Modicon M262 Logic/Motion Controller as a Slave Device on Modbus TCP
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
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 |
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:
o%IWs are mapped from register 0 to n-1 and are R/W (n = Holding register quantity, each %IW register is 2 bytes).
o%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 standard 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 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).
Select the Bus cycle task to use:
oUse parent bus cycle setting (the default),
oMAST
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.