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

1.3 RS-485 Transceiver Use Case Variability

As shown – the RS-485 requirements define a minimal set of requirements that leaves many design questions to be answered by the designer through IC selection and system design. A designer will have to consider, at minimum, the following system and application parameters when selecting an IC: bus side supply voltage, controller voltage, how many communication nodes are present on the bus, how long is the bus going to be, what is the required data-rate, does the bus need to support unterminated and terminated communication nodes, what network topology can the system support, are emissions a concern in the design, is the application full or half duplex, is there a need for protection on the device and if yes how much, is the system design static or dynamic, and finally are there any additional features needed from the RS-485 transceiver?

Table 1-1 RS-485 Boundary Conditions
System/Application Parameter Boundary Conditions in RS-485
Bus Side Supply Voltage (VCC) 3 V to 5.5 V
Controller/Logic Side Supply Voltage (VIO) 1.65 V to 5.5 V
Number of Nodes Supported Traditionally 32, Some Modern Devices Can Support up to 256
Max Bus Length 1.2 km at 100Kbps – faster speeds can reduce max length
Data-Rate Traditionally 0 – 10Mbps, Some Modern Devices Can Support up to 50Mbps
Terminated and Unterminated Nodes 2 terminations required – both End Nodes terminated – if > 2 nodes the design requires terminated and unterminated nodes
Network Topology Daisy Chain or Spine w/ Junction Box Strongly suggested

With the other parameters not having set bounds leaves the designer many unanswered questions that will vary based on the system they are designing in. TI currently has over 250 various RS-485 and related RS-422 transceivers to serve the wide array of niches in the RS-485 bus. This leads to a very involved RS-485 design process that is further complicated by the IC selection process.