Picoscope Configuration for Modbus RTU capturing and decoding

[David_Purser]

Warning

Hazard of Electric Shock: Remove all power from all devices before removing any covers or doors of the system. Disconnect power at the device and at the power source.

Electrostatic Discharge Required: Always discharge static electricity from your person by touching metal prior to handling any hardware. Failure to do so may result in damage to devices.

Issue

Modbus RTU communication is not performing as expected, and there is a need to review electrical installation or request/response packets.

Product Line

EcoStruxure Building Operation

Environment

• Building Operation Automation Server
• Building Operation Multi-purpose Controller
• Building Operation Room Controller
• Building Operation EZLogic Room Controller

Cause

Picoscope hardware and software provide the ability to review the electrical waveforms and decode as Modbus RTU

Resolution

1. Ensure proper setup and connection of Picoscope probes
   1. Set the switch on probes to 1X
   2. Connect the probes using the color coding on each end to ensure the connection is as follows:
      1. Connect Probe A to TX/RX -
      2. Connect Probe B to TX/RX +
   3. Connect the probe black reference alligator clips to the cable shield drain wire. Ensure the drain wire is connected to EGND at one location.
2. Connect the Picoscope to the PC, then open Picoscope software and choose the device connected.
3. Set the Scope
   1. Use 1 ms/div for 38400 baud
   2. Follow this thread for more information since BACnet and Modbus are both RS-485 protocols. BACnet - PicoScope Basics - Schneider Electric Community 
4. Select Channel A
   1. Set to +/- 5V
   2. Select Channel B and repeat the setting.
   3. One may change to +/- 10V if you encounter spikes above the 5V range (overvoltage warnings). 
5. Add Math Channel
   1. Click the Math Channels icon
   2. Select A - B 
6. Add Serial Decoding for Modbus RTU
   1. Click Serial decoding icon
   2. Select MODBUS RTU
   3. On Page 1
      1. select Master A - B
      2. Change Baud Rate to known baud rate, usually 38.4 kBd
      3. Change Parity to known parity, usually NONE (1 Stop Bit)
      4. Leave the rest at default and click Next 
   4. On Page 2
      1. Recommend leaving as default of HEX, knowing you will have to convert for Slave IDs over 09.
      2. Leave the rest at default and click Finish 
7. Set Waveform
   1. Adjust thumbnail size to preference
   2. Leave Capture Time = On (default)
   3. Set Max Waveform to 2000 (example has 5000) 
8. Start and stop capture and select Waveform as example
9. Set Trigger
   1. Using a waveform, set the threshold voltage just above the blue line voltage. For example, the blue line is at 0.8V, and the threshold is set at 1.92V. Adjust based on your collected waveform(s)
   2. Set the Pre-Trigger
      1. 0% puts Yellow Diamond to the left and 100% puts Yellow Diamond to the right.
      2. The recommended setting is between 5% and 20% (for example, 20%). 
10. Start capturing
11. Set Serial Decoding for MODBUS RTU to show buffer
    1. The Current Buffer shows only the waveform on the screen. 
    2. All Buffers show everything. 
    3. NOTE: The Buffer Number column references the waveform number