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SN75C189: About relation between Response and Threshold Control Inputs and threshold voltage

Masanori Tachibana
Genius 5350 points
Part Number: SN75C189

Hello support team,

I have two questions about SN75C189 and SN75C189A.
I am trying to change the threshold voltage by connecting a pull-down resistor "RC" and bias voltage "VC" to pins of Response and Threshold Control Inputs.

1. Could you teach me the relational expression between RC, VC and the threshold voltages VIT +, VIT-?
2. Could you teach me the relational expression between the capacitance CC connected to these pins and the response of output signals?

Best regards,
M. Tachibana

  • 0
    TI__Guru 62910 points
    Hello Tachibana-san,

    The front end of this device has a resistor divider in order to attenuate the RS-232 signals present at the input down to lower voltages that are more compatible with the IC's internal circuitry. You can see this divider as well as the connection of the response control in the equivalent input circuit diagram. This extra connection allows users to shift the effective switching threshold by changing this divider ratio (either by reducing the lower resistance by connecting a parallel resistor to it via the CONT pins or by increasing the upper resistance by connecting a series resistance prior to the input). A capacitor can also be connected to introduce a low-pass filter with time constant proportional to the capacitance and the upper resistance (nominally 3.4 kOhm if no additional series resistances are used).

    Max
  • 0 in reply to Miguel Robertson
    Genius 5350 points

    Hello Max-san,

    Thank you for your prompt reply.
    Your advice is very helpful for me.

    I understood that the voltage division ratio changes when the resistor is connected to the Response Control terminal.
    However, I couldn't understand where the threshold voltage decides from this circuit diagram.

    I want to know the mathematical formula between the pull-down resistance value and the threshold voltage.
    Also, I would like to know the detailed mechanism for determining the threshold voltage.

    Could you teach me more?

    Best regards,
    M. Tachibana

  • 0 in reply to Masanori Tachibana
    TI__Guru 62910 points
    Tachibana-san,

    I am trying to find more detailed information to share with you. Sorry it is taking some time - since this device is very old, it is difficult to find documentation on it. I need to confirm a few things with my team and then I will respond back to you within one or two days.

    Max
  • 0 in reply to Miguel Robertson
    TI__Guru 62910 points
    We believe that the threshold point is detected between the 1.5 kOhm resistor and the 530 Ohm resistor. We want to confirm this through lab testing, though, since we are unable to locate the original design information. We have ordered the units and should be able to conduct the testing in the next several days. Again, I apologize that this is taking some time to resolve.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Genius 5350 points
    Hello Max-san,

    I'm sorry for troubling you.
    We are waiting for information from you.

    Best regards,
    M. Tachibana
  • 0 in reply to Masanori Tachibana
    TI__Guru 62910 points
    Tachibana-san,

    Sorry for the delay. We did finally receive the units and were able to test them. Through this testing we confirmed that the voltage at the "response control" is what determines the output state of the receiver. If the response control input is left open, then the voltage required for switching can be computed based on taking the VIT+/- specs and dividing them by 2.67 (the voltage divider ratio determined by the resistor network shown in the equivalent input circuit diagram).

    If a higher or lower threshold is needed, a pull-up or pull-down resistor can be added to this pin. For example, if a pull-down resistor were added to the response control pin, it would cause the voltage divider ratio to increase. This would mean that a higher voltage would be needed at the input pin in order to reach the same threshold voltage at the response control pin.

    Best regards,
    Max
  • 0 in reply to Miguel Robertson
    Genius 5350 points

    Hello Max-san,

    Thank you for confirming by the experiment.
    And I'm sorry to trouble you.

    However, I do not understand the principle of operation well.
    I want to know more concrete numerical values and formulas.
    Is the division ratio “2.67” the result of the following calculation?
    (3.4 kohm + 1.5 kohm + 530 ohm) / (1.5 kohm + 530 ohm) = 2.67487....

    Please let me confirm regarding the operation of SN75C189.
    From the equivalent input circuit diagram, I guessed as follows.

    When Vin is applied to the input terminal, the voltage at point A becomes Va.
    Here, Va = Vin * 530 ohm / (3.4 kohm + 1.5 kohm + 530 ohm).

    When Va is lower than the emitter voltage of the transistor Q1, the collector current of Q1 flows and operates.
    As a result, Q1 operates so that the output signal of SN75C189 becomes "H".

    On the other hand, when Va becomes higher than the emitter voltate of Q1, Q1 is cut off and collector current of it does not flow.
    As a result, Q1 operates so that the output signal becomes "L".
    Is my understanding correct?

    If so, I think that the threshold voltage of Va will be about 0.1 V from the calculation result using the specification value of VIT +/-.
    Is my understanding correct?

    Furthemore, I think that the threshold voltage of VIT decreases when a pull-up resistor is added to the Response Control terminal,
    and that the threshold voltage rises when pull down resistance is added.
    Is my understanding correct?

    Best regards,
    M. Tachibana

  • 0 in reply to Masanori Tachibana
    TI__Guru 62910 points
    Tachibana-san,

    Yes, your understanding is correct on all points. You explained it much better than I did! I hope it is all clear now, but please let me know if you have further questions.

    Max
  • 0 in reply to Miguel Robertson
    Genius 5350 points
    Hello Max-san,

    I am relieved that my understanding is correct.
    What I did not understand was almost clear.
    Thank you for your help.
    If I didn't understand again, please teach me .

    Best regards,
    M. Tachibana