Fieldbus Controller 750-841 • 63

Data Exchange

3.1.5 Data Exchange

The ETHERNET TCP/IP fieldbus controller can be configured for either

MODBUS/TCP or the Ethernet IP protocol.

MODBUS/TCP works according to the master/ model. The master (e.g., a PC or a PLC) will query a slave device and the slave will return a response to the master depending on the kind of query. Queries are addressed to a specific node through the use of the IP address.

Typically, the ETHERNET TCP/IP controller of the WAGO-I/O-SYSTEM is a slave device. But, with the use of the WAGO-I/O-PRO CAA programming tool, the PFC can additionally perform master functions.

A controller is able to produce a defined number of simultaneous socket connections to other network subscribers:

•3 connections for HTTP (read HTML pages from the controller),

•5 connections via MODBUS/TCP (read or write input and output data from the controller),

•128 Ethernet IP connections,

•2 connections via the PFC (available for IEC 61131-3 application programs)

•2 connections for WAGO-I/O-PRO CAA (these connections are reserved for debugging the application program via ETHERNET.

For debugging, WAGO-I/O-PRO CAA requires 2 connections at the same time. However, only one programming tool can have access to the controller.

The maximum number of simultaneous connections may not be exceeded. If you wish to establish further connections, terminate an existing connection first. For data exchange, the ETHERNET TCP/IP fieldbus controller uses three main interfaces:

•interface to the fieldbus (master),

•the PLC functionality of the PFCs (CPU) and

•the interface to the I/O Modules.

Data exchange takes place between the fieldbus master and the I/O modules, between the PLC functionality of the controller and the I/O modules as well as between the fieldbus master and the PLC functionality of the controller. Currently, the 750-841 supports MODBUS/TCP and ETHERNET IP based master devices. When the controller performs PLC functions, and controls various I/O modules, this is done with the use of an IEC 61131-3 application program, whereby the data addressing is different than the fieldbus addressing.

WAGO-I/O-SYSTEM 750 ETHERNET TCP/IP

64 • Fieldbus Controller 750-841

Data Exchange

3.1.5.1 Memory Areas

The PFC‘s process image contains the physical data of the I/O modules in memory words 0 to 255 and 512 to 1275.

(1)Reading data of the input modules is possible from both the controller’s CPU and from the fieldbus master (See Figure 3-8).

(2)In the same manner, writing data to output modules is possible from both the controller’s CPU and from the fieldbus master.

The controller’s process image also contains variables called “PFC Variables”. These variables are allocated based on the fieldbus protocols. The MODBUS TCP PFC variables are stored in memory from word 256 to 511. Ethernet IP PFC variables are stored in memory from word 1276 to 1531. The memory area above word 1531 is reserved for future protocols.

WAGO-I/O-SYSTEM 750 ETHERNET TCP/IP

Fieldbus Controller 750-841 • 65

Data Exchange

(3)The PFC input variables are written into the input memory space from the fieldbus master and can be read by the controller’s CPU for further processing.

(4)The variables processed by the controller’s CPU , via an IEC 61131-3 application program, can be written to the PFC Variables and then read by the fieldbus master.

In addition, with the MODBUS TCP/IP protocol, all output data has a mirrored image in memory with the address offset 0x0200 or 0x1000. This permits reading back output values after they are written by adding 0x0200 or 0x1000 to the MODBUS address.

In addition, the controller offers other memory spaces which partly cannot be accessed from the fieldbus master:

RAM The RAM memory is used to create variables not required for communication with the interfaces but for internal processing, such as for instance computation of results.

Remanent The remanent memory is non volatile memory, i.e. all values are Memory retained following a voltage failure. The memory management is

automatic. In this 24 kByte sized memory area (word 0 ...

(Retain) 12288), flags for the IEC 61131-3 program are filed together with variables without memory space addressing or variables which are explicitly defined with "var retain".

Note

The automatic memory management can cause a data overlap. For this reason, we recommend not to use a mix of flags and retain variables.

Code- The IEC 61131-3 program is filed in the code memory. The code Memory memory is a flash ROM. Once the supply voltage is applied, the program is transmitted from the flash to the RAM memory. After

a successful start-up, the PFC cycle starts when the operating mode switch is turned to its upper position or by a start command from WAGO-I/O-PRO CAA.

WAGO-I/O-SYSTEM 750 ETHERNET TCP/IP