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Entry type: FAQ Entry ID: 109780220, Entry date: 05/29/2020

SINAMICS V20: When use USS/Modbus to control V20, it can't start up directly

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When the drive is in "ON Inhibit" state (r2=6), it must get the switch off command to get in "Ready" state.

​When you use "USS/MODBUS on RS485" as control resource (p700=5), the SINAMICS V20 can be in "ON Inhibit" state and does not switch on in below situations:

  • Situation 1: The communication is not established between Host device and SINAMICS V20
  • Situation 2: F72 occurs, and the fault is not reset

This "ON Inhibit" statue prevents the SINAMICS V20 from a direct start-up. In this case, r2 shows the value 6.
The bits "ON/OFF1" and "Acknowlege" need a signal change from false to true to execute there function.

To start the drive you have to:

  • Set "ON/OFF1" bit to false and "Acknowlege" bit to true
    This will change from state "On Inhibit" to "Ready" (r2=1)
  • Set "ON/OFF1" bit to true to start the drive ( It's recommended to reset "Acknowlege" bit to false)

Typical sequence for USS in the control word is:

  • 16#04FE - switch off ("ON/OFF1" (bit 0) = false, "Acknowledge" (bit 7) = true, all other enables are set)
    This will change from state "On Inhibit" to "Ready" (r2=1)
  • 16#047F - switch on ("ON/OFF1" (bit 0) = true, "Acknowledge" (bit 7) = false, all other enables are set)

Typical sequence for Modbus RTU is:

  • Set register 40006 to 0 (Switch off) and register 40007 to 1 (Acknowledge faults)
    This will change from state "On Inhibit" to "Ready" 
  • Set register 40006 to 1 (Switch on) and register 40007 to 0 (Prepare new acknowledge )

The register 40004 must be 1 for operation (RUN enable).

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