SLLA628 September   2023 THVD1424

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Use Case Domain of RS-485
    1. 1.1 RS-485 Compliant Transmitter
    2. 1.2 RS-485 Compliant Receiver
    3. 1.3 RS-485 Transceiver Use Case Variability
  5. 2Traditional RS-485 Design Process
    1. 2.1 Design Process Overview
    2. 2.2 Requirement Definition
      1. 2.2.1 Bus Voltage and Logic Voltage (VCC and VIO):
      2. 2.2.2 Number of Communication Nodes Supported plus Static vs. Dynamic Systems
      3. 2.2.3 Max Bus Length, Network Topology, Emission Concerns, and Data Rate Required
      4. 2.2.4 Duplex
      5. 2.2.5 Protection Needs
      6. 2.2.6 Additional Features of RS-485 Bus
    3. 2.3 IC Selection, Application Design, and Validation/Qualification
  6. 3One Multi-System Design: Flexible RS-485 with the THVD1424
    1. 3.1 Flexible Multi-System Design
    2. 3.2 Simplification of RS-485 Design Process Using THVD1424
      1. 3.2.1 Bus Voltage and Logic Voltage Supplies (VCC and VIO)
      2. 3.2.2 Number of Communication Nodes Supported plus Dynamic or Static Systems
      3. 3.2.3 Max Bus Length, Network Topology, Data Rate, and Emissions Concerns
      4. 3.2.4 Duplex
      5. 3.2.5 Protection Needs
      6. 3.2.6 Additional Features
  7. 4Summary
  8. 5References

3.2.3 Max Bus Length, Network Topology, Data Rate, and Emissions Concerns

As discussed previously the max bus length, max unterminated stub length, network topology, and emissions concerns are all dependent on data-rate for the device. In general, faster devices mean shorter effective bus length and shorter unterminated stub lengths with a greater concern for EMI. In traditional design different devices would fit into different speed classifications meaning most speeds had a group of parts that serviced it. The THVD1424 on the other hand handles this issue through an integrated selectable slew rate limit option. This is controlled via 1 logic pin that when held low the device can output up to 20Mbps and when held high the devices data rate will be reduced to a maximum of 500Kbps. This allows the THVD1424 to handle faster RS-485 use cases up to 20Mbps – but in length and/or emissions sensitive systems the THVD1424 can also slow its transition times to increase the maximum stub length in a system, effective bus length, and reduce the amount of higher frequency energy generated therefore helping reduce EMI issues.