The IEC 60870-5-103 standard does not take into account that the IED could have several remote client connections. It should be noticed that a remote control operation could also be rejected if another remote client is performing a control operation at the same time. The IED handles the remote command rejection in three different ways.
- Remote command to an existing object, while the IED is in Local mode or the IED is in Remote mode, but control operation is blocked for some reason (Blocking reasons include simultaneous control being performed by another remote client):
- The command is accepted on link level (Link ACK)
- The command is rejected on application level (Negative response, COT=21)
- Remote command while the IED is still performing the previous command of the same client:
- The command is rejected on link level (Link NAK, DFC=1)
- Remote command performed on a non-existing object:
- The command is rejected on link level (Link NAK, DFC=0)
In the last two cases the DFC flag is used to distinguish the faults.