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THVD4431: Using the same two pins for RS-232 and RS-485 half duplex - can I short R1 and R3?

Mads Marstrand
Prodigy 185 points
Part Number: THVD4431

Hello!

(I am very likely missing something obvious in the datasheet.)

I'd like to use the THVD4431 in an application where the same two pins of a D-Sub connector must be able to act as either RS-485 A and B or RS-232 Receive and Transmit, respectively.

In RS-485 half duplex mode, L2 and L3 are connected to R1 and R2.
In RS-232 mode, L2 also connects to R2 - but L3 now connects to R3.

Is it possible to short R1 and R3 as illustrated below? In the RS-232 case, I can just ignore the signal on L1, but can I be sure that R3 is high impedance and will not mess with the bus when the THVD4431 is in RS-485 half duplex mode?

Or is there some better or more obvious way to achieve the same result?

Thanks!!

  • +1
    TI__Mastermind 46327 points

    Hi Mads,

    So, the issue is that RS-232 is point to point and this setup essentially forces the driver on "R3" to drive 2 receivers instead of the intended 1. 

    An RS-232 driver is designed to drive an RS-232 receiver which is going to have an input impedance of no less than 3000 ohms - but typically it should be 5000 ohms. The problem comes that in RS-232 mode the driver will not only be driving the intended receiver but also "R1" which has an integrated nominal 5k resistance. So, 2 nominal input impedances of RS-232 receivers in parallel is 2.5k which is under the 3k minimum that is required. 

    Essentially best case scenario you see more attenuation than expected and add more wear/tear to driver and worst case scenario is communication failure. This connection most likely won't cause immediate damage to THVD4431 - but you could see issues where RS-232 communication fails and would increase the wear/tear on the part due because RS-232 driver isn't anticipating that much current draw. 

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

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 185 points

    Hi Parker,

    Thank you for your prompt reply! While it was not the answer I was hoping for, it is at least clear to me now that this is not the way to go. So thanks for saving me a lot of trobule. I appreciate it!

    Sincerely,

    Mads