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ISO1176: output load parameter design

Part Number: ISO1176
Other Parts Discussed in Thread: PROFIBUS, ISO1452, ISO1432

Hi team

Here customer use ISO1176 in their products, and had below queries need your help: to select the common mode inductor for differential output and bypass capacitance on BUS? As it affect the signal quality transmission, the speed is 12Mbps

2.What does the termination resistor 390/220ohm used for? As i noticed on our reference design schematic below it has such structure, and customer found that at firstly with 2 nodes on the BUS w/o the termination resistor, it works fine. But with 3 nodes, it faild to communicate. Then add such termination resistor, it returns normal. So what does such structure influence?

3.what is limitation and which decide how many nodes are allowed on the BUS? Should every nodes add termination resisitors?

Best Regards


  • Hi Gene,

    Please find below my inputs.

    1. For most RS-485 / Profibus applications, we don't believe a common-mode choke (CMC) is needed though having it on the bus shouldn't be an issue. Since CMC is primarily used to suppress common-mode noise in either direction, it is chosen based on the expected common mode noise frequencies from the EMC tests to be conducted. The CMCs are chosen to offer high common-mode impedance to the noise at these frequencies.
      1. The filter capacitance on the bus is also chosen to filter the noise transients that are expected on the bus. It may not be as effective as other EMC protection components like TVS diodes on the bus.
      2. if your EMC requirements are stringent, then I recommend using the protection options from the below reference design.
    2. Bus termination is needed to avoid any signal reflections on the bus and minimize its impact on reliable signal transmissions. The absence of termination resistance can lead to high bit error rate and limits the max datarate that can be achieved. Please refer to section 4 in the below application note that describes bus terminations in detail.
    3. The termination resistance is only needed on the two far ends of the RS-485 bus irrespective of the number of nodes on the bus. The RS-485 standard defines that upto 32 nodes (unit loads, U.L.) can be connected on the bus but the latest RS-485 devices like ISO1432 (12Mbps) or ISO1452 (50Mbps) support nodes upto 256 with each node acting as 1/8 unit load instead of 1 unit load by offering higher input impedance.

    Please do recommend customer to go through the referred documents and let us know if you or customer have any questions, thank you.

    Koteshwar Rao