This guide discusses ICDM--RX/MOD Industrial Gateway configuration for the ICDM--RX/MOD running the
Modbus Router application.
The Modbus Router application provides enhanced connectivity from a wide variety of Modbus masters to a
wide variety of Modbus slaves, advanced Master-to-Master connectivity, and connectivity from private Modbus
serial networks to public Modbus networks.
Supported Modbus masters include:
•Modbus/TCP
•Modbus/RTU serial
•Modbus/ASCII serial
•Modbus/ RTU
•Modbus/ASCII over Ethernet TCP/IP
Supported Modbus slaves include:
•Modbus/TCP
•Public and private Modbus/RTU serial
•Public and private Modbus/ASCII serial
Connectivity can be easily achieved between any master(s) and any public slave(s) anywhere on an Ethernet
network. Combined with a ICDM-RX/MOD running the Modbus/TCP application, both serial and Ethernet TCP/
IP Raw/ ASCII devices can be accessed anywhere on a network from any Modbus master.
Modbus Router was designed to greatly enhance system maintenance capabilities. Included are
comprehensive device and port specific diagnostic web pages that display status, message response timing,
time-outs, other error counts, and overall message statistics. A serial log is also included to provide message
level diagnosis for serial devices.
1.2. Terms and Definitions
This subsection defines terms and definitions used in this guide.
Te r mD ef i ni t io n
Alias Device ID
Device ID
Device ID Offset
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The device ID that the original received ID is changed to when an Alias Device ID is
configured.
The Modbus device identification number. Device IDs include:
0 = Broadcast
1-247 = Standard device IDs
248-255 = Reserved device IDs (generally used for vendor specific gateway functions)
An offset applied at the slave serial port interface that changes the message device
ID range to match the serial device(s) device ID range.
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TermDefinition
Ethernet TCP/IP
Local Slave Device A local slave device is one that is connected directly to a serial port on the gateway.
Master (or Client)
Mode
Modbus/ASCII
Modbus/RTU
Modbus/TCP
Polling
Public Slave
Device
Private Slave
Device
Received Device
ID
Remote Slave
Device
Slave (or Server)
Mode
Sockets
A form of Ethernet connectivity that provides a level of guaranteed delivery and data
verification. This is used for many upper layer protocols such Modbus/TCP and can
be also used for transferring Modbus/RTU and Modbus/ASCII messages.
The method of operation when a PLC, a gateway, or an application is operating as a
Master or the message originator.
An ASCII, or character based, form of Modbus. The base message is the same as
Modbus/RTU, but the format is somewhat different.
The standard Modbus messages, in hexadecimal format, that are typically
transmitted over serial lines but can also be transmitted over other communication
methods such as wireless or Ethernet TCP/IP socket connections.
Note: Modbus/RTU over Ethernet TCP/IP is not the same as Modbus/TCP.
An Ethernet based form of Modbus communication. The base message is the same
as Modbus/RTU, but a special Modbus header is included for packet identification
and routing purposes.
The process where a PLC or Application requests data on a continual basis. In this
operation the Master sends the request messages while the Slave responds to the
messages.
A public slave device is one that can communicate to all master(s) on the Modbus
network.
Note: If a device is not specifically specified as a “public” or “private” slave device,
then it can be assumed to be a public device.
A private slave device is one that can only communicate to a master that is connected
on the same serial bus, such as RS-485 or RS-422. Private slave device cannot
communicate to other masters on the Modbus network.
The original device ID received in the Modbus message from a Modbus master.
A remote slave device is either a slave Modbus/TCP device or a serial slave device
attached to another gateway and accessed as a remote Modbus/TCP device.
The method of operation when a PLC, a gateway, or an application is operating as a
Slave or the message receiver.
The method used to communicate between devices while communicating over
Ethernet TCP/IP.
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1.3. What is Modbus?
This subsection discusses:
•Modbus/RTU
•Modbus/ASCII on Page 7
•Modbus/TCP on Page 8
1.3.1. Modbus/RTU
Modbus/RTU is native Modbus in hexadecimal format. These are the base Modbus messages that contain
simple read and write requests. The format is as follows:
Where:
•The terms Master or Client are used to identify the sender of the message.
•The terms Slave or Server are used to identify the devices responding to the message.
Modbus/RTU is used primarily for:
•Serial port connectivity
RS-485 is the most common serial mode, but RS-232 and RS-422 are also widely used. Commonly used
by both Master and Slave devices.
•Ethernet TCP/IP socket connections
This is not the same as Modbus/TCP on Page 8, but does provide a very simple method of interfacing to
remote devices. It is used by many applications and some OPC servers.
Note: This communication method typically is not supported by PLCs.
1.3.2. Modbus/ASCII
Modbus/ASCII is native Modbus in ASCII format. This protocol is used primarily by legacy devices and is no
longer supported as widely as Modbus/RTU.
Like Modbus/RTU, Modbus/ASCII contains the base Modbus messages that contain simple read and write
requests. The differences between Modbus/ASCII and Modbus/RTU are:
1. The message data is sent in ASCII format, so the message length is twice as long. It requires two ASCII
characters for each byte of data.
2. An 8-bit LRC is attached to verify the message instead of a 16-bit CRC. The LRC is also transmitted in
ASCII format.
3. There are defined starting and ending characters to determine a Modbus/ASCII messages.
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The format is as follows:
Where:
•The terms Master or Client are used to identify the sender of the message.
•The terms Slave or Server are used to identify the devices responding to the message.
Modbus/ASCII is used primarily for:
•Serial port connectivity
RS-485 is the most common serial mode, but RS-232 and RS-422 are also used. Used primarily by legacy
Slave devices.
•Ethernet TCP/IP socket connections
This is not the same as Modbus/TCP, but does provide a very simple method of interfacing to remote
devices. It is used by some applications and some OPC servers.
Note: This communication method typically is not supported by PLCs.
1.3.3. Modbus/TCP
Modbus/TCP is an Ethernet network based protocol that contains a Modbus/RTU message, with the exception
of the 2-byte CRC. The Modbus/TCP message contains a header with information designed to provide
message identification and routing information. The format is as follows:
Where:
•The terms Master or Client are used to identify the sender of the message.
•The terms Slave or Server are used to identify the devices responding to the message.
•Modbus/TCP messages are typically sent to and received on a defined Ethernet TCP/IP socket of 502.
•Modbus/TCP implementations provide more capability, but also require more processing than simpler
Modbus/RTU implementations.
Modbus/TCP is used for connecting advanced Ethernet based devices, such as PLCs, HMIs, SCADA Systems,
and most OPC Servers to:
•Other Ethernet devices supporting Modbus/TCP.
•Remote serial Modbus/RTU and/or Modbus/ASCII devices through gateways (such as the ICDM-RX/MOD
running the Modbus Router or Modbus/TCP applications).
•Remote serial or Ethernet TCP/IP ASCII devices (barcode scanners, printers, RFID readers, visions
systems, etc) through a gateway (such as the ICDM-RX/MOD running the Modbus/TCP application).
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1.4. Modbus Router Functionality
The Modbus Router application provides the following functionality:
•Provides robust connectivity from all supported master devices to all supported slave devices:
-Supported Masters:
-Modbus/TCP Master - accepts Modbus/TCP messages on up to eight TCP/IP ports. This includes
502 and seven configurable ports.
-Modbus/RTU Serial Master
-Modbus/ASCII Serial Master
-Modbus/RTU over Ethernet TCP/IP Master
-Modbus/ASCII over Ethernet TCP/IP Master
-Supported Slaves:
-Modbus/RTU Serial Slaves
-Modbus/RTU ASCII Slaves
-Remote Modbus/TCP Slaves (addressable to device ID and IP address/port)
-Remote Modbus/RTU Serial Slaves (via Modbus/TCP using another gateway)
-Remote Modbus/ASCII Serial Slaves (via Modbus/TCP using another gateway)
•Multiple ICDM-RX/MOD chassis can be used together to form a Modbus network.
•Supports up to 255 public Modbus devices per gateway and/or Modbus network. Both standard, (1-247),
and reserved, (248-255), device IDs are supported.
•All Modbus devices not configured remotely are assumed to be locally connected to the gateway.
•The locations of all local Modbus devices are determined automatically.
•Modbus Device ID Aliasing. Modbus message device ID can be aliased when messages are received from
a Modbus master.
•Device ID Offset. At the serial port interface, device IDs can have a positive or negative offset applied to
change the device ID range.
•Supports up to 96 Modbus/TCP connections. This can include any combination of slave and master
connections.
•Modbus/RTU and Modbus/ASCII over Ethernet TCP/IP Master specific:
-Can support up to six TCP/IP connections per serial port configuration.
-All messages received from any Ethernet TCP/IP Master connection enter the routing functionality and
can be routed to any local or remote device.
-Combined with a serial port redirector, such as the Pepperl+Fuchs Comtrol Secure Port Redirector, can
provide COM port functionality from a computer to the Modbus network.
•Supports connectivity to private Modbus serial buses, such as a serial master and slave(s) on a RS-485
loop:
-The Modbus master is provided connectivity to the public Modbus network.
-Only the master has direct access to the serial devices on the private serial network.
-Private slaves are protected from intrusion by other master(s).
-Both slave device specific and port level diagnostics are provided for private network communication.
•Advanced Master-to-Master connectivity is provided via the Shared Memory sub-system. Features are:
Modbus/TCP
Slave (remote
gateway or
slave Modbus/
TCP device)
Local Public
Modbus/RTU
Serial Slave
Local Public
Modbus/ASCII
Serial Slave
Remote
Slave
Devices
Modbus/RTU
Serial Slave (via
remote
gateway)
Modbus/
TCP Master
XX XXX
XX XXX
XX XXX
XX XXX
Modbus/
RTU Serial
Master
Masters
Modbus/
ASCII Serial
Master
Modbus/RTU
Master over
Ethernet TCP/IP
Modbus/ASCII
Master over
Ethernet TCP/IP
Remote
Modbus/ASCII
Serial Slave (via
remote
gateway)
Local Private
Modbus/RTU
Serial Slave
Local Private
Modbus/ASCII
Serial Slave
XX XXX
X
X
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1.5. Multiple Gateway Modbus Networks
A multiple gateway Modbus Network is created by combining two or more Modbus slave(s) and master(s) with
two or more ICDM-RX/MODs running the Modbus Router and/or Modbus/TCP firmware applications.
•Typically, at least one chassis will be running the Modbus Router firmware application.
•The Modbus/TCP application can provide connectivity to local or remote serial and Ethernet TCP/IP Raw/
ASCII devices such as barcode scanners, RFID readers, printers, vision systems, and weigh scales.
•All Modbus masters connected to a ICDM-RX/MOD running Modbus Router can communicate to all public
slaves.
The following connectivity can be provided in a Modbus network when using multiple ICDM-RX/MOD chassis
with Modbus Router and Modbus/TCP firmware applications:
Masters
Modbus Network Connectivity
Slave
Devices
Grid
Modbus/TCP Slave
(remote gateway or
slave Modbus/TCP
device)
Local Public Modbus/
RTU Serial Slave
Local Public Modbus/
ASCII Serial Slave
Remote Modbus/RTU
Serial Slave (via remote
gateway)
Remote Modbus/ASCII
Serial Slave (via remote
gateway)
Serial Raw/ASCII
Devices (remote or
local)
Ethernet TCP/IP Raw/
ASCII Devices (remote
or local)
Modbus
/TCP
Master
XXXXX
XXXXX
XXXXX
XXXXX
XXXXX
XXXXX
XXXXX
Modbus/
RTU
Serial
Master
Modbus/
ASCII
Serial
Master
Modbus/RTU
Master over
Ethernet TCP/
IP
Modbus/ASCII
Master over
Ethernet TCP/
IP
Local Private Modbus/
RTU Serial Slave
Local Private Modbus/
ASCII Serial Slave
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X
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The following diagram demonstrates a multiple gateway Modbus network utilizing Modbus Router firmware.
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The following diagram demonstrates a multiple gateway Modbus network utilizing both Modbus Router and
Modbus/TCP firmware.
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1.6. Modbus/RTU and Modbus/ASCII To-Slaves Interface
The ICDM-RX/MOD provides access to serial Modbus/RTU and Modbus/ASCII slave devices. Modbus master
messages are translated to appropriate Modbus slave messages, public slave devices are automatically
located, and appropriate responses are returned to the Modbus master.
Modbus Master(s) to Public Modbus Slave(s)
1.6.1. Communication Methodology
The ICDM-RX/MOD translates various Modbus formats and forwards them to public slave devices attached to
the Modbus To-Slaves configured serial ports. Each Modbus message is transmitted and a response is
expected. The ICDM-RX/MOD times out the Modbus messages if there is no response returned within the
configured timeout period.
The following diagram displays the Modbus message transfer. The following apply to Modbus To-slaves serial
ports.
•All valid Modbus messages are translated to the appropriate format for serial port or Modbus/TCP
transmission.
•Local public Modbus slave devices are automatically located on a ICDM-RX/MOD 2-port or 4- port.
•Local and remote public Modbus slave devices can be accessed from a Modbus master as if they were of
that master’s protocol type.
•Messages are timed out if no response is returned within the configured timeout period.
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•Appropriate Modbus responses are returned to the Modbus master.
•Broadcast Modbus messages, those with a unit identifier of zero, are transmitted out all Modbus To-slaves
serial ports on the ICDM-RX/MOD. Depending on the remote device configuration(s), remote slave devices
may or may not receive broadcast messages.
From a message routing standpoint, all local and remote public Modbus slave devices attached to a ICDM-RX/
MOD gateway (1, 2, or 4-port) must be addressed with unique Unit Identifiers. Valid Unit Identifiers are 1 to 255
and the Broadcast Identifier is zero. However, the Device ID Offset functionality can be configured to change
the addressing of serial connected slave devices to allow multiple slave devices with the same unit ID to be
connected to the same gateway, but addressed differently. For more information, see section on Alias and
Device ID Offset functionality.
To communicate to local Modbus slave device(s) through a ICDM-RX/MOD, perform the following steps.
1. Select the Serial menu and then select the appropriate port.
2. Under Serial Configuration, configure the serial port parameters such as the Mode, Baud rate, Data Bits,
and so forth.
3. Under Modbus Settings, set the Serial Port Protocol to Modbus/RTU-To-Slaves or Modbus/ASCII-To-Slaves.
4. Under Modbus Slaves To-Slaves Settings, set the Response Timeout to the desired value.
Note: 2- and 4-Port only: Set the Lost Device Search Enable setting. See Local Public Modbus Slave
Device Search Methodology.(below) for more information.
5. If desired, configure the Device ID Offset Mode and Device ID Offset.
To communicate to remote Modbus slave device(s) through a ICDM-RX/MOD, perform the following steps.
1. Select the Modbus menu and then the Remote Modbus Configuration page.
2. Click the Add Remote Configuration button.
3. Configure each remote device as needed and click Save.
To enable the Alias Device ID capability for one or more device Ids, perform the following steps.
1. Select the Modbus menu and then the Alias Configuration page.
2. Click the Add Alias Configuration button.
3. Configure each alias device ID as needed and click Save.
1.6.2. Local Public Modbus Slave Device Search Methodology
Locating a local Modbus slave device on a ICDM-RX/MOD 1-port is relatively simple. Either the Modbus slave
device is connected to the port or it is not. However, if more than one port is configured for Modbus To-Slaves
on a ICDM-RX/MOD 2- or 4-port, the device must be found. The following is an explanation of how the search
algorithm works on a ICDM-RX/MOD 2- or 4-port.
Locating a Local Modbus slave device after a reboot or port reset: When the ICDM-RX/MOD receives a
message for a public Modbus slave device for the first time since reboot or port initialization, it will transmit the
Modbus message out all Modbus To-Slaves serial ports and wait for a response to be returned. Once the
response is returned, the device port is known and all messages sent to the device will be routed through the
serial port.
Lost Devices: Lost devices, or devices that time out, are a special case. The ICDM-RX/MOD provides two
methods for handling lost devices via the Lost Device Search Enable option on the web page.
•Disabling this option on a Modbus To-Slaves port:
-Prevents the ICDM-RX/MOD from searching for a lost device on other Modbus To-Slaves ports.
-Prevents lost devices known to have been on other ports from being searched for on this port.
Note: This is the recommended setting whenever it is desired to prevent timeout delays on other Modbus
To-Slaves ports in the event that a device times out.
•Enabling this option on a Modbus To-Slaves port:
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-Allows the ICDM-RX/MOD to search for lost devices on all Modbus To-Slaves ports with the Lost
Device Search Enable option turned on.
-This will cause timeout delays on all Modbus To-Slaves ports with the Device Search Enable option
turned on until the device is found.
Note: This can be useful for locating devices if a device has been moved onto another port by moving the
serial cable or, perhaps, by moving the device onto a different Modbus To-Slaves serial bus.
1.7. Alias Device ID and Device ID Offset Functionality
One of the most common challenges people face when setting up Modbus systems are the problems caused
by the limited device ID range. The Alias Device ID and Device ID Offset functionality have been developed to
help solve those problems.
The Modbus specification has the following limitations:
•Requires all public devices attached to gateway to be addressed by a device ID.
•Allows only 256 device IDs with a range of 0 to 255.
•Not all device IDs can be used for addressing devices.
-Device ID 0 is reserved for broadcast messages
-1-247 are for device addressing
-248 to 255 are reserved for such things as gateway functions. Depending on your environment, these
device IDs may or may not be available for assignment to devices.
The following are common problems that can occur as a result of the device ID limitations:
•A gateway must route Modbus messages based on the device ID. Therefore, it cannot route to multiple
Modbus devices with the same device ID.
•It is not always possible or practical to change the device ID of serial Modbus slave devices.
•Serial and Ethernet TCP/IP Modbus RTU/ASCII masters with one connection may need to access multiple
devices with the same device ID. Furthermore, these devices may be located locally or remotely.
•It is not always possible or practical to modify the device IDs on existing Modbus master programs. This is
often true when adding a SCADA system to an existing PLC controlled system.
The Alias Modbus Device ID and Device ID Offset functionality has been developed to solve these problems.
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Alias Device ID and Device ID Functionality
Overview
This functionality is described in the following diagram:
Note 1:Originally received Modbus messages. All responses will be returned with the original device ID.
Note 2:Modbus messages sent to and responses received from Modbus Message Routing Handler.
Note 3:Modbus messages received from the Modbus Message Routing Handler. Depending on the Alias
Note 4:Modbus messages sent to Modbus serial slaves. Depending on the Device ID Offset configuration
Note 5:Modbus messages received from the Modbus Message Routing Handler. Depending on the Alias
Depending on the Alias ID configuration, these messages may contain the originally received
device ID or the alias device ID.
ID configuration, these messages may contain the originally received device ID from the Modbus
master or the alias device ID. All responses contain the device ID as received from the Modbus
Message Routing Handler.
for the serial port, these messages may be the same as those received from the Modbus Message
Routing Handler or have a device ID that has been either incremented or decremented to match
the serial device ID range.
ID configuration, these messages may contain the originally received device ID from the Modbus
master or the alias device ID. Device ID Offset functionality does not apply to Modbus/TCP
slaves.
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