Lenze I/O system 1000 - Page 570

Lenze · I/O system 1000 · Reference Manual · DMS 5.0 EN · 09/2016 · TD17 570
10 Modbus TCP communication
10.1 About Modbus TCP
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Telegram structure
MAC/DLL: While the Ethernet physics with its standardised signal levels covers layer 1, MAC/DLL
complies with the specifications for the data link layer (layer 2). With MAC (Medium Access Control)
/ DLL (Data Link Layer), communication takes place on the lowest Ethernet level using MAC
addresses. Every Ethernet-capable station has a non-ambiguous MAC address that may only exist
once. When MAC addresses are used, source and target are clearly specified.
IP: The internet protocol covers the network layer (layer 3) of the SO/OSI layer model. The task of the
IP is to send data packets from one computer to the receiver via several computers. These data
packets are datagrams. The IP neither ensures the correct order of datagrams nor the delivery at the
receiver. For a clear distinction between sender and receiver, 32-bit addresses (IP addresses) are
used that usually are written in four octets (exactly 8 bits), e.g. 172.16.192.11. In an octet, figures
between 0 and 255 can be displayed. A part of the address specifies the network, the rest serves to
identify the computer in the network. The transition between network part and host part is smooth
and depends on the network size.
TCP: The TCP (Transmission Control Protocol) is directly based on the IP and thus covers the
transport layer (layer 4) on the OSI layer model. TCP is a connection-oriented end-to-end protocol
and serves as a logical connection between two partners. TCP ensures a logical and reliable data
transfer. Each datagram is provided with a header of min. 20 bytes which, among other things,
contains a sequence number for the correct order. Thus, the single datagrams in a network are able
to reach the target in various ways.
API: API stands for Application Programming Interface. API fulfills the requirements for the
application layer (layer 7). Here, header and user data of the corresponding protocols are stored. In
the ModbusTCP bus coupler, the Modbus TCP protocol is used which will be explained in detail in
the following section.
[10-1] API structure
Layer 2 Layer 3 Layer 4 Layer 7
MAC/DLL IP TCP API ...
14 bytes 20 bytes 20 bytes Length depends on protocol
MAC/DLL
IP TCP
API
Layer 2 Layer 3 Layer 4 Layer 7
14 Byte 20 Byte 20 Byte Depending of length
...
Port 502 Modbus TCP header Modbus User data
6 Byte
...
max. 254 Byte
Modbus TCP