Switching RS-485 with analog switches

Discussion in 'The Projects Forum' started by Avshi, Aug 22, 2016.

  1. Avshi

    Thread Starter New Member

    Jan 16, 2015
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    Is it a good idea to switch an rs-485 signal with THIS analog switch or any analog switch? If no, can it be done at all in a reasonably inexpensive fashion?
    Thanks,
    Avi
     
  2. Marley

    Member

    Apr 4, 2016
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    40
    The supply voltage range is not high enough on you suggested analog switch. Expect RS485 signals to be between 0 and 5V even though a differential of only ±200mV is required for signalling.
    Why do you need to do this?
    RS485 can be used in a bus topology using multiple transmitters and receivers. However only one transmitter will be transmitting at any time.
    In fact RS485 only specifies the "Physical Layer" (the line voltages, etc), it does not specify any "Communications Protocol" that would be required to do this.
     
  3. Avshi

    Thread Starter New Member

    Jan 16, 2015
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    I'm using a transmitter that is powered by a 3.3V supply (here). My intention is to connect a bunch of sensors via wires and I don't want to limit the cable length coming from the board. The bus topology isn't right for my use. The stub length on the bus is supposed to be quite small (about 6 inches) as per the application note for rs485 from st (here). Even if I were to just communicate with one sensor at a time, the connection to other sensors would cause ringing. I'll be communication to sensors through an ethernet hub.
     
  4. Marley

    Member

    Apr 4, 2016
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    I see. You want a point-to point connection for each sensor.
    Instead of using analog switches, why not use a number of the ST4485EB ICs (one for each sensor) and select them using the /RE and DE pins?
     
  5. Avshi

    Thread Starter New Member

    Jan 16, 2015
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    Simply because the IC is large. I don't have that much space. I'm on a 10cmx10cm board and having 12 of these ICs would be impossible I think. Maybe I should look for a smaller one, but I don't think it's likely that a smaller one exists at this datarate.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    Note that while the normal signal voltage range is 0 V to +5 V, the allowed range is -7 V to +12 V.

    ak
     
  7. Avshi

    Thread Starter New Member

    Jan 16, 2015
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    I looked at the data sheet for the switch a little more closely and Vin for the switches must be 0V to 4.3V so that's a bust. I'll just going to use p and n mosfets
     
    Last edited: Sep 1, 2016
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