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about connecting the RS-485 transceiver.

Other Parts Discussed in Thread: SN75LBC180, SN55LBC180, SN65LBC180, P82B715, THVD8000

RS-485 トランシーバーの接続について質問です
I have a question about connecting the RS-485 transceiver.

I have a question about whether or not your company's transceiver can be used.

Two-way communication of multiple control boards is performed by reusing wiring in the house.

However, there are 2 lines (1 line is used for power connection)

Therefore only one signal can be used

SN55LBC180, SN65LBC180, SN75LBC180

I understand that it is not possible to guarantee product operation due to irregular connections.

Transmit and receive only with the Y and A terminals

①IC Bの反転入出力端子とY,Aの入出力端子の間にインバータICを接続する。
(1) Connect an inverter IC between the inverted input/output terminal of IC B and the input/output terminals of Y and A.

Is it possible to operate by inputting the inverted signal of Y from the inverter IC to B when receiving?

(2) Does the IC operate even if Z is left open?

(3) What is the minimum transfer speed?

Noise deterioration is acceptable

Allows for waveform deterioration due to reflection

Set the transfer speed to slow

Thank you very much 

  • A differential receiver works like a comparator. So to recognize a single-ended signal, you could tie the other input to a voltage in the middle between the low/high signal levels. (This will, of course, reduce the noise margin.)

    It might be a better idea to use one channel of an I²C buffer like the P82B715. (It is designed for 50 m and 3 nF.)

    However, neither of the suggestions above works if there is a large ground shift. In that case, consider a power line communication transceiver like the THVD8000, but then you need to be able to insert inductors between the power lines and all connected devices.

  • Hi Kiyoshi-san,

    For you direct questions:

    1. I think adding a inverter is just going to going to add complexities to the system that may cause some issues.

    1a) The inverter is another device that will load the bus so its capacitance and impedance must be taken into effect - while logic devices typically wouldn't load the bus that much - my biggest concern is that there could be more impedance mismatches due to the addition of it to the bus which could cause EMI / EMC issues.

    1b) The other concern is that the propagation delay between the input and output of the inverter could potentially cause glitches if the delay is too long so that would also need to be a consideration.

    1c) It would most likely be easier to tie "B" to mid-VCC so it could operate in a pseudo-differential fashion (as Clemens suggested) however the noise margin will be decreased. Also this solution and the inverter solution will not work in systems with large ground shifts (RS-485 compliant devices can handle a minimum common mode voltage range of -7V to 12V) it will result in either bad data or in the case of the inverter - potential damage/failure. 

    2. Z is a driven pin - leaving it floating shouldn't cause any issues with the device; however I'd terminate it to ground with a 375 Ohm resistor (which is the RS-485 defined common mode load) as this pin will still have a voltage associated with it, and a floating pin is much more likely to radiate then if its terminated. 

    3. The maximum data rate for the SNx5LBC180 devices is 10Mbps. The above solutions are more likely to cause issues with how long the bus can be - not the datarate so much, so the max data rate should be similar to the 10Mbps - however, depending on how long the bus is in the application is going to determine how fast you can realistically go. Psuedo-Differential setups (using 1-wire) typically do impact how far you can go so speed may degrade in the specific use case.

    Clemens' Analysis is correct and he has some good suggestions. 

    One part he did mention, the THVD8000, allows for communication signals and power to run on the same bus (with the use of capacitors and inductors to create filters so that the power signal and data signal go to the correct endpoints in the system). I have attached the datasheet, low power design guide, and a high power / low impedance application note so you can see a bit more information about the application - it may be helpful as you are using a system with two wires for power and data which is exactly what this part was made for. 


    Low Power (Power Source <= ~30V) Design Guide:

    HV (Power Source >= ~30V) / Low Impedance Design Guide:

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


    Parker Dodson