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MAX3221-EP: about input resistance value of receiver

minoru.awamura.xu
Prodigy 40 points
Part Number: MAX3221-EP

He Guys,

Our project requires to convert an RS232 signal into TTLorCMOS and vice versa.

RS-232 standard indicate to input resistance of receiver.(value:3kΩ~7kΩ)

Why is this device's input resistance of receiver out of range?(value:3kΩ~11kΩ)

Could you teach me, please.

  • 0
    TI__Mastermind 46607 points

    Hi Minoru,

    So RS-232 requires a minimum of 3K Ohms -  and generally the vast majority of devices are spec'd 3K to 7K. However some devices, such as this one, can have a higher maximum input impedance. It's not going to negatively impact the application - as the worry for communication issues and/or damage is when the RIN pin has less than 3K ohms - an RS-232 driver will be able to drive the higher impedance load as its actually less stressful for the driver overall so there really isn't a concern.

    Essentially the limiting factor is the minimum impedance not the maximum. Most devices have a typical of 5K with +/- 2K - but some select devices can have a higher max - but this should come as no detriment to the system at large.

    Please let me know if you have any other questions!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 40 points

    Hi Parker,

    Thank you for your prompt reply.

    I understood that important thing is the minimum value of input resistance because High input impedance can maintain voltage level.

    I have an additional question.

    Why did RS232 standard determined upper limit of input resistance?

    Is the upper limit of input resistance set in consideration of noise?

    Best regards.

    Minoru Awamura

  • 0 in reply to minoru.awamura.xu
    Guru 275410 points

    The nominal input impedance is 5 kΩ. The minimum of 3 kΩ ensures that the input current is no more than 5 mA (at the maximum voltage of 15 V); the maximum of 7 kΩ was chosen somewhat arbitrarily, just to get the same ±2 kΩ tolerance.

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points

    Hi Clemens,

    Thank you for your prompt reply.

    I understood why there is an upper limit for input impedance.

    My problem solved.

    Thank you very much Clemens and Parker.

    Minoru Awamura