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Peripheral---Max distance — Parallax Forums

Peripheral---Max distance

StevezilaStevezila Posts: 35
edited 2009-03-20 18:30 in BASIC Stamp
Can anyone tell me what the maxium allowable distance is for peripheral devices over a cat 5 data cable? I want to connect several devices to a single stamp and in order to do this I will need to run cable out to the remote devices.

Thanks

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-03-20 15:59
    It depends on too many things like the speed of transmission of the data, the format of the data, the signalling technique and voltages involved, etc.

    What distance do you need and what's the signalling speed?

    True RS232 is good for hundreds of feet up to thousands of feet depending on the Baud. At the further distances, things like RS485 are better than RS232.

    You will not be able to just use logic level signalling for more than a few feet.

    Do have a look at 1-wire protocol from Dallas Semiconductor. This is designed for, among other things, wiring a large building with sensors. They have application notes on large networks, dealing with noise, cable losses, signal degradation, etc. (www.maxim-ic.com/products/1-wire/)

    Post Edited (Mike Green) : 3/20/2009 4:06:13 PM GMT
  • StevezilaStevezila Posts: 35
    edited 2009-03-20 16:22
    Mike,

    Thanks! Initially I will only need to drive an output or read an input and the concern was line loading and the ability to that read an input or drive an output at some distance before it drops below the TTL threshold. Timing for the input/outputs is not of concern at least at this point in time.
  • Mike GreenMike Green Posts: 23,101
    edited 2009-03-20 16:52
    Timing is an important issue. A long line has a lot of capacitance. An ordinary logic output working at 5V will take a long time to charge or discharge the capacitance of the line. You'll also get ringing on any signal changes. Without careful attention to timing, signal levels, ringing, impedance, drive capability, you are limited to a few feet.
  • StevezilaStevezila Posts: 35
    edited 2009-03-20 17:38
    O.K. so it's more involved than I thought. I really need to turn on/off a device and determine if a device is running or not. I don't really want to run conduit and power wires to do this. I was thinking of using a darlington array with local power and switch it via a ss relay·deiven by·an output and then possibly reading status via an input. What do you think is the best way to do this? Eventually I will want to send/read data but I lack the ability to do that at this point.
  • Mike GreenMike Green Posts: 23,101
    edited 2009-03-20 18:08
    What you're describing is quite doable over several thousand feet. You need a switching transistor like a 2N3904 or 2N2222A connected as shown in Nuts and Volts Column #6 downloadable from Parallax. The collector is connected to one wire in your cable and another wire connected to your Vdd supply. The other end of the two wires is connected to your SSR input which should be rated for anything up to about 24VDC and probably uses 10-20mA of current.

    Returning status would involve the same sort of circuit, but you could use an ordinary optocoupler instead of a SSR with the emitter and collector of the optocoupler's output connected to one pair of wires at the distant end. At the Stamp end, the emitter would be connected to ground and the collector connected through a 220 Ohm resistor to Vdd (+5V). The collector would also be connected through a 1K resistor to a Stamp I/O pin.
  • StevezilaStevezila Posts: 35
    edited 2009-03-20 18:30
    Mike,
    Thanks that's just the·piece that·I was missing. Thanks so much for your help.

    Steven
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