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THVD1450: Can THVD1450 drive the signal with 38400bps and 1200m distance?

Kazuya Nakai59
Guru 20805 points
Part Number: THVD1450

Hello guys,

One of my customers is looking for RS485 half duplex transceiver which can drive the signal with 38400bps and 1200m distance.
At this moment, they have the following questions. Could you please give me your reply?

Q1. They are looking at the following application report (Literature Number: SLLA036D)
      https://www.ti.com/lit/an/slla036d/slla036d.pdf?ts=1670216373119&ref_url=https%253A%252F%252Fwww.google.com%252F

     On page 15, the relation ship between signal transition time (tt) and signaling rate as equation 6.

     They want to know what this "tt" measurement way and conditions. could you please tell me these?

Q2. In order to satisfy their goal (1200m and 38400bps), what other things must be considered? For example is it the signal attenuation in the cable?

Q3. Can THVD1450 achieve their target? If the answer is no, is there any device in TI for the goal achievement?

Your reply would be much appreciated.

Best regards,
Kazuya

      

  • 0
    Guru 281530 points

    1. It refers to the driver output rise/fall time, which is at most 6 ns for the THVD1450.

    2. In practive, the bottleneck is jitter. See figure 33 of the datasheet; 1200 m (4000 ft) is possible at 38 kbps. The signal attenuation will not break it as long as the voltage drop over the termination resistors is high enough, but reduces the noise margin. If it matters (which it probably does not), a cable with lower DC resistance will help.

    3. Yes. (This is mostly a question of the cable, not of the transceiver.)

  • 0
    TI__Mastermind 46797 points

    Hi Kazuya,

    1. Clemens is correct - tt stands for transition time which is another name for both rise and fall time. 

    2. Clemens is correct - in general as long as you have a properly terminated and impedance matched system (which is just following standard RS-485 implementations -one 120 Ohm resistors at each end of bus (so 2 resistors total) + a cable that has the same characteristic impedance (120) so that reflections are minimized. RS-485 is rated at 1.2km up to 100kbps so it should be possible to do that by following standard protocol. If more than 2 devices are on the line it is imperative that a daisy chain or spine with junction boxes are used and stub length is minimized.

    3. The device can but it is very dependent on system setup as 1.2km is the max distance for RS-485 so system setup, cable selection, and layout are all very large factors in whether or not the system will work. But the THVD1450 (as well as any compliant RS-485 transceiver) can meet 100kbps at 1.2km when using a proper system setup.

    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
    Guru 20805 points

    Hello Parker, Clemens,

    Thank you very much for your reply and I'm sorry to be late my reply. 
    Your replies are very helpful for the customer and me.

    Thank you again and best regards,
    Kazuya.