1-Wire is part of the real world. There's always a but.) The good "but": You can be fancier than simply having everything in one big net of parallel connected devices. More on this in a moment. The bad "but:" The bigger your net gets, the greater the chances of it not working as it should.

More complex nets can be built using hubs, which I explain on my hubs page. They are usually based on the DS2407 switch.

There's an essay about using hubs in pdf format available from the preceding link. (I spent many hours on that, posted it to the web, and then forgot to put up any links to it for 5 months! I was beginning to think I'd lost more than the document when I started trying to find it.) There's also a zip file with the Delphi source code described in the article.
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