CHAPTER 4: SETPOINTS DEVICE
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 4–33
MODBUS TCP PORT NUMBER
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 without the communications card,
•five for an 845 with the communications card.
MODBUS ACTIVITY TIMEOUT
Range: 0 to 3600 s in steps of 1
Default: 0 s
The Modbus Activity Timeout specifies the minimum time without Modbus
communication. This timeout is used to declare the Modbus ‘Loss of Communication’
state.
The Modbus state is always Active if the Modbus Activity Timeout is 0 s.
MODBUS ERROR RESPONSES
The following exception response codes are implemented.
Function Code 03H (Read Setpoints)
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:
The master/slave packets have the following format:
Table 4-5: Master Packet Format for Function Code 03H
Error ID Exception Description
01 ILLEGAL FUNCTION The function code transmitted is not one of the
functions supported by the 845.
02 ILLEGAL DATA ADDRESS The address referenced in the data field transmitted
by the master is not an allowable address for the 845.
03 ILLEGAL DATA VALUE The value referenced in the data field transmitted by
the master is not within range for the selected data
address.
Address Data
006B 022B
006C 0000
006D 0064
MASTER TRANSMISSION BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 FE message for slave 254
FUNCTION CODE 1 03 read registers
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