ABB RS485 Modbus adapter Installation manual

RS485 Modbus adapter for RVT
Touchscreen
Installation and start-up guide

2 – Table of contents RS-485
Table of contents
1Introduction .................................................................................................................. 4
1.1 Intended audience ................................................................................................. 4
1.2 Before you start ..................................................................................................... 4
1.3 How to use this manual ......................................................................................... 4
2Overview ....................................................................................................................... 5
2.1 Introduction to Modbus .......................................................................................... 5
2.2 The Modbus adapter ............................................................................................. 5
2.3 Compatibility .......................................................................................................... 5
3Serial interface considerations................................................................................... 6
3.1 Communication mode............................................................................................ 6
3.2 SINGLE ENDED versus DIFFERENTIAL data transmission ................................ 6
3.3 RS-232 interface.................................................................................................... 6
3.4 RS-422 interface.................................................................................................... 7
3.5 RS-485 interface.................................................................................................... 7
3.6 Bias resistors ......................................................................................................... 7
3.7 Termination resistors ............................................................................................. 7
3.8 Shielding and grounding considerations ............................................................... 8
3.9 Cable requirements ............................................................................................... 8
3.10 Network topology................................................................................................... 8
4RS485 Modbus adapter ............................................................................................. 10
4.1 Main features ....................................................................................................... 10
4.2 Physical dimensions ............................................................................................ 10
4.3 Mounting .............................................................................................................. 11
4.4 Technical data ..................................................................................................... 11
4.5 Commissioning process ...................................................................................... 12
4.6 To set the COMMUNICATION LOCK parameter ................................................ 13
4.7 To test the communication .................................................................................. 13
5Modbus protocol overview ....................................................................................... 14
5.1 Overview.............................................................................................................. 14
5.1.1 Transactions on Modbus Networks............................................................. 14
5.1.2 Serial Transmission Mode........................................................................... 14
5.2 Data Addresses in Modbus Messages................................................................ 15
5.3 Supported function codes.................................................................................... 15
6Data access ................................................................................................................ 16
6.1 Access levels ....................................................................................................... 16
6.2 Minimum and maximum values ........................................................................... 16
6.3 Modbus Data table .............................................................................................. 16
7Troubleshooting......................................................................................................... 17
7.1 Preferred method of testing ................................................................................. 17
7.2 Check of identical Slave – Master configuration ................................................. 17
7.3 Check the cabling of the RS485 .......................................................................... 17
7.4 Check the Transmit – Receive indicators............................................................ 18
7.5 Check the function called and the register addresses ........................................ 18
7.6 Check the data access level and the limited range of data................................. 18
7.7 Counters and Loopback diagnostics ................................................................... 19
7.8 Debugging tool and documents........................................................................... 19
8Appendix..................................................................................................................... 20
8.1 List of abbreviations............................................................................................. 20

RS-485 Table of contents - 3
8.2 References .......................................................................................................... 22
8.3 Adequate choice of an RS232-RS485 converter at the computer side .............. 22
Contact us ....................................................................................................................... 24

4 – IntroductionRS-485
1 Introduction
1.1 Intended audience
This manual is intended for installers, commissioning people, network managers who
need to install an RS485 network, start or maintain a supervision system based on the
Modbus protocol.
1.2 Before you start
This manual describes the RS485 Modbus adapter.
To be able to access data of the Power Factor Controller RVT Touchscreen consistently,
a basic knowledge of it is needed. Functionality of the RVT, meaning of various
measurements, logging of data are some particular aspects that should be familiar.
If not, please refer to the RVT Installation and operating Instructions to get the sufficient
knowledge.
To know more about the RVT data access, please refer to the RVT Modbus data table.
As other devices may be connected through the RS485 Modbus adapter, the term
“controller” will be used here to define the device.
1.3 How to use this manual
Chapter 2 is a short description of the Modbus adapter.
Chapter 3 contains a description of the physical layer. Connection to a controller is
described.
Chapter 4 gives all technical details concerning the Modbus adapter.
Chapter 5 is an overview of the Modbus protocol and how Modbus is implemented in the
controller.
Chapter 6 contains the table reference and formats to access data.
Chapter 7 is a reference in case of problems
Chapter 8 is dedicated to annexes
.

RS-485 Overview - 5
2 Overview
This chapter contains a short description of the Modbus protocol and the RS485
interface.
2.1 Introduction to Modbus
Modbus is a serial, asynchronous protocol. The Modbus protocol does not specify the
physical interface. The typical physical interfaces are RS-232 and RS-485. The Interface
Module uses the RS-485 interface.
Modbus is designed for integration with PLCs or other automation devices, and the
services closely correspond to the PLC architecture.
The Modbus commands and services supported by the Modbus adapter are discussed in
Chapter 5.
2.2 The Modbus adapter
The Modbus adapter is an optional device for the Power Factor Controller RVT
Touchscreen which enables the connection of the RVT to a Modbus system. The
controller is considered as a slave unit in the Modbus network.
Through the Modbus adapter it is possible to:
Read measurements and logged values
Read and write parameters settings of the controller
Activate output relays
Read status information
Read device identification and type numbers
2.3 Compatibility
The Modbus adapter is compatible with any Power Factor Controller RVT Touchscreen.
Other ABB controllers may be connected with the adapter.
CAUTION: Be careful that the RS485 MODBUS ADAPTER is the one with a GREEN text
colour (3.3V power supply). The one with a WHITE text colour is reserved for the old
model (5V power supply).
•The Modbus protocol is compatible with all master stations that support the
Modicon-defined Modbus serial communication protocol.

6 – Serial interface considerations RS-485
3 Serial interface considerations
The Modbus protocol communicates with the instrumentation by means of an industry
standard serial interface. This interface may be RS-232, RS-422 or RS-485. Some
systems may also support the protocol over other busses or networks, such as Ethernet.
An RS-232 interface allows only two devices to be connected together.
RS-422 supports 1 driver and up to 10 receivers on a single network.
For bi-directional communications, special tri-state circuitry is provided. RS-485 supports
up to 32 driver/receiver pairs. With special hardware, the RS-422 and RS-485 limits can
be expanded to allow as many as 248 devices on a single network. Each device on a
network must have a unique address, which may be soft configured. Address zero is
reserved for broadcast messages from the host to all slaves.
All devices on a network must also be configured with the same parameters, such as
baud rate and parity. In designing the communication architecture, one should consider
communications performance when deciding how many devices to connect to a host port.
Generally, nearly twice the performance can be achieved by splitting the devices from
one port, onto two ports.
3.1 Communication mode
MODBUS protocol uses half-duplex communications, regardless of the hardware.
Half-duplex hardware shares the same lines for transmit and receive, whereas, full-
duplex hardware has dedicated transmit and receive lines.
3.2 SINGLE ENDED versus DIFFERENTIAL data transmission
Single-ended transmission is performed on one signal line, and the logical state is
interpreted with respect to ground. The main disadvantage of the single-ended solution is
its poor noise immunity.
For differential transmission, a pair of signal lines is necessary for each channel. On one
line, a true signal is transmitted, while on the second one, the inverted signal is
transmitted. The receiver detects voltage difference between the inputs and switches the
output depending on which input line is more positive. Differential data transmission
schemes are less susceptible to common-mode noise than single-ended schemes.
3.3 RS-232 interface
An RS-232 interface is rated for distances up to 15 meters (50 feet). At least three wires
are required for an RS-232 interface. Wires are required for Transmit, Receive and Signal
Ground. Some devices support additional wires for communication handshaking. RS-232
hardware is a full-duplex configuration, having separate Transmit and Receive lines.

RS485 Serial interface considerations - 7
Each signal that transmits in an RS-232 data transmission system appears on the
interface connector as a voltage with reference to a signal ground. The RS-232 receiver
typically operates within the voltage range of +3 to +12 and -3 to -12 volts.
3.4 RS-422 interface
An RS-422 interface requires at least four wires. Two wires each are used for Transmit
and Receive. A fifth wire is usually required for Signal Ground, when connecting non-
isolated devices together. Handshaking lines may also be supported by some hardware.
This interface is full duplex, allowing use of the same software drivers as for RS-232. The
differential drivers allow for distances up to 1200 meters (4000 feet). The receivers of an
RS-422 device are always enabled.
3.5 RS-485 interface
For multi-drop operation, drivers must be capable of tri-state operation.
An RS-485 interface requires at least two wires. In a two-wire configuration, the same
pair of wires is used for Transmit and Receive. The two-wire configuration utilizes half-
duplex communications. Transmit driver circuits are always taken off-line or tri-stated,
when not in use. This tri-state feature reduces the load on the network, allowing more
devices, without the need of special hardware. This interface also uses differential
drivers, supporting distances up to 1200 meters (4000 feet).
In a differential system the voltage produced by the driver appears across a pair of signal
lines that transmit only one signal. A differential line driver will produce a voltage from 2 to
6 volts across its A and B output terminals and will have a signal ground (C) connection.
Although proper connection to the signal ground is important, it isn't used by a differential
line receiver in determining the logic state of the data line.
A differential line receiver senses the voltage state of the transmission line across two
signal input lines, A and B. It will also have a signal ground (C) that is necessary in
making the proper interface connection. If the differential input voltage Vab is greater than
+200 mV the receiver will have a specific logic state on its output terminal. If the input
voltage is reversed to less than -200 mV, the receiver will create the opposite logic state
on its output terminal.
3.6 Bias resistors
RS-422 and RS-485 networks often require bias, or pull-up and pull-down resistors.
These resistors are used to stabilize the network. By definition, in a MODBUS RTU
network, it is the responsibility of the Master to provide this function.
Some systems may function without these stabilizing resistors, but may be more
susceptible to communication errors. Though the pull-up and pull-down resistors are the
same, the value of these resistors varies from device to device. The actual recommended
resistance may be calculated, and varies with the number of devices on the bus.
3.7 Termination resistors
Termination resistors are often used to reduce reflections on the network. This problem
occurs most with long wires and high baud rates. Due to variations in wire and
equipment, whether or not to use these terminators is usually determined by system
testing. The general rule is to add them only if needed. The resistors are typically 120
ohms, and installed across the Transmit and Receive wire pairs. Normally, one resistor is
installed at each end of each pair of wires.

8 – Serial interface considerations RS-485
3.8 Shielding and grounding considerations
The signal ground conductor is often overlooked when ordering cable. An extra twisted
pair must be specified to have enough conductors to run a signal ground. A two-wire
system then requires two twisted pairs.
It is often hard to quantify if shielded cable is required in an application or not. Since the
added cost of shielded cable is usually minimal it is worth installing the first time.
3.9 Cable requirements
The type of wire to use will vary with required length. Wire with twisted pairs and an
overall shield is used most often. The shield is tied to earth ground or chassis, and
typically at one end only (generally at the Modbus Master side). The shield is not to be
used as a signal common or ground. The table below lists typical cable
recommendations.
RS-232
Up to 15m (50ft) virtually any standard shielded twisted pair with drain (Belden 9502 or
equivalent).
RS-422 and RS-485
Up to 1200m (4000ft) 24 AWG twisted pair with foil shield and drain wire on each pair
(Belden 9841 for 2-wire and 9729 for 4-wire or equiv.).
3.10 Network topology
Various kinds of network topologies may be done on the basis of an RS485 Modbus
network.
The Modbus network may be managed by a computer collecting data. Typically this
computer runs an OPC server connected to a plant intranet. OPC client applications may
present this information to any supervision program that will perform the Human machine
interface, data logging, and data setting …
It is also possible to manage the RS485 Modbus network through a PLC.

RS485 Serial interface considerations - 9
Various kind of RS485 to RS232 converters exists.
To bridge the Modbus network to another kind of software protocol, various kinds of
protocol converters may be used. They are often called ‘protocol gateway: Modbus to
Profibus, Ethernet, CAN …

10 – RS485 Modbus adapter RS485
4 RS485 Modbus adapter
The RS485 Modbus adapter enables the connection of the controller to an RS485
Modbus network.
4.1 Main features
The adapter is self powered through the power supply of the controller.
Advantage: an external power supply is not needed.
Advantage: low power consumption.
The adapter is fixed directly on the back side of the RVT.
Advantage: the Modbus adapter does not need any rail DIN or such fixation
methods.
The adapter is electrically isolated from the RVT power supply.
Advantage: the RVT is protected against common mode voltage levels applied on
the RS485 network.
Advantage: no ground loop.
The adapter has an integrated terminator resistance which may be connected with a
switch.
Advantage: no external device to be added.
The adapter contains transient voltage suppressors.
Advantage: the device and the network are protected against voltage surges.
The adapter is fitted with transmission and reception indication LED’s.
Advantage: it allows visualizing Modbus queries and Modbus answers.
The controller allows a software adjustment of communication parameters.
Advantage: no multiple hardware dipswitches to handle.
Advantage: permits self-tuning of communication parameters with a higher level
software application.
4.2 Physical dimensions
The Modbus adapter is especially suitable for application where space requirements are
important.

RS485 RS485 Modbus adapter - 11
57 mm
Fixing screw
TX –RX led’s
48 mm
35 mm
Resistor
Termination switch
Extra depth to
the controller
max. 10 mm
Screwed terminal for
bus cable connection
4.3 Mounting
The Modbus adapter is specially shaped to fit the controller case and to plug into its
dedicated connectors without any other additional fixation part than just a screw.
3D view
3D view inserted in back plate
Controller optical
transceiver
Controller power
supply connector
4.4 Technical data
Operating ambient temperature: -20 to +70 °C
Number of nodes (Tx drive): 32 max
Rx loading: receiver impedance is 1 unit load per RS485 Modbus adapter
Size of the Link: 247 stations including repeaters (31 stations and 1 repeater per
segment)
Medium: Shielded, twisted pair RS485 cable (Belden 9841 typical)
Maximum Bus Length: 1200 m
Topology: Multi-drop
Serial Communication Type: Asynchronous, 2 wires half Duplex
Baud rate: 110, 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600 bauds
selectable by the user (can be adjusted through the controller menus)
Termination resistor: built in, selectable by a switch. 120 ohms resistor is needed
at both end of the line and must be switched or not depending on the location in
the Modbus network topology.

12 – RS485 Modbus adapter RS485
Transient voltage suppressors
Screwed terminals on the RS485 side
TX – RX Led indication for easy debugging and troubleshooting
Slew-Rate Limited for Error-Free Data Transmission (minimizing EMI and
reducing reflections caused by improperly terminated cables)
Drivers are short-circuit current limited
The receiver input has a fail-safe feature in case of broken connection
Compatible Devices: Any Modbus device capable of Modbus communication as a
master.
Speed reply time: better than 5 ms @ 57600 bauds
Max data packet: Any complete table provided in the Modbus data table
4.5 Commissioning process
To install the RS485 Modbus adapter, follow the hereafter commissioning process.
Disconnect power supply from the controller
Wait the required time to ensure all capacitors are discharged
Connect the signal ground (if present) to GND
Connect the twisted pair to the A and B connections
Insert the RS485 Modbus adapter in the controller
Fix it with the appropriate screw
Ensure that all connections are tight.
If the RS485 Modbus adapter is one of the two ending station on the
communication line put the load termination switch in the ‘ON ‘position. If not, put
it in the ‘OFF ‘position.
Make sure all other Modbus stations (master and slaves) are properly connected
Apply power on the controller
Enter the MODBUS menu under:

RS485 RS485 Modbus adapter - 13
oCOMMUNICATION - I/O CONFIGURATION – COM. SETTINGS –
MODBUS
Choose a slave address (the address must be unused in the
Modbus network it is inserted in)
Select the right communication speed (baud rate)
Select the right parity checking
Select the right number of stop bits
The Modbus slave is now ready to receive Modbus queries from the Modbus
master and to send Modbus response accordingly
If something goes wrong, see the trouble shooting section hereafter
4.6 To set the COMMUNICATION LOCK parameter
If the RVT’s parameters should be changed only from the Modbus network, set
the Communication lock item as locked. It can be found under the following
menu:
oCOMMUNICATION - I/O CONFIGURATION – COM. SETTINGS
Set the Communication lock parameter to ‘Locked’
4.7 To test the communication
Depending on the way the controller is connected to the Modbus network, different ways
of testing may be chosen.
The user can find much application on the internet to scan Modbus devices and test
communication.
In case of problem, please refer to the troubleshooting paragraph.

14 – Modbus protocol overview RS485
5 Modbus protocol overview
5.1 Overview
MODBUS RTU is a non-proprietary serial communications protocol that is widely used in
the process control industry. The protocol was developed by Modicon for PLC
communications and later released for public use.
This protocol is available in all major Human Machine Interface (HMI) software packages
and terminals. Many of the major controller and PLC manufacturers also offer MODBUS
protocol as a standard or optional protocol in their instrumentation.
The hardware over which MODBUS RTU communications are performed is not defined
by the protocol. MODBUS RTU is supported on RS-232, RS-422, RS-485, Ethernet and
other electrical standards. It should be noted that MODBUS RTU, MODBUS ASCII and
MODBUS Plus are unique communication formats, and are not compatible with each
other. This document will discuss MODBUS RTU only.
5.1.1 Transactions on Modbus Networks
Modbus protocol uses a master–slave technique, in which only one device (the master)
can initiate transactions (called ‘queries’). The other devices (the slaves) respond by
supplying the requested data to the master, or by taking the action requested in the
query. Typical master devices include host processors and programming panels. Typical
slaves include programmable controllers.
The master can address individual slaves, or can initiate a broadcast message to all
slaves.
Slaves return a message (called a ‘response’) to queries that are addressed to them
individually. Responses are not returned to broadcast queries from the master.
The Modbus protocol establishes the format for the master’s query by placing into it the
device (or broadcast) address, a function code defining the requested action, any data to
be sent, and an error–checking field. The slave’s response message is also constructed
using Modbus protocol. It contains fields confirming the action taken, any data to be
returned, and an error–checking field. If an error occurred in receipt of the message, or if
the slave is unable to perform the requested action, the slave will construct an error
message and send it as its response.
5.1.2 Serial Transmission Mode
The transmission mode defines the bit contents of message fields transmitted serially on
the networks. It determines how information will be packed into the message fields and
decoded.
Modbus defines two transmission modes: ASCII or RTU.
Only RTU mode will be used here. The mode and serial parameters must be the same for
all devices on a Modbus network.

RS485 Modbus protocol overview - 15
5.2 Data Addresses in Modbus Messages
Modbus defines 4 address spaces: 2 address spaces for bit addressable data and 2
address spaces for 16 bits addressable data.
Address space Data Readable/writable Modbus name
0XXXX Output bit Read & write Coil Status
1XXXX Input bit Read Input Status
3XXXX Input word Read Input Register
4XXXX Output word Read & write Holding Register
5.3 Supported function codes
The following table gives the Modbus functions, which are implemented and supported.
The code is the one used in function field of the Modbus message.
The address space concerned and the purpose of the function are given below.
Code Function Address range/remark
1 Read Coil Status 0XXXX Reads the on/off status of discrete outputs
2 Read Input Status 1XXXX Reads the on/off status of discrete inputs
3 Read Holding Registers 4XXXX Reads contents of output registers
4 Read Input Registers 3XXXX Reads contents of input registers
5 Force Single Coil 0XXXX Sets the status of a discrete output
6 Preset Single Register 4XXXX Sets the value of a holding register
7 Read Exception Status Device specific (see Modbus data table of a device)
8 Diagnostics Checks the communication system between the master and the slave
11 Fetch Comm. Event Ctr. Returns amount of successful read/write operations on data points
12 Fetch Comm. Event Log Returns log registers of communication events
15 Force Multiple Coils 0XXXX Sets the status of multiple discrete outputs
16 Preset Multiple Registers 4XXXX Sets the value of multiple holding registers
17 Report Slave ID Device specific (see Modbus data table of the device)
22 Mask Write 4X registers 4XXXX And/Or write of a holding register
23 Read/Write 4X registers 4XXXX Reads a set of holding registers and writes a set of holding
registers in one query

16 – Data access RS-485
6 Data access
6.1 Access levels
Some access levels must be set to allow parameters to be changed. Here are some
access levels used in the RVT.
SET MODE: The RVT must be in the Set Mode to allow parameter modifications.
LOCKING SWITCH: the locking switch has to be released
BANK SETTINGS: the parameter bank settings must be set as Unlocked (for more
information on these settings, please refer to the RVT Installation and Operating manual).
Another parameter (COMMUNICATION LOCK) is used to add access rights to Modbus
users.
The access levels of the Modbus writings are coded in the Modbus data table.
6.2 Minimum and maximum values
Variables have a limited range. If a write operation to a variable exceeds the minimum
and maximum allowable values, the variable will be overridden with its minimum or
maximum value.
An ILLEGAL DATA VALUE exception error is sent back.
See the Modbus data table for more details.
6.3 Modbus Data table
Data are arranged in several tables for convenience. Individual tables contain similar
information. Table data may be read only or read/write access.
Data in each table is pointed to in a Modbus command by two consecutive data address
bytes. The first byte defines the table number, and the second byte the offset of the data
in the table. These two bytes are what is called the ‘Modbus address’ or the ‘Modbus
register’.
A specific Modbus data table is dedicated to a specific product type.
Access (read or write) to a non-referenced Modbus address result in an ILLEGAL DATA
ADDRESS exception error.
The Modbus Data table gives all information on the various data and how to access them.
To handle RVT data, please refer to the RVT Modbus Data table.

RS485 Troubleshooting - 17
7 Troubleshooting
7.1 Preferred method of testing
In most cases the Modbus Master (usually a PC software package) will be running on a
personal computer or on hardware which has RS232 communications ports. In these
instances a converter will be needed to convert the RS232 signals to the RS485 standard
used by the controller. Only when the Modbus Master is running on hardware fitted with
RS485 ports is a converter unnecessary.
The preferred method of testing communications configuration is to operate with a single
controller located close to the Master (PC, PLC etc) and minimum cable length.
Once cable connections, controller configuration and Modbus Master configuration have
been successfully tested the system may be connected to the final installation (multiple
slaves and extended cabling systems as needed to meet user requirements). Full system
configuration and testing can then take place.
The software used to provide the Modbus Master functionality is not supplied. The
commissioning engineer should be familiar with the use of the software both while testing
connection to controllers and during full system configuration. In general software used
during testing can be classed as either application software or Modbus test software, the
principal differences between these two types are as follows.
Application software (such as Data Loggers or SCADA systems) is intended to use
Modbus devices to perform a measurement, control or logging function. It often provides
tools for quick configuration of known types of instrument. Such packages make it easy
for a user to start a system without needing to know Modbus commands, but may not
provide the tools for a commissioning engineer to diagnose problems when a system
does not respond as anticipated.
Modbus test software normally allows Modbus commands to be generated as required
and the responses analyzed. Some knowledge of the Modbus protocol and the
instrument register addresses will be required before using such a package. Test
packages will assist a commissioning engineer in diagnosing communication problems.
7.2 Check of identical Slave – Master configuration
In any case the master must have the same communication parameters as the controller:
Enter the MODBUS menu under:
oCOMMUNICATION - I/O CONFIGURATION – COM. SETTINGS –
MODBUS
Choose a slave address
Select the right communication speed (baud rate)
Select the right parity checking
Select the right number of stop bits
7.3 Check the cabling of the RS485
The non-inverting and the inverting output wires should be cabled respectively on
non-inverting and inverting input respectively. No communication will take place if
non-inverting wires are mixed with inverting wires.
Load termination resistors are important at both end of the network to avoid
distortion due to reflections in the communication line. Check that the ‘Load
switch’ is at the right position (ON or OFF) according to the network configuration.

18 – Troubleshooting RS-485
Connection of a ground wire and presence of a shielding will improve reliability.
7.4 Check the Transmit – Receive indicators
This is a convenient way to check the presence of TX – RX signals:
The Receive Led (yellow Led) indicates that a Modbus query is being received by
the RS485 Modbus adapter.
The Transmit Led (green Led) indicates that a Modbus response is being
transmitted from the RS485 Modbus adapter.
If the Rx Led and the Tx Led never lit, the problem may come from:
The wires of the RS485 cable are not properly fixed or cabled
The query is not sent (see the application software)
The controller and then the RS485 are not powered
If only the Tx Led never lit, the problem may come from:
The controller settings are not correct (check communication parameters
settings)
The controller entered the ‘listen only’ mode (reset the controller or send the
required command to disable the ‘listen only’ mode.)
oThe master sends Broadcast messages (address 0) which are to be
acted on by all connected devices and do not require a reply
7.5 Check the function called and the register addresses
To ensure that a read or write message to a Modbus address will produce a normal
response, check that the register address corresponds exactly to the desired data
requested.
In Modbus register addressing there are several categories of register, including holding
registers (numbered from 40001 upwards) and input registers (numbered from 30001
upwards). The category of register addressed is implicit in the Modbus function used, e.g.
function 03 addresses holding registers and function 04 addresses input registers.
Irrespective of the function used, the register address 30001 or 40001 would be
addressed in the transmitted message as register 0, 30002 or 40002 as register 1, 30010
or 40010 as register 9 and so on.
In the Modbus communications all these register addresses are not active and don’t
contain valid information. A read or write to an unused register address will produce an
exception error.
7.6 Check the data access level and the limited range of data
To ensure that a write message to a Modbus address will produce a normal response,
check that the data value and access level is valid.
Writings in the controller memory though Modbus are restricted to the setting of some
access levels which need to be set. Look at:
SET MODE
LOCKING SWITCH
BANK SETTINGS

RS485 Troubleshooting - 19
Writings in the controller memory through Modbus are restricted to a limited range
applied on that data. An exception error is sent in case of a writing exceeding the allowed
limited range of a data.
7.7 Counters and Loopback diagnostics
Modbus offers some interesting functions to help the user to locate errors:
Fetch comm event counter (function 11) to catch successful messages.
Fetch comm event log (function 12) to look at the history of errors.
Diagnostics function and subfunctions (function 8)
Loopback diagnostics (subfunction 0) returns a response identical to the query
Restart communication (subfunction 1)
Force Listen only mode (subfunction 4)
Read various diagnostic counters
7.8 Debugging tool and documents
The user can find much application on the internet to scan Modbus devices and test
communication.
Look in the documentation of the controller and the corresponding Modbus data table for
appropriate information.
Look in the Modbus protocol for more information on Modbus.

20 - Appendix RS485
8 Appendix
8.1 List of abbreviations
ASCII American Standard Code for Information Interchange
Baud rate Unit for measuring transmission speed in bits/s;
Bit A binary digit, representing a one or zero
Bus An electrical circuit over which data is transmitted
Byte A whole number value represented by eight bits (0 to 255)
Chassis or Chassis Ground A connection to an electrically conductive housing or frame
of a device. It may or may not be connected to Earth
Ground.
Coil The telegram structure for Modbus transmission is
implemented in registers (WORD) or coils (bits). A coil may
be either 8 or 16 bits in length.
Common The voltage reference point of a circuit. It may or may not be
connected to earth ground, though it is generally assumed to
be at zero volts, unless otherwise indicated. In floating
circuits, the common is sometimes at a relatively high
potential. This term is sometimes used interchangeably with
the term “Ground” or GND.
CRC Cyclic Redundancy Check. Complex error checking on a
message block.
CTS ClearToSend hardware handshaking signal. Used with
RequestToSend.
Earth or Earth Ground Global zero voltage reference point. Physical connection is
made to the earth through a grounding rod, water pipe or
other reliable connection.
Ground Voltage reference point of a circuit
It may or may not be connected to earth ground, though it is
generally assumed to be at zero volts. Sometimes used
interchangeably with the term “Common”.
Handshaking method of data flow control for serial communications
Hexadecimal or HEX A number system using a decimal 16 as its base. A single
digit number in HEX ranges from 0 to 15, represented by 0
to 9 and A to F.
HMI Human-Machine Interface (formerly MMI)
IndustrialIT Umbrella concept for ABB’s vision for enterprise automation.
IndustrialIT Architecture The architecture of the Industrial IT system. The architecture
defines how the system is built, in terms of basic concepts,
underlying technologies, system topology, modularity, and
mechanisms for interaction between different parts of the
system. It also defines concepts, rules, and guidelines that
a component must comply with in order to fit in the
IndustrialIT system. A central feature of the IIT architecture is