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RF Link question — Parallax Forums

RF Link question

Ken PetersonKen Peterson Posts: 806
edited 2007-10-22 12:09 in Propeller 1
I see Sparkfun.com has RF link transmitter/receiver pairs for a very reasonable price. Has anyone here used them? They have two frequencies: 318 and 434 MHz. Any idea which one would be better? Both are 4800 baud.

www.sparkfun.com/commerce/categories.php?cPath=16_79

Thanks!

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The more I know, the more I know I don't know.· Is this what they call Wisdom?

Comments

  • WhelzornWhelzorn Posts: 256
    edited 2007-10-22 02:32
    It doesn't really matter, but 318 seems to be less common (i'm not sure though). So you might get less interference than with 434, which is used in garage-door openers etc.
    anyway, I've used the 434MHz 4800 baud link. it works well, but interference was a serious problem for me. So I wouldn't use it to transmit files or anything like that. I'd try sending some text from one stamp/prop whatever to another one, and send the output to your PC to watch it. If you can send a good amount of text without many junk characters, you should be fine. If you do get a lot of trash though, you might want to consider another wireless solution. But then again, it completely depends on your application.
  • Ken PetersonKen Peterson Posts: 806
    edited 2007-10-22 02:53
    Whelzorn, what kind of range were you operating at? I'm just hoping to get a signal from one side of my house to the other. It's sort of a home automation type application that I have in mind.

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    The more I know, the more I know I don't know.· Is this what they call Wisdom?
  • Timothy D. SwieterTimothy D. Swieter Posts: 1,613
    edited 2007-10-22 11:10
    I am not sure about the range of the SparkFun device. I thought they had an app note or tutorial on the range of their RF products?

    On a differnt RF 434MHz. system, one·that·was designed from scratch,·I am getting 80+ M without an amplifier on the transmitter. This is strong enough to go through several walls to another floor on the opposite end of an office building where I tested it. I thought of the engineers said it was 10 dB?

    As far as data integrity - when transmitting by RF you should always add some sort of checksum or CRC calculation to the data. I created some CRC-8 code the other day for my RF system. I need to review it and finalize it and maybe get permission before I share it with the forum. There are some great references to CRC on the web where you can learn to code your own routine by looking at C++ examples. The transmitter would add a CRC to the end of the data being transmitted. The receiver would receive the data, calculate a CRC and then compare it to the CRC it received over RF. If it is the same CRC place the data into a buffer to get used by the rest of the software. If it is a different CRC toss the received data out and notify the main program of a error in reception (if error reporting is important).

    I like CRC because of the math behind them.· I don't fully understand the math, but I trust it.· There are other ways to verify data integrity.· You could use a checksum, but there are faults with that.· I suppose a "poor mans" way of varifying a transmission would be transmit a packet x times.· The receiver then needs to receive the exact same packet y times before saying it is good and passing it on to the rest of the program.· Y is slightl smaller than x.· For instance, setup the transmitter to send a packet 10 times.· The receiver needs to receive 8 of the exact same packet before it passes it on.· One could get fancy with the software on the receiving end or simply look for y number of correct receptions in a row.

    I hope this helps!

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    Timothy D. Swieter
    tdswieter.com
    One little spark of imagination is all it takes for an idea to explode

    Post Edited (Timothy D. Swieter) : 10/22/2007 11:21:09 AM GMT
  • Ken PetersonKen Peterson Posts: 806
    edited 2007-10-22 11:57
    Thanks, T.D.S.. I am familiar with checksums and CRC checks and I would most likely use some form of error detection. I guess what you use depends on your computing resources and how critical it is to get error-free transmissions. X-10 just uses a redundant message, i.e., the message is sent twice. Perhaps a parity and simple checksum would be sufficient to catch most errors.

    I guess if people are having OK success with them, then $17 isn't a bad price to pay. I'm just wondering why there are two different frequencies available in the same product, unless it is to enable two-way transmission.

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    The more I know, the more I know I don't know.· Is this what they call Wisdom?
  • Timothy D. SwieterTimothy D. Swieter Posts: 1,613
    edited 2007-10-22 12:09
    There are different frequencies because your particular application may require different frequencies or different throughput. The allowable ISM bands (Industrial, Scientific, Medical) may vary from country to country so there are two different products to work in those different regions. I am not 100% sure on this, but the FCC for the USA may allow different power requirements for these different frequencies.

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    Timothy D. Swieter
    tdswieter.com
    One little spark of imagination is all it takes for an idea to explode
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