TIDUDI9A January   2018  – May 2025 ISOM8610

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 ISO121x
      2. 2.2.2 SN74LV165A
      3. 2.2.3 SN74LVC1GU04
      4. 2.2.4 TVS3300
      5. 2.2.5 ISOM8600
    3. 2.3 System Design Theory
      1. 2.3.1 Digital Input Stage
      2. 2.3.2 Broken Wire Detection
        1. 2.3.2.1 Case 1: Wire Intact and Input State '1'
        2. 2.3.2.2 Case 2: Wire Intact and Input State '0'
        3. 2.3.2.3 Case 3: Broken Wire
      3. 2.3.3 Readout of Digital Outputs
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Required Hardware and Software
      1. 3.1.1 Hardware
      2. 3.1.2 Software
    2. 3.2 Testing and Results
      1. 3.2.1 Test Setup
      2. 3.2.2 Test Results
        1. 3.2.2.1 Group-Channel Configuration
        2. 3.2.2.2 Single-Channel Configuration
      3. 3.2.3 Conclusion
  10. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Layout Prints
    4. 4.4 Altium Project
    5. 4.5 Gerber Files
    6. 4.6 Assembly Drawings
  11. 5Software Files
  12. 6Related Documentation
    1. 6.1 Trademarks
  13. 7About the Author
    1. 7.1 Acknowledgments
  14. 8Revision History

2.3.2 Broken Wire Detection

Broken wire detection is explained for a single-channel configuration. This means that only one channel of a ISO121x device is used. Only one additional opto-emulator switch is needed. The rest of the circuit stays the same. As shown in Figure 2-6, the opto-emulator switch must be placed in between the FGND pin of the respective ISO121x channel and the field ground input.

For the broken wire detection to work, when the wire is intact, a small current is available to charge CIN even when the input state is '0'. This assumption is made because the IEC61131-2 allows for Type 1, 2, and 3 inputs in '0' state currents of up to a few mA (see Figure 2-7).

This is only valid for powered field devices and not passive inputs like a simple, standalone mechanical switch. Furthermore, the output of the sourcing device must have a pullup resistor to its supply.

TIDA-01509 Switching Characteristics for IEC 61131-2 Type 1, 2, and 3 Proximity SwitchesFigure 2-7 Switching Characteristics for IEC 61131-2 Type 1, 2, and 3 Proximity Switches

The broken wire detection sequence works as follows:

  1. In normal operation, the field ground pin FGND of the input channel is connected to the actual field ground through the opto-emulator switch.
  2. The opto-emulator switch is opened by switching its control line low.
  3. The control line is switched high again to close the optical switch.
  4. Afterwards, the output state of the ISO121x channel is read. If a wire is connected, the output state of the channel will be '1'. This is the case for input states '1' and '0. However, if the wire is broken, the output state of the respective channel is '0' constantly.

For more information on these different cases, see Section 2.3.2.1 to Section 2.3.2.3. Therein, Break FGND is the control state of the optical switch (0 V means the switch is off, 3.3 V means the switch is turned on), IN is the input signal from the field device, CIN is the voltage at the input capacitor, and OUT is the output state of the ISO121x. The respective times are given in brackets (tx).