Fieldbus controller 750-842 • 59
Process data architecture for MODBUS/TCP
Modular I/O System
ETHERNET TCP/IP
3.2.4.3
Process data architecture for MODBUS/TCP
For some bus modules or their variations, the process data architecture is spe-
cific for the fieldbus controller used.
In the case of the ETHERNET controller with MODBUS/TCP the con-
trol/status byte is always masked in addition to the data bytes. This is required
for the two-directional data exchange of the bus module with the higher-
ranking control system. The control byte is transmitted from the control system
to the module and the status byte from the module to the control system. This
allows, for example, the display of overshooting or undershooting of the area.
Attention
Please refer to the respective bus module description in Chapter 4 "I/O modu-
les" for the specific architecture of the respective control/status byte.
The following shows the representation of some selected modules in the proc-
ess image.
In the examples, the order in which the modules are physically arranged in the
node reflects the order in the image table starting with register address 0x0000.
If the module is located at any other position in the fieldbus node, the process
data of all previous byte-wise oriented modules have to be taken into account.
In the process image, this results in a basic register address for the module. To
address its process data words, the quoted offset is added to this basic address.
If an analog input or output module is added, it takes up 2 x 16 Bit input or
output data.
With the ETHERNET fieldbus controller with MODBUS/TCP the process im-
age is organized word-by-word (word-alignment) and the control/status byte is
always a low byte.
