As the indication signals related to protection applications often change rapidly, the Modbus client may not detect all the changes.
Momentary position and momentary change detection bits
In this protection relay, indications are shown as two adjacent Modbus bits in the Modbus memory map. The two bits represent the momentary position and the momentary change detection state of the indication.
If the momentary value of an indication bit has changed twice or more since the client last read it, the MCD bit is set to one. When the MCD bit has been read, it is reset to zero. Since the indications usually are 1 (active), it is easy to detect an indication activation by combining the MOM and MCD bits using a logical OR operation (MOM+MCD). The momentary position bit and the MCD bit of a certain indication point always occur as pairs in the Modbus memory map. The momentary bit is always located on an even bit address. MCD bit on the next odd bit address.
The MCD bit states are client-dependent. The MCD bit is only reset for the specific Modbus client that reads it. Thus, other Modbus clients may still receive value 1 from the same MCD bit when it is read.
MOM indication changes are captured in the protection relay's sequential Modbus event buffer. Additionally, the Modbus event buffer provides a time stamp and chronology of indication changes.
Latch bit
Some of the digital MOM+MCD points also incorporate a latch bit alternative. The latch bit hides the MOM and MCD bits and instead returns the result of combining the two bits with a logical OR operation. The MOM+MCD bits are generally used for this.