Multiple RS232 devices to RS485 converter & vice versa in end

Discussion in 'General Electronics Chat' started by embedded.world, May 26, 2014.

  1. embedded.world

    Thread Starter Member

    Feb 27, 2014
    36
    0
    I have to design a machione which multiple boards sensing data & then send it out to user.

    1. Each sensor will read data & then send data out on RS232.
    2. 16 such sensor are there.
    3. I have to design a single bus out. So each sensor send data on RS232 which I want to convert to RS485.
    4. Now this RS485 coming out has to read back & individual data can be read of sensor.


    Query:
    1. How to convert each RS232 data ouit to single RS485 bus, is there any IC for this.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    Use a MAX232 or similar to communicate with the RS-232 sensor.
    Interface this to a microcontroller.
    Repackage the data into a communications protocol that includes senor address, data and checksum.
    Send the data via RS-485 using SN75176 or similar.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,532
    1,251
    1. There are many complete devices you can buy that do this. At the circuit level, usually this is done with two chips. One converts RS-232 to TTL and the other converts TTL to RS-485. For multiple channels there are dual and quad versions of these chips. Maxim has a single ship - MAX3162 - that looks like it will do what you want for one channel in one chip, but I've not used it before.

    ak
     
    Last edited: May 28, 2014
  4. embedded.world

    Thread Starter Member

    Feb 27, 2014
    36
    0
    I have selected SN65HVD78.

    1. since I am sending data from uart. So all MCU has varaition of internal osc of 1%. So should I connect external oscillator in these.

    2. Do termination resistance is required on both ends of cable or only one termination resistance is requited. Datasheet of SN65HVD78 shows two as on Page 13 of datasheet
     
  5. Alberto

    Active Member

    Nov 7, 2008
    169
    36
    If you will operate at 9600 bauds or less you will have no issue using the MCU internal oscillator. At higher bauds rate an external quarz or resonator is required.

    Yes , with rs485 you will need resistor terminators at both end

    Cheers

    Alberto
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I disagree about the termination networks. Maxim app note on RS-485 termination:

    http://pdfserv.maximintegrated.com/en/an/AN1090.pdf page 5

    Basically, if your cable length is under 100 ft., termination isn't needed. Either Linear Tech or TI have similar app notes.

    ak
     
  7. embedded.world

    Thread Starter Member

    Feb 27, 2014
    36
    0
    1. Do termination resistance depend upon length of cable or it only depends upon Z0 of cable which is given & fixed.

    2. Suppose I get a 100meter cable with z0 = 120ohm labelled & cut that cable into 10meters which is my requirement will Zo change or still I have terminate with 120ohm resistor
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,532
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    The characteristic impedance of a cable is independent of length. A cable with Z0=120 ohms will have a 120 ohm impedance no matter how long it is.

    However, whether or not the characteristic impedance is an important factor to deal with is dependent on cable length, the bandwidth of the signal, etc. A cable that is not properly terminated will generate reflections, but the reflections might be so small that they can be ignored in some situations. That is what the app note explains.

    My guess is that at a length of only 10 meters you can ignore terminations.

    ak
     
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