ABB LVS Digital User manual

—
DISTRIB U TIO N S OLU T IONS
LVS Digital
with M10x Motor Controller
Interface Manual Modbus

NOTICE
This document contains information about one or more ABB products and may include a description of or a
reference to one or more standards that may be generally relevant to the ABB products. The presence of any
such description of a standard or reference to a standard is not a representation that all of the ABB products
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determine the specific features supported by a particular ABB product, the reader should consult the product
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The information in this document is subject to change without notice and should not be construed as a com-
mitment by ABB. ABB assumes no responsibility for any errors that may appear in this document.
Products described or referenced in this document are designed to be connected and to communicate infor-
mation and data through network interfaces, which should be connected to a secure network. It is the sole re-
sponsibility of the system/product owner to provide and continuously ensure a secure connection between the
product and the system network and/or any other networks that may be connected.
The system/product owners must establish and maintain appropriate measures, including, but not limited to,
the installation of firewalls, application of authentication measures, encryption of data, installation of antivirus
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ABB performs functionality testing on the products and updates that we release. However, system/product
owners are ultimately responsible for ensuring that any product updates or other major system updates (to
include but not limited to code changes, configuration file changes, third-party software updates or patches,
hardware change out, and so on) are compatible with the security measures implemented. The system/ prod-
uct owners must verify that the system and associated products function as expected in the environment in
which they are deployed.
In no event shall ABB be liable for direct, indirect, special, incidental or consequential damages of any nature or
kind arising from the use of this document, nor shall ABB be liable for incidental or consequential damages
arising from use of any software or hardware described in this document.
This document and parts thereof must not be reproduced or copied without written permission from ABB, and
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The software or hardware described in this document is furnished under a license and may be used, copied, or
disclosed only in accordance with the terms of such license. This product meets the requirements specified in
EMC Directive 2014/30/EU and in Low Voltage Directive 2014/35/EU.
TRADEMARKS
MNS is a registered or pending trademark of ABB.
Ability is a trademark of ABB.
All rights to copyrights, registered trademarks, and trademarks reside with their respective owners.
Copyright © 2020 ABB.
All rights reserved.
Release: October 2020
Document Number: 1TGC908004
Revision: M0203

TA BLE O F CONTENTS
I
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Table of Contents
1. General .........................................................................................................................................1
1.1. Target Group.................................................................................................................................1
1.2. Use of Warning, Caution, Information and Tip icon.............................................................1
1.3. Terminology ..................................................................................................................................2
1.4. Related Documentation .............................................................................................................5
2. Introduction ............................................................................................................................... 6
2.1. Hardware Types and Technical Data ...................................................................................... 8
2.2. Modbus Standard ......................................................................................................................10
2.2.1. Basics..........................................................................................................................10
3. Interfaces...................................................................................................................................13
3.1. Front View....................................................................................................................................13
3.2. Power Supply ..............................................................................................................................15
4. Digital Gateway Installation ....................................................................................................16
4.1. Digital Gateway Mounting .......................................................................................................16
4.2. CF Card Installation................................................................................................................... 17
5. Communication Interface Connection....................................................................................18
5.1. Switchgear Bus Network..........................................................................................................18
5.1.1. M10x-M motor controller ........................................................................................18
5.1.2. M10x-TCP motor controller ....................................................................................19
5.2. Switchgear Control Network.................................................................................................. 20
5.2.1. Connection Examples of Switchgear Control Network.................................. 20
5.3. Time Synchronization ...............................................................................................................22
5.3.1. Option 1......................................................................................................................22
5.3.2. Option 2......................................................................................................................23
5.4. DCS Fieldbus.............................................................................................................................. 24
5.4.1. Modbus RTU Topology........................................................................................... 24
5.4.2. Modbus TCP Topology ............................................................................................27
6. Redundancy .............................................................................................................................. 28
6.1. Redundant Architecture .......................................................................................................... 28
6.2. Redundancy Configuration..................................................................................................... 30
6.2.1. Redundant Digital Gateway connection............................................................. 30
6.2.2. M10x Redundant Configuration............................................................................31
6.2.3. Digital Gateway Redundant Configuration ........................................................31
6.3. Handling of redundancy faults .............................................................................................. 33
6.4. MView / Web Interface ............................................................................................................ 34
7. Configurations ..........................................................................................................................35
7.1. Initial Values – IP Configuration............................................................................................. 35
7.2. Settings – IP Configuration .................................................................................................... 36
7.2.1. Definition of IP Addresses .....................................................................................37
7.3. Settings – Serial Switchgear Bus........................................................................................... 38
7.4. Settings – DCS Modbus RTU Communication ................................................................... 39
7.5. Settings – DCS Modbus TCP Communication.....................................................................41
7.5.1. Multiple Master in Modbus TCP applications ................................................... 42
7.6. Failsafe ........................................................................................................................................ 42
7.7. Start-up of Digital Gateway and application download .................................................. 43
7.7.1. Power-On procedure .............................................................................................. 43
7.7.2. Power On the control voltage supply ................................................................. 43

TA BLE O F CONTENTS
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7.7.3. Confirm operation .................................................................................................. 43
8. Function Codes ........................................................................................................................ 44
8.1. Message Format .......................................................................................................................44
8.1.1. Query Messages ......................................................................................................44
8.1.2. Response Messages ............................................................................................... 44
8.2. Function Code 02 – Read Input Status................................................................................. 45
8.3. Function Code 03 – Read Holding Registers....................................................................... 45
8.4. Function Code 04 – Read Input Registers ........................................................................... 45
8.5. Data Presentation for Function Code 03 and 04 ............................................................... 46
8.6. Function Code 06 – Preset Single Register......................................................................... 46
8.7. Function Code 08 – Preset Single Register......................................................................... 46
8.8. Function Code 16 – Preset Multiple Register ...................................................................... 47
8.9. Restrictions................................................................................................................................ 47
8.9.1. General....................................................................................................................... 47
8.9.2. Modbus RTU ............................................................................................................. 47
8.10. Exception Code Handling........................................................................................................ 48
9. Data Mapping........................................................................................................................... 49
9.1. User Data Map ........................................................................................................................... 49
9.2. Default Data Map ...................................................................................................................... 49
9.2.1. Monitoring (Inputs from M10x)............................................................................ 49
9.2.2. Monitoring with Function Code 02 ..................................................................... 50
9.2.3. Monitoring with Function Code 03 and 04 ....................................................... 55
9.2.4. Extended Status Description ............................................................................... 56
9.2.5. Control Commands................................................................................................. 59
9.2.6. Switching Commands ............................................................................................ 60
9.2.7. Switching Commands – Bit Control......................................................................61
9.2.8. Redundant Digital Gateway Modbus Data ........................................................ 62
9.2.9. Control Access ......................................................................................................... 63
10. Troubleshooting and Maintenance ........................................................................................ 65
10.1. Digital Gateway LED Indication ............................................................................................. 65
10.2. Troubleshooting........................................................................................................................ 68

LIST OF FIG URES
III
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List of Figures
Figure 1: Digital Gateway ............................................................................................................................ 6
Figure 2: General LVS Digital System Configuration with M10x-M.................................................... 6
Figure 3: General LVS Digital System Configuration with M10x-TCP .................................................7
Figure 4 Master Slave Query Response Cycle........................................................................................10
Figure 5 Digital Gateway front view .......................................................................................................13
Figure 6 Power Supply Connector ..........................................................................................................15
Figure 7 Digital Gateway Mounting Kit for MNS ...................................................................................16
Figure 8 CF Card Insertion......................................................................................................................... 17
Figure 9 CF Card Insertion Detail............................................................................................................. 17
Figure 10 Hardware connection between Digital Gateway and M10x-M.........................................18
Figure 11 Shielding Clamp for RS485 ......................................................................................................19
Figure 12 Hardware connection between Digital Gateway and M10x-TCP .....................................19
Figure 13 Digital Gateway directly connected to CMES Edge .......................................................... 20
Figure 14 Network connection of Digital Gateway and CMES Edge ................................................21
Figure 15 Example, showing 3rd party Network Switch and 3rd party NTP Server in a
Switchgear Control Network ....................................................................................................................22
Figure 16 Example, using Digital Gateway as NTP Server ..................................................................23
Figure 17 Digital Gateway RS232 connection via Serial 2 to DCS ..................................................... 24
Figure 18 Digital Gateway RS422 connection via Serial 2 to DCS .................................................... 25
Figure 19 Digital Gateway RS485 connection via Serial 2 to DCS .................................................... 26
Figure 20 Example for RS485 bus termination and biasing: ............................................................ 26
Figure 21 Digital Gateway Modbus TCP connection with Crossover cable ....................................27
Figure 22 Digital Gateway Modbus TCP connection with standard CAT5 cable ...........................27
Figure 23 Redundant configuration and possible failure scenario – M10x-M ............................... 28
Figure 24 Redundant configuration and possible failure scenario – M10x-TCP ........................... 29
Figure 25 Serial 1 to serial 1 redundant link connections with ferrite core ................................... 30
Figure 26 MNavigate IP address Parameterization for Primary and Backup Digital Gateway...31
Figure 27 MNavigate Fieldbus Slave address Parameterization for Primary and Backup Digital
Gateway ........................................................................................................................................................ 32
Figure 28 MView Redirecting to Redundant Digital Gateway........................................................... 34
Figure 29 Redundancy error shown in MView by a red square......................................................... 34
Figure 30 IP Address Settings ................................................................................................................. 36
Figure 31 Time Sync Settings................................................................................................................... 36
Figure 32 Parameter Window for Serial Switchgear Bus in MNavigate.......................................... 39
Figure 33 Parameter Window for MODBUS RTU parameters in MNavigate .................................40
Figure 34 Parameter Window for MODBUS TCP parameters in MNavigate.................................. 42

LIST OF TAB LES
V
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List of Tables
Table 1 Digital Gateway Modbus Hardware ............................................................................................ 8
Table 2 M10x motor controller variants ............................................................................................... 8
Table 3 Digital Gateway Technical Data................................................................................................... 9
Table 4 Modbus RTU mode of Transmission ........................................................................................12
Table 5 Front View Connectors, LED and Push Buttons .....................................................................14
Table 6 M10x Communication setting ....................................................................................................18
Table 7 Serial Redundant Link Cable ordering code ........................................................................... 30
Table 8 M10x-M redundant communication setting............................................................................31
Table 9 Primary and Backup IP address setting .................................................................................. 32
Table 10 Primary and Backup IP address setting ................................................................................ 33
Table 11 Digital Gateway Default Parameters – IP Configuration.................................................... 36
Table 12 Digital Gateway Serial Switchgear Bus.................................................................................. 38
Table 13 Digital Gateway Default Parameters – Modbus RTU configuration................................40
Table 14 Digital Gateway Default Parameters – Modbus RTU configuration.................................41
Table 15 Function Codes ...........................................................................................................................44
Table 16 Address ranges of Function Codes ........................................................................................ 45
Table 17 Byte representation for function code 03 and 04 ............................................................... 46
Table 18 Byte representation for Float .................................................................................................. 46
Table 19 Byte representation for Float Big Endian.............................................................................. 46
Table 20 Default Modbus Map Life Bit of M10x ................................................................................... 50
Table 21 Default Modbus Map Bit Status of M10x............................................................................... 54
Table 22 Default Modbus Map Measured Values of M10x ................................................................. 55
Table 23 Default Modbus Map Extended Status for M10x ................................................................ 56
Table 24 Extended Status Byte 1............................................................................................................. 56
Table 25 Extended Status Byte 2............................................................................................................. 57
Table 26 Extended Status Byte 3............................................................................................................. 57
Table 27 Extended Status Byte 4............................................................................................................. 58
Table 28 Default Modbus Command Registers ................................................................................... 59
Table 29 Switching Commands sent from DCS to Digital Gateway ................................................ 60
Table 30 Default Modbus Bit Command Registers..............................................................................61
Table 31 Default Bit Map Control Commands Low Byte .....................................................................61
Table 32 Default Bit Map Control Commands High Byte................................................................... 62
Table 33 Redundant data for monitoring by the Modbus master................................................... 62
Table 34 Redundant Command possible from the Modbus Master ............................................... 62
Table 35 Command and Status for Control Access............................................................................. 63
Table 36 Digital Gateway LED indication............................................................................................... 67
Table 37 Digital Gateway Troubleshooting........................................................................................... 69

GEN ERA L TARG ET GR OUP
1TGC908004 1
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1. General
1.1. Target Group
This document describes communication and control interfaces used in LVS Digital (includes
MNS Digital and NeoGear Digital switchgear) and LVS Digital Upgrade (upgrade from INSUM1
/ 2) projects utilizing ABB motor controller M10x [M101 / M102].
The manual is primarily intended for those requiring information on how to access infor-
mation and data provided from Digital Gateway.
Furthermore, the document provides information for control system and application engi-
neers how to integrate MNS Digital [Upgrade] as Fieldbus component into PLC or higher-level
Process Control Systems.
It is assumed that the reader of this manual is familiar with basic terms of Fieldbus and con-
trol communication (e.g. basic knowledge about Modbus etc.).
1.2. Use of Warning, Caution, Information and Tip
icon
This publication includes Warning, Caution, and Information icons where appropriate to
point out safety related or other important information. It also includes Tip icons to point out
useful hints to the reader. The corresponding symbols should be interpreted as follows:
The electrical warning icon indicates the presence of a hazard that could result in
electrical shock .
The warning icon indicates the presence of a hazard that could result in personal
injury
The caution icon indicates important information or warnings related to the con-
cept discussed in the text. It might indicate the presence of a hazard that could re-
sult in corruption of software or damage to equipment/property
The information icon alerts the reader to pertinent facts and conditions
The tip icon indicates advice on, for example, how to design your project or how to
use a certain function
Although Warning notices are related to personal injury, and Caution notices are associated
with equipment or property damage, it should be understood that the operation of damaged
equipment could, under certain operational conditions, result in impaired process
performance leading to personal injury or death. It is therefore, imperative that you comply
fully with all Warning and Caution notices.

TERM INOLOGY G ENER AL
21TGC908004
1.3. Terminology
List of the terms, acronyms, abbreviations and definitions that the document uses.
Abbreviation
Term
Description
Alarm
Alarm is defined as status transition from any
state to abnormal state. Status transition to ab-
normal state can be data crossing over the pre-
defined alarm limit.
Bus Local
A Control Access term describing that the M10x
accepts its commands from a device on the
switchgear control network, e.g. the Web Inter-
face, MView.
COTS
Commercial off the
shelf
Commercial off the shelf product, term to de-
scribe products available on the market, ready to
use
DCS
Distributed Control
System
See also PCS
Eth.
Ethernet
Ethernet is a local area network (LAN) technology.
The Ethernet standard specifies the physical me-
dium, access control rules and the message frames.
Event
An event is a status transition from one state to
another. It can be defined as alarm, if the state is
defined as abnormal or as warning as a pre-alarm
state.
FD
Field Device
Term for devices connected to the Fieldbus (e.g.
motor control units or circuit breaker protection)
GPS
Global Positioning
System
System to detect local position, universal time
and time zone, GPS technology provides accurate
time to a system
Hardware Local
A Control Access term describing that the M10x
accepts its commands from the Hardwired inputs,
when the respective Local control input is set to
true.
HMI
Human Machine In-
terface
Generic expression
LVS
Low voltage switch-
gear
A factory-built assembly built to conform with IEC
61439-1

GEN ERA L TERMINOLOGY
1TGC908004 3
M10x
M101-M
M102-M
M101-TCP
M102-TCP
A microprocessor-based intelligent motor man-
agement system that provides users with com-
plete and specialized low voltage motor control,
protection and monitoring.
‘-M’ indicates that the device is equipped with a
Modbus RTU fieldbus interface.
‘-TCP’ indicates that the device is equipped with a
Modbus TCP fieldbus interface.
MCC
Motor Control Centre
Common term for switchgear used for motor con-
trol and protection.
MNavigate
Configuration and parameterization tool for MNS
Digital
MNS
Modular Low Voltage Switchgear family from ABB
MODBUS
Fieldbus communication protocol
MODBUS RTU
Fieldbus communication protocol
MODBUS TCP/IP
Fieldbus communication protocol based on Ether-
net hardware
Motor Starter
Consists of motor controller and electrical com-
ponents to control and protect a motor, part of
Motor Control Center
NeoGear
Modular Low Voltage Switchgear family from ABB
NLS
Native Language Sup-
port
Providing the ability to change the language of
software tools in order to support native lan-
guages (English is basis, others are optional)
OPC
The industrial standard for exchange of infor-
mation between components and process control
application.
PLC
Programmable Local
Controller
Low level control unit
RCU
Remote Control Unit
Local control unit with pushbutton and indicator
to operate a device (e.g. motor) from field level.
RS232
Standard No. 232 for PC communication, estab-
lished by EIA (Electronics Industries Association,
USA)

TERM INOLOGY G ENER A L
41TGC908004
RS485
Communication interface standard from EIA (Elec-
tronics Industries Association, USA), operating on
voltages between 0V and +5V. RS-485 is more
noise resistant than RS-232C, handles data trans-
mission over longer distances, and can drive more
receivers.
RTC
Real Time Clock
Integrated clock function in devices used to gen-
erate time and date information if a remote clock
system is not present
Software Local
A Control Access term describing that the M10X
accepts its commands from the hardwired inputs
as a result of either the PCS or MView passing the
Control Access Authority to Soft-Local.
Note: Does not require the hardwired local input
to be set to true.
SNTP
Simple Network Time
Protocol
A protocol used for time synchronization in Con-
trol Network through Ethernet
Switchgear Bus Net-
work
Term used to describe the internal switchgear
communication network, between Digital Gate-
way and M10X
TCP/IP
Transmission Control
Protocol / Internet
Protocol
TCP/IP is a high-level connection oriented, relia-
ble, full duplex communication protocol devel-
oped for integration of the heterogeneous sys-
tems.
Trip
A consequence of an alarm activated or an exter-
nal trip command from another device to stop the
motor or trip the circuit breaker.
UTC
Coordinated Uni-
versal Time
Coordinated Universal Time is the international
time standard. It is the current term for what was
commonly referred to as Greenwich Meridian
Time (GMT). Zero (0) hours UTC is midnight in
Greenwich England, which lies on the zero longitu-
dinal meridian. Universal time is based on a 24-
hour clock.
Warning
A warning is defined as status transition from any
state to pre-alarm state to inform in advance be-
fore an alarm level is reached.

GEN ERA L REL AT ED DOCUM ENTATI ON
1TGC908004 5
1.4. Related Documentation
1TNC911505
M10x-M Modbus Implementation
1TNC911204
M10x-M Control System Integration Guide
1TGC908001
ABB Ability Condition Monitoring for electrical systems - CMES -
User Manual
1TNA810039
Manufacturing Instruction - Installation of MService and MNS
Digital Gateway in MNS
1TGC908028
LVS Digital Interface Manual Web Server

REL AT ED DOCUM ENTATI ON INTR ODUCTIO N
61TGC908004
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2. Introduction
The Digital Gateway is an industrial PC equipped with interface cards and ports required for
communication internally to M10x and externally to process control systems as well as to the
CMES Edge which provides condition monitoring features and functions as well as edge con-
nectivity to ABB Ability™ cloud based solutions.
Figure 1: Digital Gateway
One Digital Gateway can communicate internally to up to 128 M10x / field devices by Modbus
RTU or Modbus TCP . If more than 128 M10x / field devices are installed, then additional Digi-
tal Gateway must be configured.
Figure 2: General LVS Digital System Configuration with M10x-M

INTR ODUCTIO N RE LAT ED DOC UMENTATI ON
1TGC908004 7
Figure 3: General LVS Digital System Configuration with M10x-TCP

HARDWARE T YPES AND T ECHNI CAL DATA INTR ODUCTIO N
81TGC908004
2.1. Hardware Types and Technical Data
The configuration of Digital Gateway depends on one hand on the selected communication
protocol to the DCS control system or SCADA system and on the other hand on the fieldbus
protocol to the field devices which can be Modbus RTU or Modbus TCP.
Field Device
Fieldbus Protocol
Modbus RTU
Modbus TCP
DCS / SCADA
Fieldbus protocol
Modbus RTU / Modbus TCP
DCS / SCADA
Fieldbus Hard-
ware Interface
RS 485/ RS422 / RS232
& Ethernet TCP/IP IEEE 802.3
Digital Gateway ID
1TGE120021R0810
1TGE120021R0610
Picture
Table 1 Digital Gateway Modbus Hardware
M10x variant
Communication protocol to Digital Gateway
M10x-M
Modbus RTU (RS485)
As one segment of Modbus RTU is only able to support 32 devices,
Digital Gateway for M10x-M is supplied with an additional Serial inter-
face card which provides four Modbus RTU port to support a maxi-
mum of 128 number of devices
M10x-TCP
Modbus TCP
One Digital Gateway supports up to 128 field devices connected via
Ethernet TCP/IP (Modbus TCP protocol). Here the LAN3 port of the
Digital Gateway is utilized for the field device communication.
Table 2 M10x motor controller variants

INTR ODUCTIO N HAR DWARE T YPES A ND TECH NICA L DATA
1TGC908004 9
ELECTRICAL DATA
Power Supply
24V DC (19 – 31V DC)
Power Consumption
Typical 800mA, maximum 1000mA
MECHANICAL DATA
Weight
2.5 kg
Dimensions H x W x D
140 x 160 x 246 mm
ENVIRONMENTAL DATA
Storage Temperature
-20oC to + 70oC
Operating Temperature
0oC to 55oC
Degree of Protection
IP51
MTBF (Mean Time Between Failures)
46 years @ 40oC
Table 3 Digital Gateway Technical Data

MODBUS STAN DARD INTR ODUCTIO N
10 1TGC908004
2.2. Modbus Standard
MODBUS is a serial data communication protocol and was originally developed as a
communication language for MODICON programmable controllers, its rights now reside with
the Modbus-IDA organization.
The software on the Digital Gateway supports the pure Master-Slave operation as defined in
the MODBUS RTU specification. This manual describes the Digital Gateway communication
with MODBUS protocol in RTU and TCP modes.
The MODBUS communication protocol is implemented within the Digital Gateway to enable
LVS Digital to provide interface possibilities to process control systems or any other external
systems that supports MODBUS RTU / TCP protocol handling.
The MODBUS configuration can be used in point to point configuration or in multi-drop
mode. In Master-Slave MODBUS architecture, the Digital Gateway is always used in a slave
(TCP: Server) mode. The master station controls the traffic on the bus, in this case, by
PCS/DCS or PLC system. The Digital Gateway responds to the queries received from master
station as per the MODBUS specification.
.
Basics
Figure 4 Master Slave Query Response Cycle

INTR ODUCTIO N MOD BUS STA NDARD
1TGC908004 11
2.2.1.1. The Query
The function code (FC) in the query tells the addressed slave device what kind of action to
perform. The data bytes contain any additional information that the slave will need to
perform the function. For example, function code 03 will query 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.
2.2.1.2. The Response
If the slave makes a normal response, the function code in the response is an echo of the
function code in the query. 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.
2.2.1.3. Characteristics
Certain characteristics of the MODBUS protocol, as specified by the reference document, are
fixed such as the frame format, frame sequences, handling of communication errors and
exception conditions, and the functions performed. In case of the Digital Gateway, the trans-
mission mode is also limited to RTU or TCP.
Other characteristics are user selectable. These include a choice of transmission medium,
baud rate and character parity, number of stop bits. These parameters cannot be changed
while the communication interface is active.
The OSI layers 1, 2, and 7 are implemented in the Digital Gateway
Layer 1, 2:
In these layers the physical sending and receiving of bytes, i.e. triggering of the interface
hardware including monitoring of timeouts and CRC-Check generation as well as processing
of addresses is realized. Upon receipt, the fault states, time-out and CRC-Error are being
detected and treated according to the MODBUS RTU specification.
Layer 7:
In this layer the analysis and treatment of the function codes (FC) is implemented. This
includes processing the received commands (read and write of MODBUS-registers) and
generation of the response-message together with the addressed data.
2.2.1.4. Mode of Transmission
The mode of transmission is the structure of the individual units of information within a
message, and the numbering system used to transmit the data. Two modes of transmission
are available for use in a standard MODBUS communication, ASCII (American Standard Code
for Information Interchange), and RTU (Remote Terminal Unit). Both modes provide the same
capabilities for communication. Selecting ASCII or RTU mode defines the bit contents of
message fields, and how information is packed and decoded.
Digital Gateway does not support ASCII transmission.

MODBUS STAN DARD INTRODUCTION
12 1TGC908004
Characteristic
RTU (8-bit)
Coding System
8-bit
Number of bits per character:
Start bits
Data bits (least significant first)
Parity
Stop bits
1
8
1 (1 bit set for even or odd parity, no bits for no
parity)
1 or 2
Error Checking
CRC (Cyclical Redundancy Check)
Table 4 Modbus RTU mode of Transmission
2.2.1.5. Error Detection
There are two types of errors, which may occur in a communication system:
•Transmission error and
•Programming or Communication error
The Digital Gateway deals with either type of error as specified in MODBUS specification.
The most frequent cause of communication error is noise, unwanted electrical signals in a
communication channel. These signals occur because of electrical interference from
machinery, damage to the communication channel, impulse noise (spikes), etc. Character
framing, a parity check, and a redundancy check detect these errors. When the error occurs,
the message is unreliable and the processing of the last received erroneous message stops.
Programming or operational errors are those involving illegal data in a message or difficulty in
communicating with a slave. These errors result in an exception response either from Master
or Slave station.