Modbus Tcp/Ip; Data And Control Functions - GE 345 Communications Manual

CHAPTER 1: COMMUNICATIONS GUIDE

MODBUS TCP/IP

Data and control
functions
345 TRANSFORMER PROTECTION SYSTEM – COMMUNICATIONS GUIDE
This section describes the procedure to read and write data in the 350 relay using MODBUS
TCP protocol. The MODBUS communication allows the 350 relay to be connected to a
supervisor program or any other device with a master MODBUS communication channel.
The 350 will be always a slave station.
MODBUS TCP is a variant of the MODBUS protocol, intended for supervision and control of
automation equipment. It covers the use of MODBUS messaging in an 'Intranet' or
'Internet' environment using the TCP/IP protocols.
MODBUS TCP basically embeds a MODBUS frame into a TCP frame in a simple manner. This
is a connection-oriented transaction which means that every query expects a response.
When the relay communicates using MODBUS TCP, it does not require a checksum
calculation of the MODBUS frame as does the MODBUS RTU.
The 350 relay supports only a subset of the MODBUS protocol functions.
The following functions are supported:
01H Read Coil Status
Just respond, no action required for now.
Outgoing message for this function is the same as input one.
02H Read Input Status
Just respond, no action required for now.
Outgoing message for this function is the same as input one.
03H Read Holding Registers
Reads the binary contents of holding registers in the slave.
Query:
The query message specifies the starting register and quantity of registers to be read.
Registers are addressed starting at zero: registers 1 to 16 are addressed as 0 to 15.
Here is an example of a request to read registers 40172 to 40175 from slave device 254:
Field Name
Slave Address
Function
Starting Address Hi
Starting Address Lo
No. of Points Hi
No. of Points Lo
Response:
The register data in the response message are packed as two bytes per register, with the
binary contents right justified within each byte. For each register, the first byte contains the
high order bits and the second contains the low order bits.
The response is returned when the data is completely assembled.
ETHERNET INTERFACE
Hex
FE
03
00
AB
00
04
1–27