This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ISOW7844: The best selection between isolator and optocoupler

Part Number: ISOW7844
Other Parts Discussed in Thread: ISO1211, ISO7741

Hi Everyone,

I want to know the correct selection to use between isolator and optocoupler  where microcontroller needs a PTT control signal received for a long cable and activated by a switch.

Thanks

  • Hi Samuel,

    Thank you for reaching out and for showing interest in ISOW7844.
    Did you mean to ask if a digital isolator would be a better fit for an application to isolate a PTT control signal or if an optocoupler is better?

    I am not familiar with PTT control signal, could you please share more details to be able to comment better?
    What are the HIGH / LOW voltage levels of this signal and how long is the cable?


    Regards,
    Koteshwar Rao

  • Hi Koteshwar,

    PTT stand for Push To Talk, this signal is received by the microcontroller to recognize that peripheral or remote device needs to establish a communication. Signal is discrete and has two states,H/L where peripheral has an electromechanical switch that selects any state. Length of cable between microcontroller and remote device could be 2 meters.

    Below reference block diagram:

    Thanks

  • Hi Samuel,

    Thank you for sharing detail related to PTT and your application, this helps me understand the requirement better. May I know what are the H/L voltage levels that are expected on PTT signal? If this is going to be similar to TTL/CMOS logic levels with voltages 3.3V or 5V then any digital isolator can be used to isolate the PTT signal and interface it to MCU.

    Such inputs are called Field Digital Inputs or DI channels in standard industrial terminology and these signals can be of 24V (as defined in IEC 61131-2), 36V, 110VAC, 230VAC or any other voltage. We do have a dedicate device to isolate such signals with ease, ISO1211. The device ISO1211 also doesn't require any power supply from the field side and is self-powered by the input control signal itself. ISO1211 is compliant to IEC 61131-2 standard and additionally can support much higher voltages as well.

    Please do take a look at the ISO1211 datasheet and below technical article to get more idea on how to use the device in your applications. I have also included a link to a calculator that helps you select resistor values to choose desired input thresholds.
    Let us know if you have any further questions, thanks.

    https://www.ti.com/product/ISO1211

    How to Design Isolated Comparators for ±48V, 110V and 240V DC and AC Detection

    ISO121x Threshold Calculator for 9V to 300V DC and AC Voltage Detection


    Regards,
    Koteshwar Rao

  • Hi Koteshwar,

    Levels are indifferent but could be +5VDC for high and 0VDC for low state. I want to know if MCU is activated by low voltage and the isolator received the signal from remote unit by means of closing the contact of switch where it closes loop or  0VDC return across run cable. In other words, if activation is low, Could output of isolator guarantee 0VDC over long cable?.

    Thanks

  • Hi Samuel,

    Thanks for sharing additional inputs.
    I am not sure if I understand what you meant when you said "if MCU is activated by low voltage".

    Please allow me share more details on how digital isolators work and maybe that might answer your questions. All TI digital isolators have TTL/CMOS logic I/Os, these are the same I/Os (commonly referred to as GPIOs) that you generally see on MCUs. All digital signals that MCU transmits or receives can also be transmitted or received exactly the same. In most cases, the logic I/O thresholds for MCU and digital isolators are going to be almost the same. Hence, digital isolators are truly digital devices that can isolate digital signals.

    If the long cable creates 0V/5V voltage levels at the input of digital isolator upon opening or closing the switch irrespective of how long the cable is, the digital isolator is going to accept it as a valid signal and accordingly provides the isolated output. This is the basic function of the isolator and hence it is expected to do the same 100% of the time. Please refer to the Function Table in ISO7741 datasheet to understand device function.

    All digital isolators CMOS inputs have their input thresholds as fixed function of their supply voltage while ISO1211 is designed to allow the thresholds be adjustable by external resistors. This enabled ISO1211 accept >5V and with proper design, can also accept voltages upto 300V.

    Let me know if this answers your questions or if any of the above statements aren't clear. Thanks.


    Regards,
    Koteshwar Rao

  • Hi Koteshwar,

    I'm so sorry for the delay.

    I apologize If my expression  "MCU is activated by low voltage" caused confusion, MCU input receives a low level voltage to establish a communication.

    My design needs a control signal where it is generated by Remote Station but this signal is indirectly connected an MCU input.by means of long cables and a digital isolator . I'm worried because of a long cable could produce different potential between remote Station and digital isolator or  a ground-induced interference. Now I can rest knowing that isolator has CMOS inputs.

    Thanks 

  • Hi Samuel,

    Thanks for your confirmation! Please let us know if we can provide additional support by replying or clicking the red or yellow buttons in the top right corner of this window.

    Have a great week!


    Respectfully,
    Manuel Chavez

  • Hi Manuel,

    I clicked the green button "This resolved my issue".

    Best Regards,

    SAMUEL