Control bits are write-only coil (0X) data. In addition, they are assigned in parallel to holding registers (4X). Control bits can be accessed in the Modbus Communication Management tool in PCM600, under the tabs 0X – writable coils and 4X – writable registers.
The controllable write-only coil point definitions are separated from the read-only coil point definitions. It is possible to define a write-only object X and a read-only object Y to the same 0X coil bit address Z. While this may be a violation of the 0X Modbus area intention in some cases, it is safe from the relay's point of view. It means that the reading of 0X coil address Z returns the value of object Y, and writing to 0X coil address Z activates object X.
In the configuration example, a physical output on coil address 100 needs to be controlled. The state of the physical output should be readable from the same coil address 100.
The state of the physical output is wired to an MVGAPC input. The MVGAPC “Momonly” signal alternative is chosen for the Modbus indication. The MVGAPC user-definable address is configured as “100”. The SPCGAPC output is wired to the physical output. The writable coil address is also adjusted to “100”. Alternatively, the physical output state (XGGIO) Modbus signal could be mapped to coil address 100. However, as this signal also contains the MCD bit, the indication would then cover two coil addresses, 100 and 101.