GE 845 Instruction Manual page 135

CHAPTER 4: SETPOINTS
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
The following ModBus parameters are configurable:
Path: Setpoints > Device > Communications > Modbus Protocol
MODBUS SLAVE ADDRESS
Range: 1 to 254 in steps of 1
Default: 254
For the RS485 ports each 845 must have a unique address from 1 to 254. Address 0 is
the broadcast address to which all Modbus slave devices listen. Addresses do not have
to be sequential, but no two devices can have the same address, otherwise conflicts
resulting in errors occur. Generally, each device added to the link uses the next higher
address starting at 1.
MODBUS TCP PORT
Range: 1 to 65535 in steps of 1
Default: 502
The TCP port number used with Modbus over Ethernet. Note that the maximum number
of simultaneous Modbus connections supported over Ethernet is:
• three for an 845 relay without the communications card,
• five for an 845 relay with the communications card.
MODBUS ERROR RESPONSES
The following exception response codes are implemented.
Error ID Exception
01 ILLEGAL FUNCTION
02 ILLEGAL DATA ADDRESS
03 ILLEGAL DATA VALUE
Modbus Protocol
Function Code 03H
Modbus Implementation: Read Holding Registers
845 Implementation: Read Setpoints
For the 845 implementation of Modbus, the function code can be used to read any
setpoints ("holding registers"). Holding registers are 16 bit (two byte) values transmitted
high order byte first. As a result all 845 Setpoints are sent as two bytes. The maximum
number of registers that can be read in one transmission is 125.
The slave response to this function code is the slave address, function code, a count of the
number of data bytes to follow, the data itself and the CRC. Each data item is sent as a two
byte number with the high order byte sent first.
For example, consider a request for slave 17 to respond with 3 registers starting at address
006B. For this example the register data in these addresses is as follows:
Address
006B
006C
006D
Description
The function code transmitted is not one of the
functions supported by the 845.
The address referenced in the data field transmitted
by the master is not an allowable address for the 845.
The value referenced in the data field transmitted by
the master is not within range for the selected data
address.
Data
022B
0000
0064
DEVICE
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