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should i use rs485 or other ?

Discussion in 'General Electronics' started by Quack, May 12, 2004.

  1. Quack

    Quack Guest

    hi,

    I need a group of PIC circuits (upto 32 in a group) to send data back
    to a 'master circuit' located elsewhere (they dont need to receive
    data, only send). Lets call this group a 'bank'.
    Communications must be reliable, and only a slow data speed is
    necesary, not a lot of data will be transfered, but there is a chance
    that some circuits will try to send at the same time.

    Each node in the group would be close (under 5 meters away from each
    other), but the master would be upto 300 meters away.

    I think RS485 would be the best thing to implement here, because of
    its signal range on a single twisted pair.
    So i could have a single twisted pair cable coming from the master to
    the first node, then to the second and so on.

    Should i be looking at something other than RS485 ?
    I have considered a standard TCP LAN, but the costs per-circuit for
    the interface, plus the extra configurations etc, all seems total
    overkill.
    should i be using standard rs485 or some other variation which allows
    128?
    impedence).
    using a standard max485, is there any chance of data collisions ? this
    must be very reliable!

    Do i need to use a max485 chip for each node, because they are only
    transmitting, can i not just use some simple parts to emulate the
    RS485 levels easily enough ?
    Its very difficult to get parts where i am, and i would go out and buy
    some max485's if i could, which i will order online - but for the
    prototype i am hoping i can just use a few transistors/resistors to
    get it going.

    For the master, i will probably use some kind of rs485 hub, which then
    converts to rs232 into a standard PC, because i may have multiple
    'banks'.

    I would like to find a RS485 hub which will accept 8 inputs (each of
    32 nodes), and 1 rs232 output. Anyone know of any ?
    A bit of googling has come up with 'one to many' hubs, ie: the master
    transmits, and all nodes will get the signal.
    I am after the opposite, where each node will send independently, and
    the master will get all incoming signals.

    I dont want to have to make one, im sure it must exist, or maybe they
    all do this, and its just far too trivial for them to specify it in
    their product specs :). Just want to be sure.

    Also, it would be nice to get one with memory. IE: the PC gets shut
    down, crashes, and the RS485 hub will store all incoming
    communications (upto a curtain amount) until the PC comes back online
    to receive them.
    Could be done easily enough (sbc + rs485hub), but again, must exist
    already ?

    Thanks for any help!

    Alex.
     
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