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THVD1426: RS422 to RS485 using auto direction control device like THVD1426

Mihai Vintila
Intellectual 590 points
Part Number: THVD1426
Other Parts Discussed in Thread: THVD1451

Hello, we have to connect an RS422 device (a sensor) to an RS485 system (data logger).

The connection works in one direction (receive data from the sensor) but not bi-directional, we have tried to connect the RS422 wires in parallel but it does not work.

So we would like to create a small interface board between RS485 (2 wires) to RS422 (4 wires).

The solution that we consider would be with a device with auto direction control like THVD1426 (on the RS485 side) connected back to back with a full duplex device like THVD1451 (on the RS422 side).

Another option would be to use two other THVD1426 on the RS422 side.

Is this solution OK ?

Thank you,

Mihai

  • +1
    TI__Guru 52262 points

    Hi Mihai,

    Just to be sure I am understanding you correctly - does the proposed system look something like this:

    If this is the case the the interface should be pretty straight-forward.

    I wasn't exactly sure if your system is only sensor to system or if the problem was that you needed bi-direction communication - so I am showing you the solution for both variations to make sure I cover the correct implementation that you are looking for.

    Variant 1: Sensor to System communication only - system never needs to communicate with sensor - sensor only sends data. 

    This will keep the THVD1426 in auto-direction mode - so over standard operation a high to low transition on D will allow the THVD1426 to transmit data back to the RS-485 system. If D is held high between 400ns (min), 800ns (typ), and 1450ns (max) the driver will then be disabled and the device will return to RX mode. Since you don't care about data from 485 to 422 D on the THVD1451 should be grounded (or tied to VCC) and R on the THVD1426 can be left floating.  

    Variant 2: You need bi-directional Communication between sensor and system 

    Pretty much the same implementation as variant 1 - but R on the THVD1426 is connected to D on the THVD1451 - I added a pull-down so there is a default state (which is generally best practice) 

    The biggest concern is timing - just as in any auto-direction control application. There is a delay between data transmission and driver disabling - up to 1.45us - so if that is too much for the system please let me know as we may need to do a multi-chip solution to provide a quicker delay - but I think conceptually its okay.

    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
    Intellectual 590 points

    Hello Parker, many thanks for your fast and comprehensive response !

    We need bi-directional communication so we should use the second variant.

    I hope the timing issue will not cause problems, we will make a prototype and keep you updated about the results.

    Thanks again !

    Mihai