Omron CJ1W-EIP21 用户手册

The FINS/UDP method is a FINS communications method that uses the UDP/
IP protocol. UDP/IP is a connectionless communications protocol. When a
message is sent from one node to another, the two nodes have an equal rela-
tionship and there is no clear connection. If using TCP is like making a tele-
phone call, then UDP is more like delivering a memo by hand. Although the
UDP protocol is fast, data communications are less reliable than with TCP.

In particular, when sending large amounts of data involving significant routing,
the user must program measures, such as retries, into applications in order to
improve reliability.

The FINS/UDP method has the following features:

• Because FINS/UDP is a connectionless protocol, there is no limit to the

number of corrections.

• FINS/UDP can be used for broadcasting.

• When data is sent via an IP network with multiple layers (such as the

Internet), communications reliability drops.

The following diagram shows the structure of a UDP packet used for sending
and receiving data on an Ethernet network.

As the diagram shows, a nested structure is used with the FINS/UDP method,
i.e., Ethernet Ver. 2, IP frame, UDP frame, and FINS frame. A UDP data sec-
tion (FINS frame) that exceeds 1,472 bytes is split into packets for transmis-
sion. The split UDP data is then joined automatically at the UDP/IP protocol
layer. There is normally no need to pay attention at the application layer to this
split, but it may not be possible to send 1,472-byte UDP packets over an IP
network with multiple layers. When using the FINS communications service in
a system such as this, select the FINS/TCP method.

Node

Node

Data transmission 1

Data transmission 2

Data transmission 3

Data is sent in one direction, with no 
confirmation of whether the data was 
received. Because there are few procedures 
involved, data can be sent at high speed but 
with less reliability than with TCP.

Ethernet Ver. 2

IP

UDP

FINS frame

FCS

UDP packet