Specifications

Head Station STL-ETH1 (Ethernet) Ethernet
Quick Start Guide for the MELSEC STlite Series 4 - 7
4.2.7 Process Data Architecture
After switching on the supply voltage, the head station identifies all I/O modules connected with the
node that send or receive data (data width/bit width > 0). In the maximum total extension the node
can consist of a mixed arrangement of a maximum of 64 analog, digital I/O modules or special func-
tion modules connected on the head station.
The data of the digital I/O modules are bit-oriented; i.e., digital data are sent bit by bit. The data of the
analog and special function modules are byte-oriented; i.e., data are sent byte by byte.
The head station stores the process data in the process images. The head station works with a process
output data image (PIO) and a process input data image (PII).
The PIO is filled by the fieldbus master with the process output data. The PII is filled by the head station
with the process input data.
Into the input and output process image the data of the I/O modules are stored in the sequence of its
position after the head station in the individual process image. First, all the byte-oriented I/O modules
(analog I/O and special function modules) are stored in the process image, then the bit-oriented dig-
ital I/O modules. The bits of the digital I/O modules are grouped into bytes. If the amount of digital
I/O information exceeds 8 bits, the head station automatically starts a new byte.
Data Exchange
With the head station, data is exchanged either via the MODBUS/TCP protocol or via Ethernet/IP.
MODBUS/TCP works according to the master/slave principle. The master controller can be a PC or a
PLC. The head station STL-ETH1 is a slave device.
The master requests communication. This request can be directed to certain nodes by addressing.
The nodes receive the request and, depending on the request type, send a reply to the master.
The STL-ETH1 is essentially equipped with two interfaces for data exchange:
– the interface to the fieldbus (Master)
– the interface to the I/O modules.
Data exchange takes place between the fieldbus master and the I/O modules. If MODBUS is used as
the fieldbus, the MODBUS master accesses the date using the MODBUS functions implemented in the
head station.
NOTES Avoid equipment damages due to addressing errors!
To avoid equipment damages within the field range, you must consider that, depending on the
specific position of an I/O module in the fieldbus node, the process data of all previous byte or
bit-oriented modules must be taken into account to determine its location in the process data
map.
Consider the Process Data size for each module!
Observe the number of input and output bits or bytes for the individual I/O modules.
NOTE In this Quick start manual, only the data exchange via MODBUS/TCP is covered.