Modicon M262 Logic/Motion Controller as a Slave Device on Modbus TCP

Overview

This section describes the configuration of the M262 Logic/Motion Controller as a Modbus TCP Slave Device.

To configure your M262 Logic/Motion Controller as a Modbus TCP Slave Device, you must add Modbus TCP Slave Device functionality to your controller (see Adding a Modbus TCP Slave Device).

This functionality creates a specific I/O area in the controller that is accessible with the Modbus TCP protocol. This I/O area is used whenever an external master needs to access the %IW and %QW objects of the controller. This Modbus TCP Slave Device functionality allows you to provide the controller I/O objects to this area, which can then be accessed with a single Modbus read/write registers request.

The Modbus TCP Slave Device adds another Modbus server function to the controller. This server is addressed by the Modbus client application by specifying a configured Unit ID (Modbus address) in the range 1...247. The embedded Modbus server of the slave controller needs no configuration, and is addressed by specifying a Unit ID equal to 255. See the Modbus TCP Configuration.

Inputs/outputs are seen from the slave controller: inputs are written by the master, and outputs are read by the master.

The Modbus TCP Slave Device can define a privileged Modbus client application, whose connection is not forcefully closed (embedded Modbus connections may be closed when more than eight connections are needed).

The timeout duration associated to the privileged connection allows you to verify whether the controller is being polled by the privileged master. If no Modbus request is received within the timeout duration, the diagnostic information i_byMasterIpLost is set to 1 (TRUE). For more information, see the Ethernet Port Read Only System Variables.

For further information about Modbus TCP, see the www.modbus.org website.

Adding a Modbus TCP Slave Device

To add Modbus TCP slave device functionality to your M262 Logic/Motion Controller:

Step	Action
1	Add a TMSES4 expansion module to your configuration.
2	From the TMSES4 node in the Devices tree, add the Modbus TCP Slave Device by selecting it in the Hardware Catalog, dragging it to the Devices tree, and dropping it on the TMSES4 node. For more information on adding a device to your project, refer to: • Using the Drag-and-drop Method • Using the Contextual Menu or Plus Button

Step

Action

Add a TMSES4 expansion module to your configuration.

From the TMSES4 node in the Devices tree, add the Modbus TCP Slave Device by selecting it in the Hardware Catalog, dragging it to the Devices tree, and dropping it on the TMSES4 node.

For more information on adding a device to your project, refer to:

• Using the Drag-and-drop Method

• Using the Contextual Menu or Plus Button

Configuring a Modbus TCP Slave Device

To configure the Modbus TCP slave device, double-click COM_Bus > TMSES4 > ModbusTCP_Slave_Device in the Devices tree.

This dialog box appears:

Element	Description
IP Master Address	IP address of the Modbus master The connections are not closed on this address.
Watchdog	Timeout in 500 ms increments NOTE: The timeout applies to the IP Master Address unless the address is 0.0.0.0.
Slave Port	Modbus communication port (502)
Unit ID	Sends the requests to the Modbus TCP slave device (1...247), instead of the embedded Modbus server (255).
Holding Registers (%IW)	Number of %IW registers to be used in the exchange (2...120) (each register is 2 bytes)
Input Registers (%QW)	Number of %QW registers to be used in the exchange (2...120) (each register is 2 bytes)

Modbus TCP Slave Device I/O Mapping Tab

The I/Os are mapped to Modbus registers from the master perspective in the following way:

%IWs are mapped from register 0 to n-1 and are R/W (n = Holding register quantity, each %IW register is 2 bytes).
%QWs are mapped from register n to n+m -1 and are read only (m = Input registers quantity, each %QW register is 2 bytes).

When a Modbus TCP Slave Device has been configured, Modbus commands sent to its Unit ID (Modbus address) are handled differently than the same command would be when addressed to any other Modbus device on the network. For example, when the Modbus command 3 (3 hex) is sent to a Modbus device, it reads and returns the value of one or more registers. When this same command is sent to the Modbus TCP Slave, it facilitates a read operation by the external I/O scanner.

When a Modbus TCP Slave Device has been configured, Modbus commands sent to its Unit ID (Modbus address) access to the %IW and %QW objects of the controller, linked to the Modbus TCP device, instead of the regular Modbus words (accessed when the Unit ID is 255). This facilitates read/write operations by a Modbus TCP IOScanner application.

The Modbus TCP Slave Device responds to a subset of the Modbus commands with the purpose of exchanging data with the external I/O scanner. The following Modbus commands are supported by the Modbus TCP Slave Device:

Function Code Dec (Hex)	Function	Comment
3 (3)	Read holding register	Allows the master to read %IW and %QW objects of the device
6 (6)	Write single register	Allows the master to write %IW objects of the device
16 (10)	Write multiple registers	Allows the master to write %IW objects of the device
23 (17)	Read/write multiple registers	Allows the master to read %IW and %QW objects of the device and write %IW objects of the device
Other	Not supported	–

NOTE: Modbus requests that attempt to access registers above n+m-1 are answered by the 02 - ILLEGAL DATA ADDRESS exception code.

To link I/O objects to variables, select the Modbus TCP Slave Device I/O Mapping tab:

Channel		Type	Description
Input	IW0	WORD	Holding register 0
	...	...	...
	IWx	WORD	Holding register x
Output	QW0	WORD	Input register 0
	...	...	...
	QWy	WORD	Input register y

The number of words depends on the Holding Registers (%IW) and Input Registers (%QW) parameters of the Modbus TCP tab.

NOTE: Output means OUTPUT from the client/master controller (%IW for the server/slave controller). Input means INPUT from the client/master controller (%QW for the server/slave controller).

Bus Cycle Options

Select the Bus cycle task to use:

Use parent bus cycle setting (the default),
MAST

There is a corresponding Bus cycle task parameter in the I/O mapping editor of the controller that contains the Modbus TCP slave device. This parameter defines the task responsible for refreshing the %IW and %QW registers.