YASKAWA GPD 315/V7 User manual

GPD 315/V7 Modbus RTU
Technical Manual

GPD315/V7 Modbus RTU Technical Manual TM 4325 Page ii
Technical References
Refer to the following publications for further information about the GPD315/V7:
• GPD315/V7 Technical Manual
• Publication TM4315
Refer to the following Modicon publication for technical information on Modbus RTU:
• Modicon Modbus Protocol Reference Guide
• Publication PI-MBUS-300 Rev. D
Technical Support Center:
Provide telephone assistance related to installation, start-up, programming, and troubleshooting
For technical phone support call 1-800-541-0939.

GPD315/V7 Modbus RTU Technical Manual TM 4325 Page iii
Contents
Technical References ....................................................................................................................ii
Technical Support Center: .............................................................................................................ii
Field Service/Support: ...................................................................................................................ii
Chapter 1......................................................................................................................................1
GPD315 and Serial Communication.............................................................................................1
Introduction...................................................................................................................................1
GPD315 Modbus RTU Specifications ...........................................................................................1
Chapter 2......................................................................................................................................2
GPD315 Modbus RTU Connections .............................................................................................2
Wiring...........................................................................................................................................2
Connecting Multiple Drives ...........................................................................................................4
Chapter 3......................................................................................................................................5
GPD315 Communication Parameters...........................................................................................5
Run/Stop and Frequency Selection...............................................................................................5
Serial Communication Set up Parameters....................................................................................6
WARNING....................................................................................................................................8
Chapter 4......................................................................................................................................9
Modbus RTU Message Format.....................................................................................................9
Message Functions.......................................................................................................................9
Read Multiple Registers – Function 03h .....................................................................................10
Loop-back Test - 08h..................................................................................................................12
Write Multiple Registers - 10h.....................................................................................................14
No Response Message...............................................................................................................17
CRC-16 ......................................................................................................................................17
Chapter 5....................................................................................................................................20
Parameter Tables........................................................................................................................20
Simultaneous Broadcast Registers (Write Only).........................................................................20
Command Registers (Read/Write)..............................................................................................20
Monitor Registers (Read Only) ...................................................................................................21
Drive Parameter Registers (Read/Write) ....................................................................................23
Notes:.........................................................................................................................................33
Error Codes and Troubleshooting...............................................................................................34
Communication Error..................................................................................................................34
Modbus Error Codes...................................................................................................................34
Chapter 7....................................................................................................................................35
Command Priority.......................................................................................................................35
Command Priority.......................................................................................................................35
Set up for Serial Communication Control....................................................................................36
Set up for External Terminals Control .........................................................................................37
Set up for Digital Operator Control..............................................................................................38
Appendix A .................................................................................................................................39
Product Specifications ................................................................................................................39

GPD315/V7 Modbus RTU Technical Manual TM 4325 Page iv
Tables
Table 1-1 Serial Setup .................................................................................................................1
Table 2-1. Functions of Terminals.................................................................................................3
Table 2-2. Applicable Wire Size for Terminal Connections............................................................3
Table 3-1. Run/Stop Selection.......................................................................................................5
Table 3-2. Frequency Reference Selection...................................................................................5
Table 3-3. Modbus Time out Detection..........................................................................................6
Table 3-4. Modbus Frequency Reference Unit Selection ..............................................................6
Table 3.5 Modbus Baud Rate Selection.......................................................................................7
Table 3.6 Modbus Parity Selection...............................................................................................7
Table 4-1 Supported Modbus RTU Function Codes.....................................................................9
Table 4-2 Read Command Message..........................................................................................10
Table 4-3 Read Normal Response Message.............................................................................. 11
Table 4-4 Read Fault Response Message.................................................................................12
Table 4-5 Loop Back Command Message.................................................................................12
Table 4-6 Loop Back Normal Response Message .....................................................................13
Table 4-7 Loop Back Fault Response Message.........................................................................13
Table 4-8 Write Command Message...........................................................................................14
Table 4-9 Write Registers Normal Response Message..............................................................15
Table 4-10 Write Registers Fault Response Message ...............................................................16
Table 5-1 Simultaneous Broadcast Registers ............................................................................20
Table 5-2 Command Registers ..................................................................................................20
Table 5-3 Monitor Registers.......................................................................................................21
Table 5-4 Drive Parameters.......................................................................................................23
Table 6-1 n151 Settings.............................................................................................................34
Table 6-2 Modbus Error Message..............................................................................................34
Table 6-3 Modbus Error Codes..................................................................................................34
Table 7-1 n003 Settings.............................................................................................................35
Table 7-1: Set up for Serial Communication Control..................................................................36
Table 7-2: Set up for External Terminals Control........................................................................37
Table 7-3: Set up for Digital Operator Control............................................................................38
Table A-1 Product Specifications................................................................................................39
Figures
Figure 2-1 Location of Terminals and Dip switches......................................................................2
Figure 2-2 Shielded Wire Termination..........................................................................................2
Figure 2.3 Interconnection of Drives using RS-485......................................................................4
Figure 2.4 Interconnection of Drives using RS-422......................................................................4
Figure 4-1 CRC Calculation in Quick Basic................................................................................18
Figure 4-2 CRC Calculation in C................................................................................................19

GPD315 Modbus RTU Technical Manual TM 4325 Page 1
Chapter 1
GPD315/V7 and Serial Communication
Introduction
This manual describes the set-up and protocol for Modbus Communication. The GPD315/V7
offers RS-485 and RS-422 serial communication as a standard.
The Modbus RTU protocol requires that the controller communicates using a master-slave
technique, in which only one device (the master) can initiate transactions. The other devices
(slaves) respond by supplying the requested data to the master, or by taking the action
requested. The drive must act in the slave mode.
A complete understanding of the drive programming and operation is required before attempting
serial communication operation. A full discussion of programming and operation is covered in the
GPD315/V7 technical manual, TM4315.
GPD315 Modbus RTU Specifications
The data that may be sent or received from the drive consists of:
Run Command
Frequency Reference
Fault Contents
Drive Status
Drive Parameter Settings
The following table illustrates whether the serial communication specifications are fixed or user
selectable. If the specification is fixed, the fixed value is shown in the last column. If the
specification is selectable, the range of allowed values is shown in the last column.
Table 1-1 Serial Setup
Parameter Description Type Range
n154 Baud Rate Selectable 2400, 4800, 9600, or 19200 bps
N/A Data Bit Fixed 8
n155 Parity Selectable none, even, or odd
N/A Stop Bit Fixed 1
n153 Node/Slave Address Selectable maximum of 31 nodes

GPD315 Modbus RTU Technical Manual TM 4325 Page 2
Chapter 2
GPD315 Modbus RTU Connections
Wiring
Locate terminals S+, S-, R+, and R- on the control terminal block.
Figure 2-1 Location of Terminals and Dip switches
Twisted pair, shielded wire should be used for all RS-485 connections. The shielded wire should
be separated and connected per the drawing below to eliminate interference due to noise.
Figure 2-2 Shielded Wire Termination
R+ R-
S+ S-
S+/R+
S-/R-
S+/R+
-
S-/R-
Outer Jacket S+
S-
R+
R
-
S+
S-
R+
R-
Shiel
Outer Jacket
Two Wire RS-485 Four Wire RS-485

GPD315 Modbus RTU Technical Manual TM 4325 Page 3
The function of terminals R+, R-, S+, and S- are described below.
Table 2-1. Functions of Terminals
Terminal Symbol Functions Remarks
S+ RS-485 output (+) Use as output at
S- RS-485 output (-) parallel connection
R+ RS-485 input (+) Us as input at
R- RS-485 input (-) parallel connection
It is important to select an appropriate wire size to prevent voltage drop. The table below
indicates the suggested wire sizes.
Table 2-2. Applicable Wire Size for Terminal Connections
[mm2]AWG I[A] VAC[V]
Twisted wire 1.0 16 12 125
Single wire 1.5 16 12 125
UL - 22-16 10 300
CSA - 28-16 10 300
CSA - 28-16 10 150
When stripping the wire, approximately 5.5 mm should be exposed in order to make a good
connection.
Note: Avoid sources of electric interference capable of inducing noise into the cable.
Communication and signal wiring should be kept separate from power wiring. Four inches should
separate the communications/signal wiring from any low voltage AC or high frequency source.
Ten inches should separate the communications/signal wiring from any high voltage cables. If
communication or signal wiring must cross power wiring, it must cross at a right angle.

GPD315 Modbus RTU Technical Manual TM 4325 Page 4
Connecting Multiple Drives
Multiple drives may be connected together. A terminating resistor must be enabled at the start
and end of the network. Set SW2 switch 1 to “on” to enable the terminating resistor. SW2 is
found just above the upper row of control terminals and consists of two switches. The switch
towards the top (labeled “1”) controls the terminating resistor. The following diagram illustrates
the connection between multiple drives for RS-485 (half duplex) and RS-422 (full duplex).
Figure 2.3 Interconnection of Drives using RS-485
Figure 2.4 Interconnection of Drives using RS-422
Note: The shielded wire should be connected to the drive’s ground terminal.

GPD315 Modbus RTU Technical Manual TM 4325 Page 5
Chapter 3
GPD315 Communication Parameters
Run/Stop and Frequency Selection
The run/stop and frequency reference commands can originate from serial communication, the
digital operator, or the external terminals. The origin of the run/stop command does not have to
be the same as the origin of the frequency reference command. Parameter n003 (Operation
Method Selection) allows you to set up the origin of the run/stop commands. Parameter n004
(Reference Selection) allows you to set up the origin of the frequency reference command. The
charts below illustrate the possible run/stop and frequency reference selections.
Table 3-1. Run/Stop Selection
Parameter n003 Setting Operation Method Selection
0 Digital Operator
1 External Terminals
2 Serial Communication
The default value of parameter n003 is ‘1’.
Table 3-2. Frequency Reference Selection
Parameter n004 Setting Frequency Reference Selection
0 Digital Operator Pot
1 Digital Operator
2 Voltage Reference (0-10V)
3 Current Reference (4-20mA)
4 Current Reference (0-20mA)
5 Pulse Train Reference
6 Serial Communication
7 Multi-Function Analog Input (0-10V)
8 Multi-Function Analog Input (4-20mA)
9OptionBoard
The default value of parameter n004 is’2’.

GPD315 Modbus RTU Technical Manual TM 4325 Page 6
Serial Communication Set up Parameters
The GPD315 has parameters used for setting up serial communications. These communication
set up parameters are n151 through n155.
Parameter n151 - Modbus Time Out Detection
Parameter n151 is used to determine how the drive will respond to a time out error. A time out is
detected if the length of time between Modbus messages exceeds two seconds. The drive will
either fault, alarm, or not respond to a time out detection, depending upon how parameter n151
is set.
Table 3-3 below indicates how the drive will respond to a “time out” detection.
Table 3-3. Modbus Time out Detection
Parameter n151 Setting Description
0 Coast to Stop (fault)
1 Ramp to Stop using n020 (fault)
2 Ramp to Stop using n022 (fault)
3 Continue Operation
4 Disabled
The default setting of parameter n151 is ‘0’.
Parameter n152 - Modbus Frequency Reference Unit
Parameter n152 selects the frequency resolution of the frequency reference and the output
frequency monitor.
The output frequency resolution of the GPD315’s digital operator is settable via n035, Frequency
Reference Unit Selection. If the digital operator resolution is set to 0.1 Hz (n035=0), and the
Modbus resolution is changed to 0.01 Hz in n152, the value in the hundredths digit of 0.01 Hz of
the received frequency reference is rounded off when displayed on the digital operator.
Table 3-4. Modbus Frequency Reference Unit Selection
Parameter n152 Setting Frequency Reference Unit Selection
00.1Hz
10.01Hz
2 100% / 30,000
30.1%
The default setting of parameter n152 is ‘0’.
Parameter n153 - Modbus Slave Address
Parameter n153 selects the drive’s node or slave address. Each device on the same network
must be given a unique address. The setting range of parameter n153 is 0 to 31.
The default setting of parameter n153 is ‘0’.

GPD315 Modbus RTU Technical Manual TM 4325 Page 7
Parameter n154 - Modbus Baud Rate
Parameter n154 selects the baud rate, as indicated by the following table:
Table 3.5 Modbus Baud Rate Selection
Parameter n154 Setting Baud Rate(bps)
0 2400
1 4800
2 9600
3 19200
The default setting of parameter n154 is ‘2’.
Parameter n155 - Modbus Parity Selection
Parameter n155 selects the parity, as indicated by the following table:
Table 3.6 Modbus Parity Selection
Parameter n155 Setting Parity Selection
0Even
1Odd
2 None
The default setting for parameter n155 is ‘2’.
Note: It is necessary to recycle drive power for parameters n153 through n155 to take effect.

GPD315 Modbus RTU Technical Manual TM 4325 Page 8
“ENTER” Command
The GPD315 has two types of memory: ‘Volatile’ and ‘Non-volatile’. Data held in the Volatile
memory will be lost when power is removed from the drive. Data held in the Non-volatile memory
will be retained when power is removed from the drive. It is necessary to follow each block of
parameters with the ENTER command in order to transfer data form Volatile to Non-volatile
memory. To send an ENTER command, send a value of 0 to address 900h. See warning below.
Command Data:
The command registers (000H to 009H) are stored in Volatile memory. When writing to a
command register the new data becomes active immediately. In the case of a power loss all data
stored in these registers will be lost.
Monitor Data:
The monitor registers (020H to 03DH) are stored in Volatile memory. These registers are read
only. Any data read from the monitor registers will not be retained during a power loss.
Parameter Data:
The parameter registers (101H to 1D2H) are stored in Non-volatile memory. When writing new
data to parameter registers, an ENTER command must be sent for the new data to saved to
non-volatile memory. To send an ENTER command, send a value of 0 to address 900h. See
warning below.
Some parameter registers may only be written to when the drive is stopped. These are called the
non-run operative parameters.
WARNING
Use the ENTER (0900H) command only when necessary! The life of the EEPROM (Non-volatile
memory) on the GPD315 will only support a finite number of operations. This means that the
ENTER command can only be used a limited number of times. After the specified number of
operations, the EEPROM may fault (ERR) requiring replacement of the GPD315 control board.

GPD315 Modbus RTU Technical Manual TM 4325 Page 9
Chapter 4
Modbus RTU Message Format
Message Functions
In communicating to the GPD315 drive via Modbus RTU, there are three message functions
available. The master specifies the function to be executed by the slave according to the function
code. The following table shows the types of function codes available, and the length (quantity)
and contents of the message according to the function.
Table 4-1 Supported Modbus RTU Function Codes
Command Message Response Message
(Normal)
Function
Code (hex) Function min. (bytes) max. (bytes) min. (bytes) max. (bytes)
3 Read Multiple Registers 8 8 7 21
8 Loop-back test 8 8 8 8
10 Write Multiple Registers 11 25 8 8
The message format varies depending upon the function of the message. For each function,
there is a command message from the master and a response message initiated from the slave.
The following sections review the format of the command message and the response message
for each function.

GPD315 Modbus RTU Technical Manual TM 4325 Page 10
Read Multiple Registers – Function 03h
The multiple register read function (03h) allows the master to request information from the slave.
The command message of a multiple register read is structured as shown below.
Table 4-2 Read Command Message
Slave Address 02h
Function Code 03h
Upper 00h
Starting Register Lower 20h
Upper 00h
Quantity Lower 04h
Lower 45h
CRC-16 Upper F0h
Each GPD315’s slave address is set in advance by the drive parameter n153. Valid slave
addresses must be in the range of 1 to 31 decimal (1 to 1F hex). No two slaves may have the
same address. The master addresses the slave by placing the slave address in the address field
of the message. In the command message above, the slave is addressed at 2.
The function code of this message is 03h (read multiple registers).
The starting number is the first register to be read. In the command message above the starting
register is 20h, indicating that the first register is the Status Signal. A listing of the GPD315’s
registers is shown in Chapter 5, Registers.
The quantity indicates how many consecutive registers are to be read. The quantity may range
from 1 to 8 registers. If the quantity is greater than 8, an error code of ‘3’ is returned in the fault
response message. In this example there are four consecutive registers to be read: 20h, 21h,
22h and 23h.
A CRC-16 value is generated from a calculation using the values of the address, function code,
and data sections of the message. The procedure for calculating a CRC-16 is described at the
end of this chapter. When the slave receives the command message it calculates a CRC-16
value and compares it to the one in CRC-16 field of the command message. If these two CRC-
16 values are the same the slave has received the proper command message. If the two CRC-
16 values are not the same the slave will not respond.
If the command message has a valid slave address, function code, starting register, and quantity
value, the slave will respond with a normal response message. If the command message has an
invalid slave address, function code, starting register, and/or quantity the slave will respond with
a fault response message.

GPD315 Modbus RTU Technical Manual TM 4325 Page 11
Normal Response Message
Table 4-3 Read Normal Response Message
Slave Address 02h
Function Code 03h
Number of Data Bytes 08h
Upper 17h
Starting Register Lower 70h
Upper 17h
Next Register Lower 70h
Upper 01h
Next Register Lower 09h
Upper 00h
Last Register Lower 00h
Lower 38h
CRC-16 Upper ACh
The normal response message contains the same slave address and function code as the
command message, indicating to the master which slave is responding and to what type of
function it is responding.
The number of data bytes is the number of data bytes returned in the response message. The
number of data bytes is actually the quantity (in the command message) times 2, since there are
two bytes of data in each register.
The data section of the response message contains 8 upper and 8 lower bits of data for each
register that has been read from the drive.
A CRC-16 value is generated from a calculation using the values of the address, function code,
number of data bytes, and register data sections of the message. The procedure for calculating
a CRC-16 value is described at the end of this chapter. When the master receives the response
message it calculates a CRC-16 value and compares it to the one in the CRC-16 field of the
response message. If these two CRC-16 values are the same the master has received the
proper response message.

GPD315 Modbus RTU Technical Manual TM 4325 Page 12
Fault Response Table 4-4 Read Fault Response Message
Slave Address 02h
Function Code 83h
Error Code 02h
Lower 30h
CRC-16 Upper F1h
The fault response message contains the same slave address as the command message,
indicating to the master which slave is responding.
The function code of a fault response message is actually a value of 80h plus the original
function code of 03h. This indicates to the master that the message is a fault response message,
instead of a normal response message.
The error code indicates where the error occurred in the command message. The value of 2h in
the error code field of this fault response message, indicates that the command message
requested data be read from an invalid register. A complete listing of the error codes is shown in
Chapter 6, Troubleshooting and Error Codes.
A CRC-16 value is generated from a calculation using the values of the address, function code,
and error code sections of the message. The procedure for calculating a CRC-16 value is
described at the end of this chapter. When the master receives the fault response message it
calculates a CRC-16 value and compares it to the one in the CRC-16 field of the fault response
message. If these two CRC-16 values are the same the master has received the proper fault
response message.
Loop-back Test - 08h
The loop-back test function (08h) is used for checking signal transmission between master and
slaves. The command message format is shown below.
Table 4-5 Loop Back Command Message
Slave Address 01h
Function Code 08h
Upper 00h
Test Code Lower 00h
Upper A5h
Lower 37h
Data
Upper ACh
Lower Dah
CRC-16 Upper 8Dh
Each GPD315’s slave address is set in advance by the drive parameter n106. Valid slave
addresses must be in the range of 1 to 31 decimal (1 to 1F hex). No two slaves may have the
same address. The master addresses the slave by placing the slave address in the address field
of the message. In the command message above, the slave is addressed at 1.

GPD315 Modbus RTU Technical Manual TM 4325 Page 13
The function code of this message is 08h (loop-back test).
The test code must be set to ‘0000’. This function specifies that the data passed in the command
message is to be returned (looped back) in the response message.
The data section contains arbitrary data values. These data values are used to verify that the
slave receives the correct data.
A CRC-16 value is generated from a calculation using the values of the address, function code,
test code, and data sections of the message. The procedure for calculating a CRC-16 is
described at the end of this chapter. When the slave receives the command message it
calculates a CRC-16 value and compares it to the one in CRC-16 field of the command
message. If these two CRC-16 values are the same the slave has received the proper command
message. If these two CRC-16 values are not the same the slave does not respond.
If the command message has a valid slave address, function code, test code, and data value,
the slave will respond with a normal response message. If the command message has an invalid
slave address, function code, test code, and/or data value the slave will respond with a fault
response message.
Loop Back Normal Response
Table 4-6 Loop Back Normal Response Message
Slave Address 01h
Function Code 08h
Upper 00h
Test Code Lower 00h
Upper A5h
Lower 37h
Data
Upper ACh
Lower Dah
CRC-16 Upper 8Dh
Loop Back Fault Response
Table 4-7 Loop Back Fault Response Message
Slave Address 01h
Function Code 88h
Error Code 01h
Lower 87h
CRC-16 Upper C0h
The fault response message contains the same slave address as the command message,
indicating to the master which slave is responding.
The function code of a fault response message is actually a value of 80h plus the original
function code of 08h. This indicates to the master that the message is a fault response message,
instead of a normal response message.
The error code indicates where the error occurred in the command message. A complete listing
of the error codes is shown in Chapter 6, Troubleshooting and Error Codes.

GPD315 Modbus RTU Technical Manual TM 4325 Page 14
A CRC-16 value is generated from a calculation using the values of the address, function code,
and data sections of the message. The procedure for calculating a CRC-16 value is described at
the end of this chapter. When the master receives the fault response message it calculates a
CRC-16 value and compares it to the one in the CRC-16 field of the fault response message. If
these two CRC-16 values are the same the master has received the proper fault response
message.
Write Multiple Registers - 10h
The multiple register write function (10h) allows the master to write data to the drive’s registers.
The multiple register write message format is shown below.
Table 4-8 Write Command Message
Slave Address 01h
Function Code 10h
Upper 00h
Starting Register Lower 01h
Upper 00h
Quantity Lower 02h
Number of Data Bytes 04h
Upper 00h
First Register Data Lower 01h
Upper 02h
Next Register Data Lower 58h
Lower 63h
CRC-16 Upper 39h
Each GPD315’s slave address is set in advance by the drive parameter n153. Valid slave
addresses must be in the range of 1 to 31 decimal (1 to 1F hex). No two slaves may have the
same address. The master addresses the slave by placing the slave address in the address field
of the message. In the command message above, the slave is addressed at 1.
By setting the slave address to zero (0) in the address section of the message, the master can
send operation signals (register 1h) and frequency reference (register 2h) to all slaves on the
network. The master can send a single transmission to all the slaves simultaneously. This is
called simultaneous broadcasting. In a simultaneous broadcast message all of the slaves on the
network act upon one message. Simultaneous Broadcast registers are shown in Chapter 5,
Registers.
The function code of this message is 10h (write multiple registers).
The starting register number is the first register to be written to. In the command message above
the starting number is 01h, indicating that the first register is the frequency reference. A listing of
the GPD315’s registers is shown in Chapter 5, Registers.
The quantity indicates how many consecutive registers are to be written to. The quantity may
range from 1 to 8 registers. If the quantity is greater than 8, an error code of ‘3’ is returned in the
fault response message. In this command message there is two consecutive registers to be
written to: 01h-Operation Command and 02h- Frequency Reference.
The number of data bytes is the number of bytes of data to be written to the drive. The number
of data bytes is actually the quantity times 2, since there are two bytes of data in each register.

GPD315 Modbus RTU Technical Manual TM 4325 Page 15
The data section of the response message contains 8 upper and 8 lower bits of data for each
register that is being written to.
A CRC-16 value is generated from a calculation using the values of the address, function code,
starting register number, quantity, number of data bytes, and data sections of the message. The
procedure for calculating a CRC-16 is described at the end of this chapter. When the slave
receives the command message it calculates a CRC-16 value and compares it to the one in
CRC-16 field of the command message. If these two CRC-16 values are the same the slave has
received the proper command message. If these two CRC-16 values are not the same the slave
does not respond.
If the command message has a valid slave address, function code, starting register number,
quantity, number of data bytes, and data values, the slave will respond with a normal response
message. If the command message has an invalid slave address, function code, starting register
number, quantity, number of data bytes, and/or data values the slave will respond with a fault
response message.
Write Registers Normal Response
Table 4-9 Write Registers Normal Response Message
Slave Address 01h
Function Code 10h
Upper 00h
Starting Register Lower 01h
Upper 00h
Quantity Lower 02h
Lower 10h
CRC-16 Upper 08h
The normal response message contains the same slave address and function code as the
command message, indicating to the master which slave is responding and to what type of
function it is responding.
The starting number is the first register that was written to. In the response message above the
starting number is 01h, indicating that the first register is the operation command.
The quantity indicates how many consecutive registers were written to.
A CRC-16 value is generated from a calculation using the values of the address, function code,
starting register number, and quantity value of the message. The procedure for calculating a
CRC-16 value is described at the end of this chapter. When the master receives the response
message it calculates a CRC-16 value and compares it to the one in the CRC-16 field of the
response message. If these two CRC-16 values are the same the master has received the
proper response message.

GPD315 Modbus RTU Technical Manual TM 4325 Page 16
Write Registers Fault Response
Table 4-10 Write Registers Fault Response Message
Slave Address 01h
Function Code 90h
Error Code 02h
Lower CDh
CRC-16 Upper C1h
The fault response message contains the same slave address as the command message,
indicating to the master which slave is responding.
The function code of a fault response message is actually a value of 80h plus the original
function code of 10h. This indicates to the master that the message is a fault response message,
instead of a normal response message.
The error code indicates where the error occurred in the command message. The value of 2h in
the error code field of this fault response message, indicates that the command message
requested data to be written to an invalid register. A complete listing of the error codes is shown
in Chapter 6, Troubleshooting and Error Codes.
A CRC-16 value is generated from a calculation using the values of the address, function code,
and error code sections of the message. The procedure for calculating a CRC-16 value is
described at the end of this chapter. When the master receives the fault response message it
calculates a CRC-16 value and compares it to the one in the CRC-16 field of the response
message. If these two CRC-16 values are the same the master has received the proper
response message.