GE UR Series Instruction Manual page 358

B.1 OVERVIEW APPENDIX B
A master transmit packet with a slave address of 0 indicates a broadcast command. All slaves on the communication link
will take action based on the packet, but none will respond to the master. Broadcast mode is only recognized when associ-
ated with FUNCTION CODE 05h. For any other function code, a packet with broadcast mode slave address 0 will be
ignored.
FUNCTION CODE
This is one of the supported functions codes of the unit which tells the slave what action to perform. See the SUPPORTED
FUNCTION CODES section for complete details. An exception response from the slave is indicated by setting the high
B
order bit of the function code in the response packet. See the EXCEPTION RESPONSES section for further details.
DATA
This will be a variable number of bytes depending on the function code. This may include actual values, settings, or
addresses sent by the master to the slave or by the slave to the master.
CRC
®
This is a two byte error checking code. The RTU version of Modbus includes a 16 bit cyclic redundancy check (CRC-16)
®
with every packet which is an industry standard method used for error detection. If a Modbus slave device receives a
packet in which an error is indicated by the CRC, the slave device will not act upon or respond to the packet thus preventing
any erroneous operations. See the CRC-16 ALGORITHM section for a description of how to calculate the CRC.
DEAD TIME
A packet is terminated when no data is received for a period of 3.5 byte transmission times (about 15 ms at 2400 bps, 2 ms
at 19200 bps, and 300 µs at 115200 bps). Consequently, the transmitting device must not allow gaps between bytes longer
than this interval. Once the dead time has expired without a new byte transmission, all slaves start listening for a new
packet from the master except for the addressed slave.
B-2 L90 Line Differential Relay GE Power Management