GE 269Plus

GE Power Management 269Plus Motor Management Relay 4- 3
4 COMMUNICATIONS PACKET FORMAT
4
4.4 PACKET FORMAT
Communication with the 269Plus occurs in packets, which are groups of asynchronous bytes that are framed
by silent time gaps. The packet format of Modbus RTU is adopted by the GE Power Management 269Plus
relays. This format is as follows:
where:
• silent times are used as packet frames and separators. They are equivalent to 3.5 bytes transmission
silent time (29.2 ms for 1200 baud). The receiving device, upon detecting this silent time, readies itself to
recognize the next received byte as the SLAVE ADDRESS.
• the SLAVE ADDRESS data block is the ID of the 269Plus. The SLAVE ADDRESS setpoint must be pro-
grammed in Setpoints page 5. Only the addressed 269Plus responds to the master command. It sends its
own ID to the master to identify who is responding. The address 00 is a broadcast address used by the
master to issue a command to all slave devices (as per the Modbus protocol). The broadcast command is
not supported by 269Plus and is ignored.
• the FUNCTION data block is the command op-code sent to the slave to indicate which function to perform.
The 269Plus supports Modbus functions 03, 04, and 16 (10H).
• the DATA BYTES data block is the information needed to perform the requested function. This field may
contain values, address references, byte counts, and limits.
• the CRC data block is the error check code. It is a 16-bit cyclic redundancy check (CRC) computed by the
sender and appended to the packet. The code is recomputed by the receiver and compared to the
received code. If the received code is different from that sent, an error in transmission has occurred.
SILENT
TIME
SLAVE
ADDRESS
FUNCTION DATA BYTES CRC SILENT
TIME
1 byte 1 byte N bytes 2 bytes