Fieldbus Controller 750-841 • 47

Process Image

3.1.4 Process Image

The powered-up controller recognizes all I/O modules connected in the node that are waiting to transmit or receive data (data width/bit width > 0). The maximal length of a node is limited to 64 I/O modules.

Note

Use of the WAGO 750-628 Bus Extension Coupler Module and the 750-627 Extension End Module enables support of up to 255 I/O modules on the 750-841 controller.

Note!

Expansion to 255 I/O modules is not enabled in the initial version of the 750-841 PFC.

Note

For the number of input and output bits or words of an individual I/O module, please refer to the corresponding module description later in this chapter.

The controller generates an internal local process image from the data width and type of I/O modules, as well as the position of the I/O modules in the node. This image is divided into an input and an output area.

The data of the digital I/O modules are bit-based (i.e., the data exchange is made by bits). The analog I/O and most specialty modules (e.g., counter modules, encoder modules, and communication modules) are byte-based, in which the data exchange is made by bytes.

The process image is divided into an input and an output data area. Each I/O module is assigned a location in the process image, based on the data exchange type (i.e., bit-based or byte-based) and their position after the controller.

All of the byte-based I/O modules are filled in the process image first, then the bit-based modules. The bits of the digital modules are grouped into a word. Once the number of digital I/Os exceeds 16 bits, the controller automatically starts another word.

Note

Changing the physical layout of a node will result in a new structure of the process image. Also, the addresses of the process data will change. When adding or removing modules, the process data must be verified.

The process image for physical input and output data is stored in the first 256 words of memory (word 0 to 255). This memory actually consists of a separate area for the input and output data, but both areas are referenced in a PLC program with an index of 0 to 255 for word operations.

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

48 • Fieldbus Controller 750-841

Process Image

The MODBUS PFC variables are mapped after the process image of the I/O modules. This memory area contains 256 words (word 256 to 511).

If the quantity of I/O data is greater than 256 words, the additional data is appended after the MODBUS PFC variables in word 512 to 1275. Like the first physical I/O process image area, there is a separate memory area for input and output data, but both are referenced with an index of 512 to 1275 for word operations.

After the remaining physical I/O data is the Ethernet IP PFC variables. This memory area is word 1276 to 1531.

For future protocol additions, the area above word 1532 is reserved for additional PFC variables.

With all WAGO fieldbus controllers, the method used by PLC functions to access process data is independent of the fieldbus system. This access always takes place via an application-related IEC 61131-3 program.

In contrast to the above, access from the fieldbus side is fieldbus specific. For the ETHERNET TCP/IP fieldbus controller, either a MODBUS/TCP master or an Ethernet/IP master is used. MODBUS/TCP accesses the data via implemented MODBUS functions. Here decimal and/or hexadecimal MODBUS addresses are used. With Ethernet/IP, data access occurs with the use of an object model.

Further information

A detailed description of these fieldbus-specific data access operations is given in the sections “MODBUS functions” and “Ethernet/IP (Ethernet/Industrial Protocol)”.

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

Fieldbus Controller 750-841 • 49

Process Image

3.1.4.1 Example of a Process Input Image

The following figure is an example of a process input image. The configuration includes 16 digital and 8 analog inputs. Therefore, the process image has a total data length of 9 words (8 words for the analog data and 1 word for the digital inputs).

WAGO-I/O-SYSTEM 750

ETHERNET TCP/IP

50 • Fieldbus Controller 750-841

Process Image

3.1.4.2 Example of a Process Output Image

The following figure is an example of a process output image. The configuration includes 2 digital and 4 analog outputs. Therefore, the process image has a total data length of 5 (4 words for the analog data and 1 word for the digital outputs). When using MODBUS protocol, output data can be read back with an offset of 200hex (0x0200) added to the MODBUS address.

Note

All output data over 256 words can be read back with an offset of 1000hex (0x1000) added onto the MODBUS address.

DO: Digital Output

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