GE Industrial Solutions FG600 User Manual
DEH41181
Rev01
g
SMR2 Modbus User Manual
SMR2 Modbus User Manual
Table of Contents
Introduction .......................................................................................................................2
RTU Transmission Mode .......................................................................................................................... 2
Message frame............................................................................................................................................ 2
RTU Mode Message Frames ...................................................................................................................... 2
Message Format.......................................................................................................................................... 2
Electrical Interface .............................................................................................................2
Communication Environment...........................................................................................3
Register Map .....................................................................................................................2
Fixed Value Registers ................................................................................................................................. 2
Dynamic Value Registers............................................................................................................................ 7
Interpretation of SOS values...................................................................................................................... 9
Setpoint Registers....................................................................................................................................... 9
Event Registers......................................................................................................................................... 13
Interpretation of Event Codes.................................................................................................................. 15
Bit position in Bit map............................................................................................................................. 16
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SMR2 Modbus User Manual
Introduction
The SMR2 trip unit has an optional communications
module that facilitates monitoring of the trip unit via a
master Modbus device. Monitoring features include
breaker settings and the breaker status.
Communication with the SMR2 trip unit requires a
FAMECM module powered externally that provides
isolation and protection.
The Modbus communication protocol is a single
master multi-slave protocol that is hardware
independent and it interfaces with a 2-wire RS485
interface. A single master device initiates and controls
all communication with the other slave devices on the
network. A computer with a serial port and an external
RS-232/RS-485 converter can be used as the mastermonitoring device. The SMR II trip unit is always the
slave and cannot initiate communication. The
maximum number of devices on a Modbus network is
32, i.e. only 31 slave devices can be connected to a
master.
MODBUS devices usually include a Register Map.
MODBUS functions operate on Register map registers
to monitor, configure, and control module I/O. You
should refer to the Register map for your device to gain
a better understanding of its operation.
The transmission mode defines the bit contents of the
message bytes transmitted along the network, and how
the message information is to be packed into the
message stream and decoded.
Standard MODBUS networks employ one of two types
of transmission modes:
i) ASCII Mode ii) RTU Mode
The mode of transmission is usually selected along
with other serial port communication parameters
(baud rate, parity, etc.) as part of the device
configuration.
RTU Transmission Mode
In RTU (Remote Terminal Unit) Mode, each 8-bit
message byte contains two 4-bit Hexadecimal
characters, and the message is transmitted in a
continuous stream. The greater effective character
density increases throughput over ASCII mode at the
same baud rate.
Message frame
A message frame is used to mark the beginning and
ending point of a message allowing the receiving device
to determine which device is being addressed and to
know when the message is completed. It also allows
partial messages to be detected and errors flagged as a
result.
A MODBUS message is placed in a message frame by
the transmitting device. Each word of this message
(including the frame) is also placed in a data frame
that appends a start bit, stop bit, and parity bit.
In ASCII mode, the word size is 7 bits, while in RTU
mode; the word size is 8 bits. Thus, every 8 bits of an
RTU message is effectively 11 bits when accounting for
the start, stop, and parity bits of the data frame.
RTU Mode Message Frames
RTU mode messages start with a silent interval of at
least 3.5 character times. Implemented as a multiple of
character times at the baud rate being used on the
network. The first field transmitted is the device
address. The allowable characters transmitted for all
fields are hexadecimal values 0-9, A-F. A networked
device continuously monitors the network, including
the silent intervals, and when the first field is received
(the address) after a silent interval of at least 3.5
character times, the device decodes it to determine if it
is the addressed device. Following the last character
transmitted, a similar silent interval of 3.5 character
times marks the end of the message and a new message
can begin after this interval.
The entire message must be transmitted as a
continuous stream. If a silent interval of more than 1.5
character times occurs before completion of the frame
(not a continuous stream), the receiving device flushes
the incomplete message and assumes the next byte will
be the address field of a new message.
In similar fashion, if a new message begins earlier than
3.5 character times following a
previous message, the receiving device assumes it is a
continuation of the previous message. This will
generate an error, as the value in the final CRC field
will not be valid for the combined messages.
Message Format
The Modbus RTU Protocol is strictly based upon a
transaction scheme where a master device generates a
query and a slave device replies with a response. Each
query and response message transaction consists of the
following four parts
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SMR2 Modbus User Manual
MODBUS ADDRESSES
The master device addresses a specific slave device by
placing the 8-bit slave address in the address field of the
message (RTU Mode). The address field of the
message frame contains two characters (in ASCII
mode), or 8 binary bits (in RTU Mode). Valid
addresses are from 1-247.
When the slave responds, it places its own address in
this field of its response to let the master know which
slave is responding.
MODBUS FUNCTIONS
The function code field of the message frame will
contain two characters (in ASCII mode), or 8 binary
bits (in RTU Mode) that tell the slave what kind of
action to take. Valid function codes are from 1-255, but
not all codes will apply to a module and some codes are
reserved for future use.
MODBUS DATA FIELD
The data field provides the slave with any additional
information required by the slave to complete the
action specified by the function code. The data is
formed from a multiple of character bytes (a pair of
ASCII characters in ASCII Mode), or a multiple of two
hex digits in RTU mode, in range 00H-FFH. The data
field typically includes register addresses; count values,
and written data. If no error occurs, the data field of a
response from a slave will return the requested data. If
an error occurs, the data field returns an exception
code that the master's application software can use to
determine the next action to take.
The resulting value is compared to the error check
field.
Transmission errors occur when the calculated
checksum is not equal to the Checksum stored in the
incoming packet. The receiving device ignores a bad
packet.
Electrical Interface
The electrical interface is 2-wire RS485 where data flow
is bi-directional and half duplex. RS-485 lines from the
master device should be connected to a FAMECM
module, which is powered externally using a 19-29 VDC
power supply. Refer Figure 1 and 2.
The following wire types are recommended for network
wiring: Belden 3074F – Data Tray 600 V industrial
twinax 18 AWG (7X26); or Belden 9841 – 300 V
communication cable 24 AWG (7X32); or Alpha 6412 –
300 V Communication cable 24 AWG (7x32). Shielded
wire should always be used to minimize noise. Polarity
is important in RS485 communications and each '+'
terminal of every device must be connected together for
the system to communicate.
CRC Error Checking
RTU Mode message frames include an error checking
method that is based on a Cyclical Redundancy Check
(CRC). The error-checking field of a message frame
contains a 16-bit value (two 8-bit bytes) that contains
the result of a Cyclical Redundancy Check (CRC)
calculation performed on the message contents.
The checksum field lets the receiving device determine
if a packet is corrupted with transmission errors. In
Modbus RTU mode, a 16-bit Cyclic Redundancy Check
(CRC-16) is used. The sending device calculates a 16bit value, based on every byte in the packet, using the
CRC-16 algorithm. The calculated value is inserted in
the error check field.
The receiving device performs the calculation, without
the error check field; on the entire packet it receives.
Figure1. Connection between ETU and FAMECM
module
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SMR2 Modbus User Manual
The communication happens at 19200bps with a
Modbus address of 100
Figure2. Connection between Breaker and FAMECM
module
Communication Environment
• The trip unit is always the slave and cannot
initiate communication
• All the registers are read-only
• Hardware layer
RS485: Data moves serially on a 2-wire differential line
(used for TX as well as Rx i.e.; half- duplex)
N-8-1, no parity, 1 start bit, 8 data bits & 1 stop bit (10
bits per character)
• Communication Protocol
RTU Modbus Master-Slave protocol Commandresponse (half-duplex) Min delay between messages is
3.5 char time Max delay within message is 1.5 char
time.
Slave Response time: minimum 3.5 char;
maximum 50mS + 1mS per register
Register order: MSB first, then LSB
• In the presence of a communication
module on Trip unit
A pair of rotary switch decides the Modbus address (0
to 255)
A pair of slide switch decides the baud rate (2400, 4800,
9600 or 19200bps). These switches are available on the
backside of the module
• In the absence of a communication
module on Trip unit
Figure3. Communication modules used with trip unit
Modbus Message Frames
Master’s Query
Data
Start
Data
Byte1
(n bytes)
(n bytes)
No. of
Registers
(2 bytes)
(n bytes)
Byte2
No. of
Registers
(2
bytes)
..
CRC
(2 bytes)
(2bytes)
Byte n
CRC
bytes)
CRC
(2
Slave
Address
(1 byte)
Function
Code
(1 byte)
Address
(2 bytes)
Slave’s Response
Slave
Address
(1byte)
Functio
n Code
(1 byte)
Number
of Bytes
(1 byte)
Examples :
Read Holding Registers (Function Code 03)
Use this command to read setpoint or fixed value
registers. It specifies the starting register and the
number of registers to be read. Read the registers 30103012 from device 100:
Slave
Addres
s
(1
byte)
Function
Code
(1 byte)
Data
Start
Address
(2
bytes)
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SMR2 Modbus User Manual
A
A
64h
Response: (register contents: 3010 = 022Bh, 3010 = 0000h,
3012 = 0066h)
Slave
Addres
s
(1byte)
64h
Read Input Registers (Function Code 04)
Use this command to read dynamic value or event
registers. Read the registers 1058 – 1060 from device
100:
Query:
Slave
Address
(1 byte)
64h
Response: (register content: 1058 = FFFFh, 1059 = A01Ah,
1060 11B1h)
Slave
Addres
s
(1
byte)
64h
Force Single Coil (Function Code 05)
Use this command to perform remote control. This
command sets a single coil to enter ON or OFF.
Force coil 4 of device 100 ON (FF00h force on, 0000h
force off)
Query:
Slave
Address
(1 byte)
Functio
n Code
(1 byte)
03h
Function
Code
(1 byte)
Function
Code
(1 byte)
04h
Function
Code
(1 byte)
03h
04h
0Bh
C2h
Numbe
r of
Bytes
(1byte)
06h
02h 2Bh 00h
Data
Start
Address
(2 bytes)
04h 22h 00h 03h
Number
of Bytes
(1 byte)
06h
FFh FFh A0h
Data
Coil
Address
(2 bytes)
00h
03h
Data
(n bytes)
Byte1
Byte2
00h 00h 66h
(n bytes)
No. Of
Registers
(2 bytes)
Data
(n bytes)
Byte2
-------
Byte1
1Ah 11h B1h
(n bytes)
Data
(2
bytes)
CRCl
CRCh
CRC
-------
(2
bytes
Byte n
)
CRCl
CRC
h
CRC
(2 bytes)
CRCl
CRCh
CRC
(2
bytes)
Byte n
CRCl
CRCh
CRC
(2 bytes)
64h
05h
00h 04h FFh 00h
CRCl
CRCh
Response: (echo query message)
Data
Coil
(n bytes)
Data
(2
bytes)
CRC
CRCl
(2 bytes)
CRCh
Slave
Address
(1 byte)
64h
Function
Code
(1 byte)
05h
Address
(2 bytes)
00h 04h FFh 00h
Preset Single Register (Function Code 06)
Use this command to write to a register. This presets a
value into a single holding register.
Preset register 0002 to 00 03 hex in slave device 17.
Query:
Data
Slave
Address
(1 byte)
11h
Function
Code
(1 byte)
06h
Register
(2 bytes)
00h 02h 00h 03h
(n bytes)
ddress
No. of
Registers
(2 bytes)
CRC
(2 bytes)
CRCl
CRCh
Response (echo query message)
Data
Slave
Address
(1 byte)
Function
Code
(1 byte)
Register
(2 bytes)
(n bytes)
ddress
No. of
Registers
(2
(2 bytes)
CRC
bytes)
11h
06h
00h 02h 00h 03h
CRCl
CRCh
Retransmit Last Message (Function Code 56)
Query:
Slave Address
(1 byte)
64h
Function Code
(1 byte)
38h
CRCl CRCh
CRC
(2 bytes)
Response
Slave Function
Data
(n bytes)
CRC
4
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