ENS5130
ADVANCED INDUSTRIAL CONTROL
Modbus Protocol and Ethernet Network
Overview
Objectives
 Understand the characteristics and feature set of Modbus
 Understand the basic principles of an Ethernet Network
2
Modbus
 The Modicon Bus (Modbus) is used worldwide due to its simplicity of
implementation. Not only intelligent devices like PLCs are able to
communicate with Modbus, also many intelligent sensors are equipped with
a Modbus interface to send their data to host systems.
 While Modbus was previously mainly used on wired serial communication
lines, there are also extensions to the standard for wireless communications
and TCP/IP networks.
 The Modbus™ protocol was created in 1978 by Modicon Inc. as a simple
way for communicating control data between controllers and sensors using
an RS232 port. The protocol became widely adopted, quickly reaching the
status of de facto standard in the industrial automation field. Today, the
Modbus protocol is the single, most supported protocol among automation
devices. Schneider Electric transferred the specifications for Modbus and
Modbus/TCP (Modbus over TCP/IP) to Modbus.org. See the official website
for further information about Modbus.
 Modbus allows communication between many devices connected to the
same network, e.g., a system that measures temperature and humidity and
communicates the results to a computer. Modbus is often used to connect a
3
supervisory computer with an RTU in SCADA systems such as Citect.
Features
The popularity of Modbus stems from a commitment to simplicity, recognizing that industrial automation
applications are the most diverse and that there are advantages in delegating the handling of diversity to them:
 Simple to implement: Can be implemented in days
 Small footprint: Can run on any computer system, CPU, or microcontroller, allowing the networking of simple
devices
 Scalable in complexity: A device that has only a simple purpose needs only to implement the needed
message types.
 Scalable in scope: From real-time sensing to production data.
 Scalable in range: A collection of devices using Modbus/TCP to communicate can range up to 10,000 or more
on a single switched Ethernet network.
 Simple to administer and enhance: There is no need to use complex configuration tools when adding a new
node to a network.
 Open and low cost:
• The specifications are freely downloadable from the www.Modbus.org website.
• In addition to being a de facto standard, it has been standardized by the International Electrotechnical
Commission (IEC)
• For most Modbus underlying layers, these layers—hardware and software—are commercially available
(RS232, RS422, RS485, TCP/IP) with economy of scale, and they are already present at no extra cost
to device vendors as part of the standard compendium of the devices.
• There is no vendor-proprietary equipment or software needed, and since there is no need to modify the
underlying commercial standard layers, the protocol can readily take advantage of any improvements in
those layers.
 Connectivity: It is very easy to build Modbus networks made of different underlying layers, via widely available
flexible gateways.
 Installed base, experience, and tools: The installed base of Modbus devices is substantial, so is the
4
experience in their deployment and the availability of monitoring/debugging tools.
OSI Layers Used for Modbus
 Modbus is an application layer
messaging protocol placed at OSI
layer 7, for Client / Server (Master /
Slave) communication between
devices connected on different types
of buses or networks, as shown in the
top figure.
 Modbus is currently transported using
any of the following underlying layers:
• RS232, RS422, RS485
• TCP/IP
• Modbus Plus, a token-passing
network
• Many other stacks over a variety
of media (e.g., fibre, radio,
cellular)
 The bottom figure shows an example
of a Modbus network. Gateways are
used to connect the underlying layers.
5
Modbus Client/Server Communications
 The figure shows the messaging between one client and one server and the conventional
names given to the messages at the source and destination.
 Only one device can transmit on the bus at any time. The client manages and initiates the
exchange. No server can send a message unless it is invited to do so.
 The transfer of information between a Modbus client and a Modbus server is initiated
when the client sends a request to the server to transfer information, to execute a
command, or to perform one of many other possible functions.
 After the server receives the request, it executes the command and/or prepares the
required data. The server then responds to the client by acknowledging that the command
is complete and/or providing the requested data.
 The system response time is limited by two main factors: the time required for the client to
send the request/receive the response and the ability of the server to answer within a
specific amount of time.
 Direct server-to-server communications are not possible. Instead, the client application
must interrogate a server and send the data received to the other server.
6
Modbus Client and Server Services
 A device that implements the Modbus client service can initiate Modbus messaging
requests to another device that implements a Modbus server. These requests allow
the client to transact data with and/or send commands to the remote device.
 A device that implements the Modbus server service can respond to requests from
any Modbus client. The Modbus server service allows a device to make all its internal
and I/O data available to remote devices for both reading and writing and allows for
the execution of other commands.
 A device may implement a Modbus client service, a Modbus server service, or both,
depending on the requirements of the device and in some cases on the particular
Modbus underlying layer.
• A typical SCADA or HMI application implements a client service to initiate
communications with PLCs and other devices for information gathering.
• An I/O device implements a server service so that other devices can read and
write its I/O values. Because this I/O device does not need to initiate
communications, it does not implement a client service.
• A PLC typically implements both client and server services so that it can initiate
communications to other PLCs and I/O devices and can respond to requests
from other PLCs, SCADA, HMIs, and other devices.
• In Modbus Plus, both services are always present at the same Modbus
address/node, and the client or server role is directed by a token-passing 7
mechanism.
Client/Server Interactions
 The interactions can be of two types. The
client issues a request and starts an
application-specified time-out.
• Broadcast mode (unconfirmed): The
client broadcasts a request to all
servers without expecting a response.
The client simply waits for the
specified time-out before issuing any
other request. The time-out is the socalled turnaround delay.
• Unicast mode (confirmed): The
master repeats the request when
there is an incorrect exchange, and
declares the interrogated slave
absent if no response is received
within a given time period. If a slave
does not understand a message, it
sends an exception response to the
master. The master may or may not
retransmit the request.
8
State Diagrams
 Syntax:
 Client:
 Server:
9
General Modbus Frame
 The messages exchanged by Modbus client and server services are defined
by a simple protocol data unit (PDU) independent of the underlying
communication layers. The mapping of MODBUS protocol on specific buses
or network can introduce some additional fields on the application data unit
(ADU).
 The address field size is 1 octet.
 Server devices are addressed using the address field. This address is
assigned to servers, not to clients. A client addresses a server by placing the
server address in the address field of the message. In all allowed cases but
the broadcast - which does not have a response - when the server sends its
response, it places its own address in this address field of the response to
let the client know which server is responding.
10
Function Codes
 The function code field defines the
Modbus function to execute.
 The function code field size is 1 octet.
 Example: A PLC which sends a “preset n
registers” request (code 16) to a device,
where n = 3 and register target = 145, is
going to write values to the device’s
registers 145, 146, and 147.
 Note: Not all devices are compatible with
all function codes.
11
Data Types
 In a request, the data field contains additional information that the server must use to take the
action defined by the function code. This can include items like register addresses, quantity of
items to be handled, etc. When the server answers, if no error occurs, the data field contains
the data requested. If an error occurs, the field contains an exception code that the client can
use to determine the next action to be taken.
 The data field size is from 0 to 252 octets, depending on the value of the function code field.
 In Modbus, client and servers
exchange data between their
memories. Modbus bases its
data model on a series of
tables that have distinguishing
characteristics. The four
primary tables are:
 Each Modbus function
references only one type of
data. Memory references are
expressed as addresses
relative to the lowest possible
absolute address for that data
type:
12
Modbus Serial
 Due to its limitations, Modbus serial is typically used for
monitoring rather than processing. Typically based on
RS485, the characteristics are:
• Up to 247 servers on the network
• Transmission Speed: 9.2 Kbps and 19.2 Kbps are
standards (other speeds can be selectable, depending
on the vendor, up to 115 Kbps maximum)
• Maximum Length: 1100 m without repeaters (up to
4800 m with fibre optic)
• Maximum Number of Devices: 32 including the
Master
13
• Bus type topology: Allowed with termination resistors
Transmission Modes
 Two distinct transmission modes:
• Remote terminal unit (RTU):
Every data item is encoded
in one byte (8 bits). The main
advantage is that it achieves
higher throughput.
• American Standard Code for Information Interchange (ASCII): Every data item is
encoded in two ASCII characters (7 bits). Allows time intervals of up to 1s
between characters without causing an error. Dedicated more to devices such as
printers, modems, barcode readers.
 RTU is more efficient but slightly more complicated to implement; ASCII is very
simple.
 The transmission mode (and serial port parameters) must be the same for all devices
on a Modbus serial line.
 The Checksum field is used to control the integrity of the request. Modbus RTU uses
CRC (Cyclical Redundancy Check on 2 bytes) whereas Modbus ASCII uses LRC
(Longitudinal Redundancy Check on 1 byte) which has less performance in terms of
error detection. In a request, the checksum is calculated by the client and sent to the
server. In a response, the checksum is re-calculated by the server and compared to
the value sent by the client. If a difference is detected, the server will not construct
14 a
Modbus RTU Framing
 A Modbus message is placed by the transmitting device into a frame that has a known
beginning and ending point. This allows devices that receive a new frame to begin at the start of
the message and to know when the message is completed. Partial messages must be detected
and errors must be set as a result.
 This mode is more efficient than ASCII since each application message octet needs only one
character on the wire. Each character on the wire is sent in this order (left to right): LSB to MSB.
 If no parity is implemented, an additional stop bit is transmitted to fill out the character on-thewire frame.
 The RTU transmission mode identifies the transmission element boundaries based on time.
Message frames are separated by a silent interval of at least 3.5 character (on the wire) times.
 The entire message frame must be transmitted as a continuous stream of characters. If a silent
interval of more than 1.5 character (on the wire) times occurs between two characters on the
wire, the message frame is declared incomplete and should be discarded by the receiver.
15
Modbus ASCII Framing
 This mode is less efficient than RTU since each application message octet
needs two characters on the wire. For example, the application message
octet 0x5B is encoded as two characters for the wire: 0x35 and 0x42 (0x35 =
5 and 0x42 = B in ASCII).
 While there are time-outs involved, the ASCII transmission mode identifies
the transmission element boundaries based on character values. A message
must start with a colon (:) character (ASCII 3A hex) and end with a carriage
return–line feed (CRLF) pair (ASCII 0D and 0A hex).
 If no parity is implemented, an additional stop bit is transmitted to fill out the
character on-the-wire frame.
 Each character on the wire is sent in
this order (left to right): LSB to MSB.
 Intervals of up to one second may
elapse between characters within the
message. If a greater interval occurs,
the receiving device assumes that an
16
error has occurred.
Ethernet Modbus TCP
 Ethernet Modbus TCP combines the openness and simplicity of Modbus
with the high speed characteristic of Ethernet networks. Modbus TCP means
that the Modbus protocol is used on top of Ethernet TCP/IP.
 Ethernet is a type of Local Area Network (LAN) architecture. There are two
types of physical layouts, or topologies, that Ethernet can use to create a
LAN:
• BUS topology, where the computers are strung together in the manner of
a series of Christmas tree lights.
• STAR topology, where the computers all
connect to a central hub.
 These topologies support data transfer rates of
10 Mbps. It is one of the most widely
implemented LAN standards.
 A later version of Ethernet, called Fast Ethernet, supports data transfer
rates of 100 Mbps. Newer versions, including Gigabit Ethernet support17data
rates of 1 gigabit per second and beyond.
Ethernet Networks
 Ethernet technology has evolved to meet new bandwidth and market
requirements. In addition to computers, Ethernet is now used to interconnect
appliances and other personal devices. It is used in industrial applications
and is quickly replacing legacy data transmission systems in the world’s
telecommunications networks.
 Two tin cans joined together by a string create a network. In its simplest form
a network is simply two devices joined together by a wire so that they can
talk to each other.
 Networks enhance the capabilities of a computer system. A network will
allow sharing of devices such as printers, information in the form of
documents and applications such as Control Expert.
18
OSI Layers Used by Modbus TCP
 It is important to remember the distinction that Modbus itself is an application
protocol, as it defines rules for organising and interpreting data, but remains
simply a messaging structure, independent of the underlying physical layer.
As it happens to be easy to understand, freely available, and accessible to
anyone, it is thus widely supported by many manufacturers.
 Ethernet Modbus TCP/IP is basically Modbus frames encapsulated into
Ethernet wrapper. It is the Modbus Protocol running on a TCP interface on
Ethernet.
19
Ethernet Frame Encapsulation
 Modbus messages on Ethernet are packaged into frames for delivery. The requests
are encapsulated within other protocols as they pass through the OSI model layers.
 As a comparison, imagine the delivery service of a factory that has to take a package
to your house. The package will go into a car; the driver will drive all the way down
from the factory to the house and deliver the package.
 Now what is the house is very far away from the factory? The driver can still deliver
the package, but it can take a lot of time driving all the way. A solution could be that
the car goes on a high speed train, which will stop into the closest city, and then the
car can go from the train station to the house.
 The same principle
applies with Ethernet
Modbus TCP/IP. The
slow Modbus frames are
in the high speed
Ethernet train:
20
Ethernet Networking




To put it simply:
• A Sever is a computer that is able to share a resource.
• A Client is a computer that is connected to a resource.
Any resource. It does not matter what operating system is installed. The
Operating System is dictated by the type of Server the computer is going to be.
Some examples of different types of Server / Client model architectures are:
• Email Server / Email Program
• Web Server / Browser
• FTP Server / FTP Client
• Modbus Slave / Modbus Master
The screenshot shows that this computer is both a Server (the second item is enabled) and a Client
(the first item is enabled).

There are four elements that need to be configured in order to allow devices to talk to each other:
Client, Service, Adaptor, Protocol

These can be viewed and configured by going to the Control Panel and double clicking the Network
and Sharing Center icon.

Windows operating systems have inbuilt functionality which allows them to connect to a wide variety
of networks.

Network adaptors are the physical cards that are installed into the computer allowing them to
communicate on the network. Newer computers have integrated adaptors built onto the motherboard.
Adaptors are typically shipped with associated drivers which are required for their use. Most network
21
adaptors are Plug and Play devices.
Ethernet Protocols
 We saw that a simple way to understand the concept of protocols is to think
of them as a language. In order for people to understand each other they
need to speak the same language. In the same way computers need to use
compatible protocols.
 A protocol is really a set of rules or standards which vendors follow to allow
computers of different types to talk to each other. This is why it is possible to
talk to someone over the Internet using TCP/IP protocol even though one
computer has Windows installed and the other is an Apple. In a Windows
environment there are primarily only three protocols commonly used:
NetBEUI, IPX/SPX, and TCP/IP.
22
TCP/IP
 Research funded by the Defence Advanced Research Projects Agency
produced the TCP/IP suite. The TCP/IP standard can be used to
communicate between any two interconnected networks.
 TCP/IP can be used simply to connect all networks within a single physical
location or for communications between geographically remote sites or Wide
Area Networks (WAN). TCP/IP is supported across a wide selection of
network operating systems and on a wide range of hardware platforms.
 The Transmission Control Protocol (TCP) provides a reliable transfer of
application data between devices. It is responsible for the establishment of a
dedicated connection between the two end devices. Acknowledgement of
each byte of data that is sent is mandatory. If this is not achieved, TCP has
the ability to retransmit any lost data. It can also manage the resources of
each end device to prevent overload. TCP is a connection based protocol,
which means that a socket must be established before data can be sent.
The socket should be closed in a manner that allows any final data to be
23
sent.
Ports and Sockets
 Each protocol uses a default port to avoid confusion by the device, and also allows
different types of information on the same media. Server devices may be running
multiple services for Modbus, Web, etc. These services listen on different TCP ports
for a request. Client devices send an IP message which includes the source port
number and destination port number to establish a connection between two devices
(called Socket connection).
 This principle is controlled by TCP. The number of TCP service connections is
limited, so the server will grant the connection request if a socket is available. Default
ports can be changed on the
devices. A Client can have
multiple connections to the same
Server service, as well as a
Server can have multiple Clients
connected to the same service.
Using different random source
port numbers, the server can
organise the correct response
data to send to each Client socket
24
connection.
IP Address
 In order to recognise different nodes on a network TCP/IP uses Internet Protocol
(IP) Addresses. The IP address of the computer may be set locally or allocated
dynamically using Dynamic Host Configuration Protocol (DHCP).
 The easiest way to check the IP address of the computer is to open a command
window and type ipconfig at the command prompt:
 IP is a messaging protocol that operates at OSI Layer 3, the Network layer. It is
responsible for transmitting the message to the destination IP address on the local
network or to the remote network.
 The IP protocol will fragment a packet that is larger than the Maximum Transmission
Unit (MTU) and the packet will be sent as multiple fragments. These fragments are
reconstructed at the destination.
 The protocol makes no guarantee that the message will reach its destination.
25
The reliability is done by the Transport layer (TCP), should it be used.
What is an IP Address?
 An IP Address is made up of four sets of three numbers. Actually they are
four sets of eight bits. An example would be:
11000000.10101000.00000000.11010100
 When those eight-bit numbers are translated into decimal they become
numbers between 0 and 255. For the example above:
192.168.0.212
 An IP Address contains two things:
• The identity of the Network
• The identity of the Host (Computer,
Printer, I/O Device, etc.)
 There are many networks around the world.
Connections are established all the time
when the Internet is used. Every network
has a unique identification and every node
on that network is also unique.
26
IP Assignment
 By default, any Ethernet device has a default Media Access Control (MAC)
address, which is a unique identifier assigned to network interfaces for
communications on the physical network segment. This address, the
physical one, is unique throughout the world.
 From this, the default factory IP address can be calculated, but a new IP
address should be assigned. There are two ways to assign the IP address:
• Static: IP address is keyed directly into the device using software or web
interface. In the case of a computer, the IP address is set using the
operating system (Control Panel inside Windows for example).
• Dynamic: A server assigns automatically the IP address to the device.
 A specific IP address 127.0.0.1 is reserved for loopback (the host
connecting to itself). It can be used, for example, to connect a web browser
to a web server running on the same PC to display the pages.
 The Broadcast IP Address 255.255.255.255 is used by the network
management for diagnostics to contact all the devices on the network.
27
IP Addresses in the Public Domain
 Within an isolated network, IP addresses may be assigned randomly as long
as each one is unique. However, connecting a private network to the Internet
requires using registered IP addresses (called Internet addresses) to avoid
duplicates.
 The four numbers in an IP
address are used in
different ways to identify a
particular network and a
host on that network. The
regional Internet registries
assign Internet addresses
from three classes.
 There are five Regional
Internet Registries operating
under the auspices of the
Internet Assigned Numbers
28
Authority (IANA):
IP Address Classes
 Class A supports 16 million hosts on each of 126 networks. The Class A
Networks are the fewest in number but are able to have the greatest number
of hosts connected to that network. A Class A network is identified by the
first number of the IP Address only. The first number in that IP address can
be from 1 to 126. An example of a Class A Address would be:
110.192.56.12
 Class B supports 65000 hosts on each of 16000 networks. A Class B
Network is identified by the first two numbers in the IP Address. The first
number can be from 127 to 191. An example of a Class B Address would be:
151.134.34.9
 Class C supports 254 hosts on each of 2 million networks. Class C
Networks are the most numerous but are able to have the fewest hosts
connected to the network. A Class C Network is identified by the first three
numbers and the first number can be from 192 to 223. An example of a
Class C Address would be:
203.162.153.8
29
Network ID and Host ID
 So what does the Subnet mask do? It identifies which part of the number is
the Network ID and which part is the Host ID. For example, examine the
Class A example:
110.192.56.12
 The first number is the Network ID. The subnet mask indicates this by
blocking out the first number:
So a computer on this network belongs to Network number 110 and it is
computer number 192.56.12
 In a Class B network the Subnet mask covers the first two numbers:
So this computer belongs to network number 151.134 and is computer
number 34.9 on that network.
 Most of the time a Class C network address is configured as they are the
most numerous:
30
Troubleshooting TCP/IP
 There are several commands which may be used to obtain information over
a TCP/IP network. One of the most commonly used is the Ping command. It
is used in the same way as two submarines use their ping to check the
distance between each other.
 Ping in computers is used to check the connection and the response times
over TCP/IP networks. If there are connection problems on the network the
first step in troubleshooting should be to ping a remote computer. If a
response is received then the network is working and the problem is located
in another area…
(see TCP/IP debugging
in the first lab!).
 Here is an example of
the ping command
after execution:
31
UDP
 The User Diagram Protocol is becoming more popular in industrial
communication. This protocol has the same features as the TCP, except that
it provides an unreliable mechanism to transport data. Messages can be lost
because no acknowledgement of the packet is requested. However, retries
and data integrity can be provided by the Application layer. The system
relies on the error detection in the Application layer to ensure the integrity of
the message.
 So why use it? Simply because it requires less processing overhead that
TCP. Without ordering messages and managing connections, this protocol is
faster than TCP, the network interface does not have as much work to do as
with TCP. Time-sensitive applications often use UDP because dropping
packets is preferable to waiting for delayed packets, which may not be an
option in a real-time system.
 In addition, UDP allows Broadcast messages, unlike TCP that requires a
dedicated socket at the end device.
32
UDP vs. TCP Transmission
 UDP is like sending a letter. It has  TCP is like a telephone
a destination address, the data
conversation. A permanent
reaches the destination via
connection is established for the
different paths and along the way it
duration of the telephone call. The
may get lost or get out of
connection is reliable and all the data
sequential order. Each letter can
gets through in the correct order. It is
take a different path to the same
said to be connection oriented.
destination. It is said to be
 TCP Advantages:
connectionless.
• Guarantees the data will get
 UDP Advantages:
through. Should be used on
• Protocol has less overhead.
unreliable and complex networks.
More throughput on a reliable
• Direct connection between client
and simple network.
and server.
 UDP Disadvantages:
 TCP Disadvantages:
• Data may become lost.
• Has more overhead. On a
Application layer will pick this
reliable network will consume
up.
more bandwidth to transfer the
same data.
33
Collisions




Any device or computer can have access to the network at any time. The Datalink layer that defines
the access allows any device to send messages, which can result in collisions (CSMA-CD). When a
collision occurs, it can be detected by TCP, by IP (in case of both sending to the same IP address) or
by the Application layer.
Collisions between two connected end devices can often happen when the devices are linked in half
duplex. Only one device can transmit at a time. Devices wait for a short period of no transmissions,
and then begin transmitting. If a device starts transmitting after another device is already transmitting,
a collision occurs. The transmitter senses the collision electrically and stops transmitting. Both devices
then wait for an interval determined by an algorithm before trying to transmit again (up to 16 times).
This can have a very high impact in terms of automation performances.
Half duplex occurs when using hubs: each and every frame is repeated from all ports by the hub. All
nodes listen to see if they are the receiver of the frame. As more nodes are added and collisions
increase, performance decreases. If two devices transmit simultaneously, a collision occurs.
To reduce collisions, the system has to work in full duplex. In this case, the network has to be
designed with switches that use two wires to transmit and two wires to receive. Moreover, managed
switches (such as the Connexium switches in the lab) are able to know which devices are connected
to their ports and can redirect the message to the appropriate devices directly, not on all the ports.
Even if they are more expensive, they are highly recommended for industrial applications.
34
Physical Media
 The most commonly used medium is twisted pair wires. They are easy to install, with
low cost. There are many typed of cables defined, but the most commonly found are:
• 10BASE-T: Four wires are used in cables that end in an RJ45 connector. Each
cable can be straight to connect between devices and hub or switch, or crossed
to have a point to point connection. Nevertheless, more and more devices have
automatic port adaptation and crossed or straight cable can be used. Speeds
supported are up to 10 Mbits/s, maximum distance of a segment is 100 m.
• 100BASE-T: Same as 10BASE-T, but with a speed of up to 100 Mbits/s.
• 100BASE-FX: Optical fibre is able to produce speeds up to 100 Mbits/s.
Expensive, they are used for long distances or harsh industrial environments.
• 1000BASE-T: 1 Gbit/s capable cable using 4 pairs of wires in full duplex. This
technology is compatible with 10BASE-T and 100BASE-T.
 These names are
derived from the speed
(in Mb/sec), the
communication type
(Baseband or
Broadband) and the
cable type (T = twisted
35
pair, FX = fibre optic).
Other Components
 Radio: Wireless communication such as Wi-Fi is possible to implement
Ethernet Modbus TCP/IP and will be used more often in the future. The main
issue to solve with wireless are the resistance against electro-magnetic
disturbances as well as security.
 Hubs and Switches are used to connect devices together.
 Transceivers are used to convert one medium to another, typically from
copper cables to fibre optic.
 Routers are used to connect and route information between networks. The
router may have an IP address that is used as gateway for the device to
jump from one network to the other. Otherwise, without the router, the device
cannot reach another network due to the limitation fixed by the subnet mask.
Inside a router, a table of the interfaces tells the router on which interface to
send the packet, and to which network.
36
Gateways
 Gateways involving Modbus have been around for a very long time, and new ones keep
being developed, with Modbus being the common second protocol available side by side
other protocols, or providing the gateways between different Modbus stacks.
 The gateway between Modbus serial and Modbus/TCP, a very successful one, enabled
many legacy products to ride the TCP/IP wave without having to be replaced or left
behind.
 Often gateways perform the duty of proxy clients, for instance, the aforementioned
gateway between Modbus serial and Modbus/TCP allows multiple clients on the
Modbus/TCP side to access quasi-concurrently servers on the Modbus serial side, by
buffering and maintaining separate queues, with no need for user synchronization.
 The major reason Modbus is a big player in these protocol activities is that it makes no
assumptions about the application semantics and it has an excellent performance /
resource ratio on generic services instead.
 Modbus has been used to access other protocols in two major ways:
• When using interpretation, the gateway is knowledgeable about Modbus and the
other protocol, or about Modbus on different stacks, and manages services and
activities on both protocols, essentially by mapping activities. An example is the
aforementioned Modbus/TCP to Modbus serial gateway.
• When using mapping, two protocols share a memory mapping, where both can read
and write or communicate activities/commands. Once the mapping is agreed, it is
very easy to write a Modbus client that can access the other protocol’s information
37
with no need to know anything about the other protocol.
Summary
 Modbus Features
 Ethernet Modbus TCP
• Basic Ethernet Principles
• TCP/IP
• UDP
• Media
• Components
• Topologies
38
39