GE 750, Инструкция по эксплуатации

Страница: 321 / 504

GE Power Management 750/760 Feeder Management Relay 16-
13
16 COMMUNICATIONS 16.3 MODBUS PROTOCOL
16
The following table shows the format of the master and slave packets for a master device sending the STORE
TIME operation code to all slave devices on the communications link as required by step three of the proce-
dure.
16.3.12 READING THE EVENT RECORDER
All Event Recorder data can be read from Modbus registers found in the address range 2000h - 20FFh.
The ‘Number of Events Since Last Clear’ register at address 2001h is incremented by one every time a new
event occurs. The register is cleared to zero when the Event Recorder is cleared. When a new event occurs, it
is assigned an ‘event number’ which is equal to the incremented value of this register; the newest event will
have an event number equal to the Number of Events. This register can be used to determine if any new
events have occurred by periodically reading the register to see if the value has changed. If the Number of
Events has increased then there are new events available.
Only the data for a single event can be read from the Modbus memory map in a single data packet. The ‘Event
Number Selector’ register at address 2000h selects the event number for which data can be read from the
memory map. For example, to read the data for event number 123, the value 123 must first be written to this
register. All the data for event number 123 can now be read from the ‘Event Record Data’ registers at
addresses 2010h to 201Dh. Only the last 128 events are actually stored in the relay’s memory. Attempting to
retrieve data for elder events that are not stored will result in a Modbus exception response when writing to the
‘Event Number Selector’.
The following example illustrates how information can be retrieved from the Event Recorder:
A SCADA system polls the Number of Events register once every minute. It now reads a value of 27 from the
register when previously the value was 24 which means that three new events have occurred. The SCADA
system writes a value of 25 to the Event Number Selector register. It then reads the data for event number 25
from the Event Record Data registers and stores the data to permanent memory for retrieval by an operator.
The system now writes the value 26 to the selector and then reads the data for event number 26. Finally, the
value 27 is written to the selector followed by reading the data for this event. All the data for the new events
has now been retrieved by the SCADA system so it resumes polling the Number of Events register.
The data for an event should be straightforward to interpret from the information given in the Memory Map
except for the ‘Cause of Event’ register. Data format F24 describes the event cause register which contains
several ‘fields’ of information. The ‘Event Type’ field determines how to interpret the remainder of the register.
Interpreting the event cause register is most easily described with several examples.
Table 16–11: MASTER/SLAVE PACKET FORMAT FOR STORE TIME OPERATION CODE
MASTER TRANSMISSION SLAVE RESPONSE
PACKET FORMAT EXAMPLE (HEX) PACKET FORMAT EXAMPLE (HEX)
SLAVE ADDRESS 00 No response from slave.
FUNCTION CODE 05
OPERATION CODE - hi 00
OPERATION CODE - lo 04
CODE VALUE - hi FF
CODE VALUE - lo 00
CRC - lo CC
CRC - hi 2A