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THVD1550: High-z value

Shu-Cheng LIN
Guru 11785 points
Part Number: THVD1550
Other Parts Discussed in Thread: THVD1500, SN75176B

Hi Sirs,

Sorry to bother you.

We have use THVD1550.

Our issue is we found our THVD1550 pin 6 (DE) if pull low, that will still to effect pin7 state.

The other word is RS232 TX waveform will be different between U83 THVD1550DR pin 6 have connect or disconnect. please refer as below.

As my understand when DE low, AB will be high-z, right?

Actually we try competitor ADM485 that didn't find this phenomenon.

Based on TI cross reference them should be P2P.

We guess the waveform is cause high-z impedance value not large enough?

So, we would like to know,

1. Why THVD1550 have this phenomenon?

2. How to check the IC in Hi-z? like use volt-ohm-milliammeter to get impedance value? how to connect volt-ohm-milliammeter ?

Postive connect to Pin6 (A) and Negitive connect to Pin5(GND)

3. What is the Hi-z correct value? 10M? 100M?

Waveform: not disconnect pin6. ch1 is pi6 and ch2 is pin7


Waveform:  Disconnect pin6 Tx. ch1 is pi6 and ch2 is pin7

Schematic:

  • 0
    TI__Guru 62920 points
    Hello Shu-Cheng,

    The input impedance is usually determined by measuring the input current for a given test voltage. This is reflected in the datasheet in the bus input leakage current specification.

    I believe what you are seeing in this case is some transient protection circuits in THVD1550 beginning to activate at the higher RS-232 voltages and clamping the signals. This is something that unfortunately cannot be disabled in the device.

    As a solution, you could replace the part with a p2p equivalent that had a different transient protection implementation. In this case I would recommend THVD1500 as long as the data rate your application needs is supported.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Guru 11785 points

    Hi Sirs,

    Thanks for your reply,

    Actually we have try different node solution SN75176B and looks it's didn't have this phenomenon

    We want to know how H-Z is affected by node qty difference (or driving strength).

    It’s hard to find the answer from spec, could you help to get more information?

    Thanks!!

     

  • 0 in reply to Shu-Cheng LIN
    TI__Guru 62920 points
    Hi Shu-Cheng,

    Sorry for the delay due to holidays. You can read more about how the high-Z input leakage of a transceiver impacts the achievable number of nodes in an RS-485 network in this blog:

    e2e.ti.com/.../rs-485-basics-how-to-calculate-unit-loads-and-the-maximum-number-of-nodes-on-your-network

    Please let me know if you have any questions on this.

    Regards,
    Max