Generally, output objects are controlled one at a time. The protection relay accepts only functions 05 (force single coil) and 15 (force multiple coils), when the 0X coils control structure is used for control operation.
Only one control bit can be operated at a time when the 4X control structures are used.
Exception codes
Only a few exception code alternatives exist for the write coil and write register requests in Modbus:
- 01 = illegal function
- 02 = illegal address
- 03 = illegal value
Internal control rejection reasons with coils may be, for example:
- The client has no write authority.
- The protection relay is in local or OFF state.
- The control operation is already reserved by another client and thus blocked.
If a positive acknowledgement is returned, the control command has been initiated by the protection relay.