Connect with us

Differential signaling RS-485

Discussion in 'General Electronics Discussion' started by rasmus, Jun 19, 2017.

  1. rasmus

    rasmus

    30
    0
    Nov 6, 2014
    Hi

    I am currently reading about serial communication and i am kind of stuck with the "Differential Signaling" part. While i do understand how "single ended" signaling works, i am unable to fully comprehend how you can transfer two different signals using only two wires without a third return wire "return path". I have searched and found that a Differential Signaling "Driver" is basically just an H bridge (Correct me here) which has a constant current source and through the use of FET's it just flips the current flow direction on those two wires. what i cant understand is what happens at the receiver end and if i am correct about the H-bridge, where does the second signals come from ???


    Thanks in advance
     
    Last edited: Jun 19, 2017
  2. martin de jong

    martin de jong

    11
    1
    Jun 19, 2017
    You cannot send two different signals at the same time over an RS-485 connection, but many RS-485 tranceivers are bidirectional, and so can disconnect from the "Bus", this allows bidirectional signalling from one "master" to anyone of several "slave" receivers. Perhaps that is what you meant.
    And yes, you can compare differential signalling to a H-bridge in the sense that one line is driven high while at the same time another line is driven low. The receiver is normally a comparator which compares one line with the other to determine which voltage is higher.
     
  3. rasmus

    rasmus

    30
    0
    Nov 6, 2014
    Hi Martin.
    I got the idea clear on the software level, what is confusing me is the hardware level.

    So now that the Driver can be thought of as an H-bridge... how does the Receiver exactly work, what does it measure, does it simply measure the voltage between the two wires and regard to the current flow direction as an indication to the logic level, for example clock wise current flow = High, counter clockwise current flow = Low ????

    RS232 simply flips the voltage polarity for High and Low and the receiver measures the polarity and translates that to ether High or Low. If true for RS485, what is then the difference between singled end and differential signaling
     
  4. rasmus

    rasmus

    30
    0
    Nov 6, 2014
    Hi Martin.
    I got the idea clear on the software level, what is confusing me is the hardware level.

    So now that the Driver can be thought of as an H-bridge... how does the Receiver exactly work, what does it measure, does it simply measure the voltage between the two wires and regard to the current flow direction as an indication to the logic level, for example clock wise current flow = High, counter clockwise current flow = Low ????

    RS232 simply flips the voltage polarity for High and Low and the receiver measures the polarity and translates that to ether High or Low. If true for RS485, what is then the difference between singled end and differential signaling
     
  5. Bluejets

    Bluejets

    3,514
    713
    Oct 5, 2014
  6. martin de jong

    martin de jong

    11
    1
    Jun 19, 2017
    For the receiver RS-485 uses an electronic device called a Comparator:
    https://en.wikipedia.org/wiki/Comparator
    it is a kind of operational amplifier (OpAmp) designed for this specific function, determining which input is higher than the other, independent of the absolute value of the inputs (to a certain degree).
    Differential signalling is more robust than single ended signaling, is less affected by surrounding electromagnetic fields, and can support higher speeds over longer distances. Its normally used with twisted wire pairs. the twists annul the influence of an alternating magnetic field, as voltages induced in the wire cancel each other out.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    9,364
    1,901
    Nov 17, 2011
    Think of a battery connected to a battery via a switch. YOu can transmit a signal (state of the switch on or off) to the light bulb using only two wires.
    With RS485 it is ismilar. Assuming an ideal receiver with infinite input resistance, all you need to sense is the voltage differential between the two wires.
    With a real, finite input resistance the current flows to the receiver through one wire and back through the other.
    In an impedance matched system, the input resistance is deliberately set to a defined rather low value of e.g. 120 Ω.

    Nevertheless, it is usually good practise to provide a twirh wire as common ground to minimize signal distortion caused by different and possibly noisy ground potentials.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-