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TRS232E: Requirement series resistor for signal line

Ryuuichi machida
Guru 12945 points
Part Number: TRS232E


Tool/software:

Hello,

Sometimes, series resistor (case of driver hundreds ohm (ex, 680ohm), case of receiver several tens of kohm (ex, 47kohm)) implement 232 line to avoid entering much current to IC.
However, according to datasheet of TRS232E, there is no description about this.
Does it mean that user can remove these resistors ?
(It seems that receive side have input impedance typ 5 kohm and drive side have output impedance typ 300ohm. can these use for current limitation for IC ?)  

Best Regards,

  • 0
    Guru 318700 points

    With additional resistors, it is no longer guaranteed that the voltages at the receiver pins are large enough to be detected correctly in all circumstances.

    The device can tolerate the voltages specified in the absolute maximum ratings. If you expecter noise with higher voltages on the bus, add protection like TVS diodes.

  • 0 in reply to Clemens Ladisch
    TI__Guru* 77101 points

    As Clemens mentioned, adding resistors can distort the output and the voltage seen at the input. Large values like 10's of kohms would cause a large voltage divider to the RS232 input side and likely cause issues. I normally see customers use 50 ohms on the drivers to add some additional current limiting. I wouldn't go too crazy with this since it would affect the distance/data rate and the signal integrity (the voltage divider). Maybe up to a few 100s of ohms would be okay though I would try to keep it below a 100 if you can.

    If you're concern is overvoltage though, you will likely need to clamp the voltage using a TVS (this is for transients). You should avoid any long term over voltage situations to prevent damage to the part.

    -Bobby

  • 0 in reply to BOBBY
    Guru 12945 points

    Hello,

    I believe the Purpose that series resistor is implemented is to control current to IC input.
    I also think that user should implement TVS diode to signal line to avoid over voltage.

    >I normally see customers use 50 ohms on the drivers to add some additional current limiting.
    I think that this is for input protection from RS232 side to output pins. Not to control output current from driver IC.
    Because, this device has current limitation function for output current. (You define "Ios"(short circuit current) spec.)

    The answer what I expect is we do not need to implement series resistor due to current limitation function and several level input / output impedance.
    I understand that the one of reason you do not recommend to implement series resistor due to voltage drop.
    However, from viewpoint of protection, I would like to know your opinion about this. (If voltage drop is only concern, I think we just use high voltage output product such as TRS232E. And implement series resistor for protection from RS232 side.)

     Best Regards,

  • 0 in reply to Ryuuichi machida
    Guru 318700 points

    Protection against what?

    Transient voltages (that exceed the absolute maximum ratings) can be clamped with TVS diodes. When you have a TVS, then a small series resistor between the TVS and the chip can provide additional protection.

    DC voltages (that exceed the absolute maximum ratings) cannot be easily protected against.

  • +1 in reply to Ryuuichi machida
    TI__Guru* 77101 points

    My understanding of the IOS spec is the device will sink/source 10mA of current when the pin is shorted to GND. I don't think the device specifically clamps the current. 

    The absolute max table for the device states that the short circuit duration can be indefinite as long as only one short is occurring. So the device itself shouldn't get damaged if only one RS232 output were to be shorted to GND.

    Adding a series resistor on the output would help in a short to GND case where both outputs were shorted to GND since it limits the current.

    -Bobby