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Thread: Voltage dividers and data signal

  1. #1
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    Voltage dividers and data signal

    Hi, I was studying Paul's DMX Signal Transmitter and Receiver for a project and it's hard for me to understand the role of the voltage dividers applied to the data signals.

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    (Sorry if the schematics are messy)

    For what I understand the input signal on the transmitter and the output signal on the receiver are tied to the center of a voltage divider (5v-47K on one side, 100K-GND on the other). Paul explains on the transmitter's OSH Park page that the output of the receiver outputs a 5v signal, so I think those voltage dividers are there to step up the signal from the Teensy on the transmitter and from the MAX485 on the receiver from 3.3v to 5v, by adding 3.4v to it, but that doesn't mean that there's never gonna be a 0? How does this works? Are my schematics wrong? Isn't also 6.7v (3.3+3.4) an excess of voltage?

  2. #2
    The transmitter input does not have a voltage divider. There is a 47K to Vcc and 100K to ground. Your input signal will pull the MAX485 pin to 0V and 3.3V, more or less, and those resistors will not make much difference at all. The MAX485 will recognize any input voltage over 2.0V as a high so nothing needs to be done to drive it from a 3.3V source.

    On the receive side I would expect there to be a resistor in series with the MAX485 output. As shown, there is no voltage division. The MAX485 output current is sufficient so that the resistors do not provide enough load to reduce the output voltage below 3.3V.

  3. #3
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    Ok, so why are those resistors there?

  4. #4
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    Quote Originally Posted by maurobarreca View Post
    I think those voltage dividers are there to step up the signal from the Teensy
    On the receiver (top diagram), the chip is being supplied by 5v (called Vcc). So the output may rise to 5 volts. That's outside the compatibility for some Teensy types (ought to be 3v3). So the 47K and 100K are acting as a divider - from what comes out of the output - and limiting this to no more than 3v3. The division ratio is 100/(100+47) = two thirds approx.

    Same is true for the transmitter. Without the divider (same ratio), then P6 input pin (high impedance) would "float" when not connected - not good practice - should always be either a logic 0 or a logic 1. Standard practice is to pull high - but that would pull up to 5 volts, which again is outside compatible range for some Teensy types. By using a divider with same ratio, then P6 never gets pulled higher that 3v3. At the instant of connecting anything to that P6 pin, you can then be sure that this board will not damage the Teensy - not even momentarily.

  5. #5
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    Ok, thank you for your kind and well explained answers. I'll look into it to try and understand it by myself. It's difficult for me to imagine the current flow in this case, but I suppose its a very basic circuit. I know now that the circuit will work for my project.

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