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THVD1450: RS-485

Part Number: THVD1450

Can we connect 3.3V RS-485 system from one end to 5V RS-485 system another end on the same line? Is that advisable.

Note: I understood that minimum differential voltage should be +/- 1.5V.

  • Yes. Both transceivers use the same differential voltage (which is at least 1.5 V over a 54 Ω load). This is independent of the supply voltage.

  • Hi Sivasankar,

    Generally speaking this should be okay as across a 54 ohm load a RS-485 compliant transceiver will hit a minimum of 1.5V and the 1450 is rs-485 compliant at both 3.3V and 5V. However there are a couple caveats.

    1. The drive strength is stronger at a 5V source and you will have a higher differential voltage from the 5V system - they both will hit a minimum of 1.5V but the typical differential may be different. This means that the max distance that the bus can communicate may be higher on the 5V system --> this should really only be a concern for systems that are already very long and have much of their signals attenuated.

    2. The data-rate of the application - since the bus will have frequency related attenuation that is proportional to the incoming signal - the device with a higher drive strength will allow farther data transmission at higher speeds. This really is point 1 again - however point 1 is for DC and AC losses - but this point is purely AC losses in the system. I.e. if you are running the application at max data-rate you may not be able to transmit as far as you would with a 3.3V system compared to a 5V system.

    In most cases it should be fine - but be aware there could be a max length reduction that could negatively impact the system in certain edge cases but this is dependent on multiple factors (how long the transmission line is in the system, the datarate you are using, transmission line characteristics, temperature, etc...).

    Please let me know if you have any questions and I will see what I can do!

    Best,

    Parker Dodson