Exception And Error Responses; Request Response Sequence - GE 350 Communications Manual

MODBUS TCP/IP

Exception and error responses

Request response sequence

3–12
Field Name
Slave Address
Function
Group Activation
Starting Address Hi
Starting Address Lo
No. of Registers Hi
No. of Registers Lo
Byte Count 04
Data Hi
Data Lo
Response:
Field Name
Slave Address
Function
Starting Address Hi
Starting Address Lo
No. of Registers Hi
No. of Registers Lo
One data frame of an asynchronous transmission to or from a 350 typically consists of 1
start bit, 8 data bits, and 1 stop bit. This produces a 10 bit data frame. This is important for
transmission through modems at high bit rates.
Modbus protocol can be implemented at any standard communication speed. The
SR350supports operation at 9600, 19200, 38400, 57600, and 115200 baud.
A complete request/response sequence consists of the following bytes (transmitted as
separate data frames):
Master Request Transmission:
SLAVE ADDRESS: 1 byte
FUNCTION CODE: 1 byte
DATA: variable number of bytes depending on FUNCTION CODE
CRC: 2 bytes
Slave Response Transmission:
SLAVE ADDRESS: 1 byte
FUNCTION CODE: 1 byte
DATA: variable number of bytes depending on FUNCTION CODE
CRC: 2 bytes
SLAVE ADDRESS: This is the first byte of every transmission. This byte represents the user-
assigned address of the slave device that is to receive the message sent by the master.
Each slave device must be assigned a unique address and only the addressed slave will
respond to a transmission that starts with its address. In a master request transmission the
CHAPTER 3: ETHERNET INTERFACE
Hex
FE
43
00
09
C1
00
01
02
00
01
Hex
FE
43
09
C1
00
01
350 FEEDER PROTECTION SYSTEM – COMMUNICATIONS GUIDE