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THVD2410: Shorting DE and RE in THVD2410

Datta Prasad Thiyagarajan
Prodigy 40 points
Part Number: THVD2410

Hi

We are planning to use THVD2410 as a replacement to MAX3440EESA+ without any change to the software. There is some pin difference between the two. Since , we do not intend to change the software , can i short the DE and RE pin (MAX3440 has a single RE/DE pin). 

1) Is it allowed to short RE and DE. Will there be any negative impact.

2) If both of these pins are shorted and microcontroller pin is undefined, would it cause any issue? Because DE pin will be pulled up 2Mohm and RE pin will be pulled down by 2Mohm. Is it better to add a strong pull down so that the pin will be at defined state in default.

Please let me know.

  • 0
    TI__Mastermind 46457 points

    Hi Datta,

    The THVD2410 is an excellent device and a good choice for a replacement. 

    For your direct questions:

    1. Yes /RE and DE can be shorted together - I'd say that is the most common implementation used for most of our RS-485 devices with two enables. The one note is that the transition time between high to low or low to high should be quick so that the pins are in an undefined range too long. 

    2. So I'd advise to add a stronger pull-down on the pin (like 4.7k to 10k would be fine) in case of undefined MCU output. That is because if the MCU output is undefined ideally the voltage on the pin would be VCC/2 - which depending on the VCC could either put the device into driver mode (typically you'd want to default to receive mode as to not hinder communication on the rest of the bus - as only 1 driver can be active at a time) or put the device into an undefined state. However that is complicated by the fact that those 2Mohm resistors are not super precise so you may not even see VCC/2 on the line - it could be different. The risk is alleviated completely with a stronger pull-down resistor. 

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

    Best,

    Parker Dodson