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Modbus Protocol Description

MODBUS© Protocol is a messaging structure, widely used to establish master-slave communication
between intelligent devices. A MODBUS message sent from a master to a slave contains the address of the
slave, the 'command' (e.g. 'read register' or 'write register'), the data, and a check sum (LRC or CRC).
Since Modbus protocol is just a messaging structure, it is independent of the underlying physical layer. It is
traditionally implemented using RS232, RS422, or RS485

The Request
The function code in the request tells the addressed slave device what kind of action to perform. The data
bytes contains any additional information that the slave will need to perform the function. For example,
function code 03 will request the slave to read holding registers and respond with their contents. The data
field must contain the information telling the slave which register to start at and how many registers to read.
The error check field provides a method for the slave to validate the integrity of the message contents.

The Response
If the slave makes a normal response, the function code in the response is an echo of the function code in
the request. The data bytes contain the data collected by the slave, such as register values or status. If an
error occurs, the function code is modified to indicate that the response is an error response, and the data
bytes contain a code that describes the error. The error check field allows the master to confirm that the
message contents are valid.

Controllers can be setup to communicate on standard Modbus networks using either of two transmission
modes: ASCII or RTU.

ASCII Mode
When controllers are setup to communicate on a Modbus network using ASCII (American Standard Code for
Information Interchange) mode, each eight-bit byte in a message is sent as two ASCII characters. The main
advantage of this mode is that it allows time intervals of up to one second to occur between characters
without causing an error.

Coding System
Hexadecimal ASCII printable characters 0 ... 9, A ... F
Bits per Byte
1 start bit
7 data bits, least significant bit sent first
1 bit for even / odd parity-no bit for no parity
1 stop bit if parity is used-2 bits if no parity
Error Checking
Longitudinal Redundancy Check (LRC)

RTU Mode
When controllers are setup to communicate on a Modbus network using RTU (Remote Terminal Unit) mode,
each eight-bit byte in a message contains two four-bit hexadecimal characters. The main advantage of this
mode is that its greater character density allows better data throughput than ASCII for the same baud rate.
Each message must be transmitted in a continuous stream.

Coding System
Eight-bit binary, hexadecimal 0 ... 9, A ... F
Two hexadecimal characters contained in each eight-bit field of the message
Bits per Byte
1 start bit
8 data bits, least significant bit sent first
1 bit for even / odd parity-no bit for no parity
1 stop bit if parity is used-2 bits if no parity
Error Check Field
Cyclical Redundancy Check (CRC)

In ASCII mode, messages start with a colon ( : ) character (ASCII 3A hex), and end with a carriage return-line
feed (CRLF) pair (ASCII 0D and 0A hex).
The allowable characters transmitted for all other fields are hexadecimal 0 ... 9, A ... F. Networked devices
monitor the network bus continuously for the colon character. When one is received, each device decodes
the next field (the address field) to find out if it is the addressed device.
Intervals of up to one second can elapse between characters within the message. If a greater interval
occurs, the receiving device assumes an error has occurred. A typical message frame is shown below.

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Start Address Function Data LRC End

: 2 Chars 2 Chars N Chars 2 Chars CR LF

RTU Framing
In RTU mode, messages start with a silent interval of at least 3.5 character times. This is most easily
implemented as a multiple of character times at the baud rate that is being used on the network (shown as
T1-T2-T3-T4 in the figure below). The first field then transmitted is the device address.
The allowable characters transmitted for all fields are hexadecimal 0 ... 9, A ... F. Networked devices monitor
the network bus continuously, including during the silent intervals. When the first field (the address field) is
received, each device decodes it to find out if it is the addressed device.
Following the last transmitted character, a similar interval of at least 3.5 character times marks the end of
the message. A new message can begin after this interval.
The entire message frame must be transmitted as a continuous stream. If a silent interval of more than 1.5
character times occurs before completion of the frame, the receiving device flushes the incomplete
message and assumes that the next byte will be the address field of a new message.
Similarly, if a new message begins earlier than 3.5 character times following a previous message, the
receiving device will consider it a continuation of the previous message. This will set an error, as the value in
the final CRC field will not be valid for the combined messages. A typical message frame is shown below.

Start Address Function Data CRC End

3.5 Char time 8 Bit 8 Bit N * 8Bit 16 Bit 3.5 Char time

Address Field
The address field of a message frame contains two characters (ASCII) or eight bits (RTU). The individual
slave devices are assigned addresses in the range of 1 ... 247.

Function Field

The Function Code field tells the addressed slave what function to perform.
The following functions are supported by Modbus Poll.

01 (0x01) Read Coils

02 (0x02) Read Discrete Inputs
03 (0x03) Read Holding Registers
04 (0x04) Read Input Registers
05 (0x05) Write Single Coil
06 (0x06) Write Single Register
08 (0x08) Diagnostics (Serial Line only)
11 (0x0B) Get Comm Event Counter (Serial Line only)
15 (0x0F) Write Multiple Coils
16 (0x10) Write Multiple Registers
17 (0x11) Report Server ID (Serial Line only)
22 (0x16) Mask Write Register
23 (0x17) Read/Write Multiple Registers
43 / 14 (0x2B / 0x0E) Read Device Identification

The data field contains the requested or send data.

Contents of the Error Checking Field

Two kinds of error-checking methods are used for standard Modbus networks. The error checking field
contents depend upon the method that is being used.

ASCII
When ASCII mode is used for character framing, the error-checking field contains two ASCII characters. The
error check characters are the result of a Longitudinal Redundancy Check (LRC) calculation that is
performed on the message contents, exclusive of the beginning colon and terminating CRLF characters.
The LRC characters are appended to the message as the last field preceding the CRLF characters.
LRC Example Code

RTU
When RTU mode is used for character framing, the error-checking field contains a 16-bit value implemented
as two eight-bit bytes. The error check value is the result of a Cyclical Redundancy Check calculation
performed on the message contents.
The CRC field is appended to the message as the last field in the message. When this is done, the low-order
byte of the field is appended first, followed by the high-order byte. The CRC high-order byte is the last byte to
be sent in the message.
CRC Example Code

Function 01 (01hex) Read Coils

Reads the ON/OFF status of discrete coils in the slave.

Request

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The request message specifies the starting coil and quantity of coils to be read.

Example of a request to read 13 coils address 10...22 (Coil 11 to 23) from slave device address 4:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 04 04
Function 01 01
Starting Address Hi 00 00
Starting Address Lo 0A 0A
Quantity of Coils Hi 00 00
Quantity of Coils Lo 0D 0D
Error Check Lo DD LRC (E 4)
Error Check Hi 98
Trailer None CR LF
Total Bytes 8 17

Response
The coil status response message is packed as one coil per bit of the data field. Status is indicated as: 1 is
the value ON, and 0 is the value OFF. The LSB of the first data byte contains the coil addressed in the
request. The other coils follow toward the high-order end of this byte and from low order to high order in
subsequent bytes. If the returned coil quantity is not a multiple of eight, the remaining bits in the final data
byte will be padded with zeroes (toward the high-order end of the byte). The byte count field specifies the
quantity of complete bytes of data.

Example of a response to the request:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 04 04
Function 01 01
Byte Count 02 02
Data (Coils 18...11) 0A 0A
Data (Coils 23...19) 11 11
Error Check Lo B3 LRC (D E)
Error Check Hi 50 None
Trailer None CR LF
Total Bytes 7 15

Function 02(02hex) Read Discrete Inputs

Reads the ON/OFF status of discrete inputs in the slave.

Request
The request message specifies the starting input and quantity of inputs to be read.

Example of a request to read 13 inputs address 10...22 (input 10011 to 10023) from slave device address 4:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 04 04
Function 02 02
Starting Address Hi 00 00
Starting Address Lo 0A 0A
Quantity of inputs Hi 00 00
Quantity of inputs Lo 0D 0D
Error Check Lo 99 LRC (E 3)
Error Check Hi 98 None
Trailer None CR LF
Total Bytes 8 17

Response
The input status response message is packed as one input per bit of the data field. Status is indicated as: 1
is the value ON, and 0 is the value OFF. The LSB of the first data byte contains the input addressed in the
request. The other inputs follow toward the high-order end of this byte and from low order to high order in
subsequent bytes. If the returned input quantity is not a multiple of eight, the remaining bits in the final data

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byte will be padded with zeroes (toward the high-order end of the byte). The byte count field specifies the
quantity of complete bytes of data.

Example of a response to the request:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 04 04
Function 02 02
Byte Count 02 02
Data (Inputs 18...11) 0A 0A
Data (Inputs 23...19) 11 11
Error Check Lo B3 LRC (D D)
Error Check Hi 14 None
Trailer None CR LF
Total Bytes 7 15

Function 03 (03hex) Read Holding Registers

Read the binary contents of holding registers in the slave.

Request
The request message specifies the starting register and quantity of registers to be read.

Example of a request to read 0...1 (register 40001 to 40002) from slave device 1:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 01 01
Function 03 03
Starting Address Hi 00 00
Starting Address Lo 00 00
Quantity of Registers Hi 00 00
Quantity of Registers Lo 02 02
Error Check Lo C4 LRC (F A)
Error Check Hi 0B None
Trailer None CR LF
Total Bytes 8 17

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.

Example of a response to the request:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 01 01
Function 03 03
Byte Count 04 04
Data Hi 00 00
Data Lo 06 06
Data Hi 00 00
Data Lo 05 05
Error Check Lo DA LRC (E D)
Error Check Hi 31 None
Trailer None CR LF
Total Bytes 9 19

Function 04 (04hex) Read Input Registers

Read the binary contents of input registers in the slave.

Request
The request message specifies the starting register and quantity of registers to be read.

Example of a request to read 0...1 (register 30001 to 30002) from slave device 1:

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Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 01 01
Function 04 04
Starting Address Hi 00 00
Starting Address Lo 00 00
Quantity of Registers Hi 00 00
Quantity of Registers Lo 02 02
Error Check Lo 71 LRC (F 9)
Error Check Hi CB None
Trailer None CR LF
Total Bytes 8 17

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.

Example of a response to the request:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 01 01
Function 04 04
Byte Count 04 04
Data Hi 00 00
Data Lo 06 06
Data Hi 00 00
Data Lo 05 05
Error Check Lo DB LRC (E C)
Error Check Hi 86 None
Trailer None CR LF
Total Bytes 9 19

Function 05 (05hex) Write Single Coil

Writes a single coil to either ON or OFF.

Request
The request message specifies the coil reference to be written. Coils are addressed starting at zero-coil 1 is
addressed as 0.

The requested ON / OFF state is specified by a constant in the request data field. A value of FF 00 hex
requests the coil to be ON. A value of 00 00 requests it to be OFF. All other values are illegal and will not
affect the coil.

Here is an example of a request to write coil 173 ON in slave device 17:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 11 11
Function 05 05
Coil Address Hi 00 00
Coil Address Lo AC AC
Write Data Hi FF 00
Write Data Lo 00 FF
Error Check Lo 4E LRC (3 F)
Error Check Hi 8B None
Trailer None CR LF
Total Bytes 8 17

Response
The normal response is an echo of the request, returned after the coil state has been written.
Example of a response to the request:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)

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Slave Address 11 11
Function 05 05
Coil Address Hi 00 00
Coil Address Lo AC AC
Write Data Hi FF 00
Write Data Lo 00 FF
Error Check Lo 4E LRC (3 F)
Error Check Hi 8B None
Trailer None CR LF
Total Bytes 8 17

Function 06 (06hex) Write Single Register

Writes a value into a single holding register.

Request
The request message specifies the register reference to be Written. Registers are addressed starting at zero-
register 1 is addressed as 0.

The requested Write value is specified in the request data field. Here is an example of a request to Write
register 40002 to 00 03 hex in slave device 17.

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 11 11
Function 06 06
Register Address Hi 00 00
Register Address Lo 01 01
Write Data Hi 00 00
Write Data Lo 03 03
Error Check Lo 9A LRC (E 5)
Error Check Hi 9B None
Trailer None CR LF
Total Bytes 8 17

Response
The normal response is an echo of the request, returned after the register contents have been written.

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 11 11
Function 06 06
Coil Address Hi 00 00
Coil Address Lo 01 01
Write Data Hi 00 00
Write Data Lo 03 03
Error Check Lo 9A LRC (E 5)
Error Check Hi 9B None
Trailer None CR LF
Total Bytes 8 17

Function 15 (0Fhex) Write Multiple Coils

Writes each coil in a sequence of coils to either ON or OFF.

Request
The request message specifies the coil references to be written. Coils are addressed starting at zero-coil 1
is addressed as 0.

The requested ON / OFF states are specified by contents of the request data field. A logical 1 in a bit
position of the field requests the corresponding coils to be ON. A logical 0 requests it to be OFF.

Below is an example of a request to write a series of ten coils starting at coil 20 (addressed as 19, or 13 hex)
in slave device 17.

The request data contents are two bytes: CD 01 hex (1100 1101 0000 0001 binary). The binary bits
correspond to the coils in the following way:

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Bit: 1 1 0 0 1 1 0 1 0 0 0 0 0 0 0 1

Coil: 27 26 25 24 23 22 21 20 - - - - - - 29 28

The first byte transmitted (CD hex) addresses coils 27 ... 20, with the least significant bit addressing the
lowest coil (20) in this set.

The next byte transmitted (01 hex) addresses coils 29 and 28, with the least significant bit addressing the
lowest coil (28) in this set. Unused bits in the last data byte should be zero-filled.

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 11 11
Function 0F 0F
Coil Address Hi 00 00
Coil Address Lo 13 13
Quantity of Coils Hi 00 00
Quantity of Coils Lo 0A 0A
Byte Count 02 02
Write Data Hi CD CD
Write Data Lo 01 01
Error Check Lo BF LRC (F 3)
Error Check Hi 0B None
Trailer None CR LF
Total Bytes 11 23

Response
The normal response returns the slave address, function code, starting address, and number of coils written.
Here is an example of a response to the request shown above

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 11 11
Function 0F 0F
Coil Address Hi 00 00
Coil Address Lo 13 13
Quantity of Coils Hi 00 00
Quantity of Coils Lo 0A 0A
Error Check Lo 26 LRC (C 3)
Error Check Hi 99 None
Trailer None CR LF
Total Bytes 8 17

Function 16 (10hex) Write Multiple Registers

Writes values into a sequence of holding registers

Request
The request message specifies the register references to be written. Registers are addressed starting at
zero-register 1 is addressed as 0.

The requested write values are specified in the request data field. Data is packed as two bytes per register.

Here is an example of a request to write two registers starting at 40002 to 00 0A and 01 02 hex, in slave
device 17:

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 11 11
Function 10 10
Starting Address Hi 00 00
Starting Address Lo 01 01
Quantity of Registers Hi 00 00
Quantity of Registers Lo 02 02
Byte Count 04 04
Data Hi 00 00

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Data Lo 0A 0A
Data Hi 01 01
Data Lo 02 02
Error Check Lo C6 LRC (C B)
Error Check Hi F0 None
Trailer None CR LF
Total Bytes 13 23

Response
The normal response returns the slave address, function code, starting address, and quantity of registers
written. Here is an example of a response to the request shown above.

Field Name RTU (hex) ASCII Characters

Header None : (Colon)
Slave Address 11 11
Function 10 10
Starting Address Hi 00 00
Starting Address Lo 01 01
Quantity of Registers Hi 00 00
Quantity of Registers Lo 02 02
Error Check Lo 12 LRC (D C)
Error Check Hi 98 None
Trailer None CR LF
Total Bytes 8 17

LRC Example Code

This function is an example how to calculate a LRC BYTE using the C language.

BYTE LRC (BYTE *nData, WORD wLength)

{
BYTE nLRC = 0 ; // LRC char initialized

for (int i = 0; i < wLength; i++)

nLRC += *nData++;

return (BYTE)(-nLRC);

} // End: LRC

CRC Example Code

This function is an example how to calculate a CRC word using the C language.

WORD CRC16 (const BYTE *nData, WORD wLength)

{
static const WORD wCRCTable[] = {
0X0000, 0XC0C1, 0XC181, 0X0140, 0XC301, 0X03C0, 0X0280, 0XC241,
0XC601, 0X06C0, 0X0780, 0XC741, 0X0500, 0XC5C1, 0XC481, 0X0440,
0XCC01, 0X0CC0, 0X0D80, 0XCD41, 0X0F00, 0XCFC1, 0XCE81, 0X0E40,
0X0A00, 0XCAC1, 0XCB81, 0X0B40, 0XC901, 0X09C0, 0X0880, 0XC841,
0XD801, 0X18C0, 0X1980, 0XD941, 0X1B00, 0XDBC1, 0XDA81, 0X1A40,
0X1E00, 0XDEC1, 0XDF81, 0X1F40, 0XDD01, 0X1DC0, 0X1C80, 0XDC41,
0X1400, 0XD4C1, 0XD581, 0X1540, 0XD701, 0X17C0, 0X1680, 0XD641,
0XD201, 0X12C0, 0X1380, 0XD341, 0X1100, 0XD1C1, 0XD081, 0X1040,
0XF001, 0X30C0, 0X3180, 0XF141, 0X3300, 0XF3C1, 0XF281, 0X3240,
0X3600, 0XF6C1, 0XF781, 0X3740, 0XF501, 0X35C0, 0X3480, 0XF441,
0X3C00, 0XFCC1, 0XFD81, 0X3D40, 0XFF01, 0X3FC0, 0X3E80, 0XFE41,
0XFA01, 0X3AC0, 0X3B80, 0XFB41, 0X3900, 0XF9C1, 0XF881, 0X3840,
0X2800, 0XE8C1, 0XE981, 0X2940, 0XEB01, 0X2BC0, 0X2A80, 0XEA41,
0XEE01, 0X2EC0, 0X2F80, 0XEF41, 0X2D00, 0XEDC1, 0XEC81, 0X2C40,
0XE401, 0X24C0, 0X2580, 0XE541, 0X2700, 0XE7C1, 0XE681, 0X2640,
0X2200, 0XE2C1, 0XE381, 0X2340, 0XE101, 0X21C0, 0X2080, 0XE041,
0XA001, 0X60C0, 0X6180, 0XA141, 0X6300, 0XA3C1, 0XA281, 0X6240,
0X6600, 0XA6C1, 0XA781, 0X6740, 0XA501, 0X65C0, 0X6480, 0XA441,
0X6C00, 0XACC1, 0XAD81, 0X6D40, 0XAF01, 0X6FC0, 0X6E80, 0XAE41,
0XAA01, 0X6AC0, 0X6B80, 0XAB41, 0X6900, 0XA9C1, 0XA881, 0X6840,
0X7800, 0XB8C1, 0XB981, 0X7940, 0XBB01, 0X7BC0, 0X7A80, 0XBA41,
0XBE01, 0X7EC0, 0X7F80, 0XBF41, 0X7D00, 0XBDC1, 0XBC81, 0X7C40,
0XB401, 0X74C0, 0X7580, 0XB541, 0X7700, 0XB7C1, 0XB681, 0X7640,

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0X7200, 0XB2C1, 0XB381, 0X7340, 0XB101, 0X71C0, 0X7080, 0XB041,

0X5000, 0X90C1, 0X9181, 0X5140, 0X9301, 0X53C0, 0X5280, 0X9241,
0X9601, 0X56C0, 0X5780, 0X9741, 0X5500, 0X95C1, 0X9481, 0X5440,
0X9C01, 0X5CC0, 0X5D80, 0X9D41, 0X5F00, 0X9FC1, 0X9E81, 0X5E40,
0X5A00, 0X9AC1, 0X9B81, 0X5B40, 0X9901, 0X59C0, 0X5880, 0X9841,
0X8801, 0X48C0, 0X4980, 0X8941, 0X4B00, 0X8BC1, 0X8A81, 0X4A40,
0X4E00, 0X8EC1, 0X8F81, 0X4F40, 0X8D01, 0X4DC0, 0X4C80, 0X8C41,
0X4400, 0X84C1, 0X8581, 0X4540, 0X8701, 0X47C0, 0X4680, 0X8641,
0X8201, 0X42C0, 0X4380, 0X8341, 0X4100, 0X81C1, 0X8081, 0X4040 };

BYTE nTemp;
WORD wCRCWord = 0xFFFF;

while (wLength--)
{
nTemp = *nData++ ^ wCRCWord;
wCRCWord >>= 8;
wCRCWord ^= wCRCTable[nTemp];
}
return wCRCWord;

} // End: CRC16

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