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

2.2.1 Bus Voltage and Logic Voltage (VCC and VIO):

The RS-485 bus side pins can always require between 3 V to 5.5 V as a supply voltage to meet base RS-485 specifications. A higher supply can lead to a higher differential output voltage which leads to a longer maximum bus length. Depending on the power rails available some applications can only be able to support 3.3 V ± 10% supply operation. In devices with only one supply pin the logic inputs and outputs, pins that interact with a logic controller, can be related to the supply voltage VCC.

In many applications the devices controller can be operating at a different supply voltage than the bus – in some cases down 1.8 V controllers – this can require either the use of a level translator between the transceiver and controller or a transceiver that allows separate logic supplies – typically denoted on RS-485 parts as VIO.