Specifications

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1.3.2 IP protocol
(1) Role of IP
IP is a network layer protocol which is processed by all devices connected to an IP
network.
The most important role of the TCP/IP (UDP/IP) network is "data transfer to a
device or PLC at the target address".
This role is achieved by IP (Internet Protocol).
Data (packets) are delivered with the information called "IP header" (tag) attached.
(2) Restrictions on IP
Although IP transfers data to a target PLC, there are some restrictions.
(a) No guarantee to reach the destination
3 1
2
13
IP network
(b) No guarantee to reach in the order of sending
IP network
2 13 1 32
(c) As the transfer size at one time is limited, a packet may be divided into
several pieces at transmission. (*1)
IP network1 1-11-2
(d) No guarantee for no data damage
IP network
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Damaged data
To put it shortly, IP bends every effort to transfer data (packets) to the target device or
PLC, however, it does not guarantee delivery of packets (Best effort).
Note that, there is no need to consider these restrictions when using TCP over IP.
When using UDP as a higher layer, care should be taken since the above restrictions
apply except the data damage detection.
*1 The size of one message (1 packet) that can be transmitted by the Ethernet
module is up to 1500 bytes (including the IP header).
Data exceeding 1500 bytes are divided in either case of the TCP/IP or UDP/IP
communication. The divided data are reassembled into one data on the receiving
side and handed to the application program.