The protection relay creates a Modbus event record when a momentary digital input bit changes its value. The protection relay then stores the changed Modbus bit location and value into the Modbus event record buffer. The event time tag is also stored into the record. The time tag includes a full time stamp from a year down to milliseconds.
Modbus event generation on/off is selectable for each individual momentary bit in the Modbus memory map. It is possible to define whether events are to be generated from the rising edge or both edges' transitions of the momentary bit.
If the Modbus indication point is mapped to the user-definable Modbus area, then the possible events from this point come from the original Modbus point location. In case the UDR mapped indication point has no original Modbus point location, then the event comes from its UDR point location.
The size of a Modbus event buffer is 500 events. Modbus event buffers are instance dependent. The Modbus event buffer’s behavior at overflow is configurable. Either the 500 oldest or the 500 newest events are kept in the buffer.
- In the "Keep oldest" mode, no new Modbus events are collected into the buffer after overflow. Event collecting is resumed only after the event buffer has been emptied of the amount defined by the Event backoff setting. No special overflow event is created at the overflow moment. Instead, the Modbus record sequence number is increased for every new event that was not stored into the buffer. Thus, a jump in the record sequence number reveals the overflow moment and the sequence number difference reveals how many events were lost after overflow.
- In the "Keep newest" mode, new Modbus events continuously overwrite the oldest events in the buffer. The record sequence number reveals that overflow has happened, but it is noticed in the sequence jumps for the oldest events.
Multiple clients support
Several Modbus clients can read out Modbus event records from the protection relay independently of one another. The Modbus event buffer keeps track of where in the event buffer the different clients read at any moment. Clients are identified either by the serial port from where the requests are issued or by the client's IP address in the TCP/IP network. Up to 25 different IP addresses, belonging to both registered and unregistered Modbus clients, can be memorized by the protection relay.