Single event record reading - Arc protection - Auto synchronization - Bay control and measurement - Merging unit - Transformer protection - 2 winding - Feeder protection - Voltage regulation - Capacitor bank protection - Petersen Coil control - Grid automation - Busbar differential protection (high impedance) - Back-up protection - Motor protection - Busbar protection (voltage and frequency) - Transformer protection - 3 winding - Interconnection protection - Generator protection - Power management/Load shedding - Modbus Communication Manual - REX640 Protection and control - PCL4 - IEC - ANSI

As long as there are unread Modbus events available for a Modbus client, bit 0 of Modbus SSR3 register remains "1".

Events are read in two steps. First, the client writes a selection code to the Event selection register at location 49251. The selection code defines the type of read operation that the client wants to perform. If this register is written a second time omitting a read event transaction, this write operation does not succeed and consequentially the relay answers with a message having the exception code: illegal data value. The selected event record is loaded by the protection relay into the following 11 registers from 9252 to 9262. Second, the client reads out the 11 registers in one multiple register read operation.

Note: Event records can be read by using two commands, function 6 for the write operation and function 3 for the read operation, or by using function 23 that includes write and read operations in the same transaction.

Note: If event records are read by using two commands, the positive confirmation to the write select operation tells the client that an event record has been loaded for reading. Another way to detect the positive confirmation is by monitoring the state of SSR3 bit 8.

Selection code 1: Reading the oldest unread record

When writing the selection code 1, the protection relay first checks the client. If the client has read events before, the protection relay knows which internal event has been sent to this specific client during the last reading. The protection relay then loads the next event, that is the oldest unread, into the next 11 registers. If this is the first time the client reads events from the protection relay, the oldest event of the Modbus event buffer is loaded into the 11 event record registers.

Selection code 2: Reading the oldest stored record

Selection code 2 always forces the event reading to go back to the oldest event in the Modbus event buffer. The oldest event record is then loaded into the 11 event record registers. After the client has read out this record, the next record becomes the oldest unread. The client can continue with the selection code 1 by reading out the oldest unread event record again.

Selection code -1...-499

A negative selection code, that is a 16 bit two's complement value, defines how many records backwards from the newest event the event record reading is to be moved. For example, the ten latest events could be read out at any time by first selecting -10, reading out the event and then continuing with the selection code 1 to read out the nine additional event records. There can be 500 event records altogether.

Selection code 3: Resetting the event read pointer

The write selection 3 is not followed by a read operation. The selection 3 means that there are no unread records in the Modbus event buffer left for the client in question, that is, the buffer is cleared. The next new event that is logged into the Modbus event buffer becomes the first unread record for this specific client.

Selection code 4: Resetting SSR3 bit 8

The write selection 4 is not followed by a read operation. The selection code only resets bit 8 in SSR3.

Note: If event records are read by using two commands, the client can re-read the 11 event record registers as many times as it wants. As long as no new selection write operation is performed, the contents of the 11 event record registers are not changed.

Selection code 5: Reading the newest stored record

Selection code 5 always forces the event reading to go to the newest event in the Modbus event buffer. The newest event record is then loaded into the 11 event record registers. The next new event that is logged into the Modbus event buffer becomes the first unread record for this specific client. Older events can be read with a negative selection code.