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Multi drop serial bus

Discussion in 'Electronic Basics' started by Doorman352, May 5, 2004.

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  1. Doorman352

    Doorman352 Guest

    I asked a week ago about how to implement a multidrop design to use a serial
    bus to communicate between multiple boards scattered across an area. I got a
    lot of great ideas...the best seemed to be RS485.

    I don't seem to be able to find anything on the development of such a
    bus....or the means to utilize it in a scenario.

    I found tons of data on what it does and can do, and a few schematics, which
    lack enough detail to derive my configuration.

    I am trying to build a master controller and a series of sub boards, that
    are addressed and controlled. I want to have 2 way communications with the
    boards and be able to reset all or 1 if necessary. I am trying to avoid
    embedding PICs in my design as I can't simulate my schematics with PICs.

    I can't find any simulation programs that can simulate an RS485 chip

    I am not a heavy designer, nor am I a heavy application developer. I just
    want to build a crude prototype so I can see if my idea is feasible.....and
    I'm not getting anywhere.

    Anybody with a whole lot of patience and a few minutes want to bounce some
    ideas back to me...... Please.

  2. CFoley1064

    CFoley1064 Guest

    Subject: Multi drop serial bus

    I read you as saying you want to have a master controller communicate with
    multiple "sub boards" without having a PIC or other processor on the "sub
    board" side to read and interpret the communication, as well as doing error
    checking and resetting the I/O on command.

    Just about the only way you're going to be able to do that with LSI/MSI ICs and
    discrete components is to make a PIC or other rudimentary processor out of
    those components. If you just want to use the RS-485 balanced transmission
    technique to set up remote shift registers and such, you're going to need a lot
    more than one pair of wire.

    The way to emulate a processor in a circuit is to use an emulator written for
    that processor. The way to test and debug a program you've written in a
    circuit is to use an ICE (In-Circuit Emulator). Both of these are available
    for very reasonable prices for the PIC.

    If this is a real project and not just a school exercise, I'd like to recommend
    that you Google "RS-485" +"Remote I/O", and look at the results. There are a
    number of manufacturers that make these remote I/O boards already built and
    tested for less than $100 USD each. And yes, all of them have PICs or other
    low end processors on board to handle the comm chores. Get an RS232-to-RS485
    adapter for your PC serial port from the same source, and you've now got
    nothing more than a programming problem.

    Good luck
  3. Hi, RS485 is only the hardware part of a bus. On that bus a prorocol
    must run. You can do asynchronous communication (what one often
    does with RS232 hardware) or synchronous communication like
    several networks, Profibus, Bitbus etc.
    So RS485 is not a bus, it is only the hardware.
    I will show an example here I often worked with:
    the good old Bitbus (from 1984, way ahaed of its time, people never
    really understood the possibility's): It uses RS485 as hardware layer.
    The software talks with synchronous SDLC packets, a subset of HDLC,
    looks like something on your Ethernet. It is a single master system
    (no collision detection) but can have outstanding messages slaves can
    reply to, and therefor still have 2-way communication. The chip I
    often use is the 8044 (Intel used to make it, we distribute it a
    replacement device for it): it already contains the iDCX51
    multitasking operating system with a pre-programmed Bitbus
    communication task. One of the things that task can do is a reset of
    the microprocessor, as you asked. It can also read/write memeory, i/o,
    download firmware, start and stop tasks etc. And that all over the
    rs485 network at 380 KBit, or 1.5 MBit.

    So you see: it probably is more work than you think! But there already
    are solutions. Several.
    So what do you want to build?

    Pieter Hoeben
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