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FINS/TCP Method Section 8-3

8-3 FINS/TCP Method

8-3-1 Overview

FINS/TCP Features

The FINS/TCP method is a FINS communications method that uses the TCP/

IP protocol. TCP/IP is a connection-type communications protocol. Before a

message is sent from one node to another, it is necessary to establish a vir-

tual circuit, i.e., a connection. Once a connection has been established, com-

munications are quite reliable. The arrival of data that is sent via the

connection is confirmed by an acknowledgement (ACK) response, and retries

are executed automatically as required.

When FINS/TCP is used, it must be determined which node is the server and

which nodes are the clients.

For communications between a personal computer and a PLC, the computer

should normally be set as the client and the PLC as the server. For communi-

cations between two PLCs, either one can be set as the client and the other

as the server.

Compared to the FINS/UDP method, the FINS/TCP method has the following

characteristics.

• Data transmission is more reliable, due to factors such as retry process-

ing at the TCP/IP layer. The FINS/TCP method is thus better suited to

dealing with communications errors in an IP network that spans several

layers.

• Remote clients can be restricted by means of settings at the server (i.e.,

the server can be protected from access by non-specified IP addresses).

• Broadcasting cannot be used.

• TCP/IP has various retry procedures, and this tends to lower its perfor-

mance in comparison with UDP/IP.

• There is a limit to the number of connections that can be made (i.e., 16

connections maximum), and any given node can communicate only with

up to 16 other nodes at a time.

Node

(Client)

Data transmission 1

Request to establish a connection

Node

(Server)

Notification of establishing

connection

Acknowledgement

An acknowledgement is received whenever a

connection is established or data is sent, so

transmissions are more reliable but somewhat slower.

Connection

established

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