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Overview of Socket Communications from Ethernet Units Section 6-1
6-1 Overview of Socket Communications from Ethernet Units
6-1-1 What are Sockets?
Sockets are interfaces that allow TCP and UDP protocols to be used directly
from the user program. With personal computers, socket are provided as C
language interface libraries, which allow TCP or UDP protocols to be
programming using library functions. With UNIX computers, socket interfaces
are supported in the form of system calls.
The CS/CJ-series PLCs support the socket service from the user program.
The user program requests service functions either by manipulating Socket
Service Request Switches in the CPU Bus Unit Area in the CIO Area or by
sending FINS commands to the Ethernet Unit by executing CMND(490)
instruction in the ladder diagram.
Socket communications services can be used to transfer arbitrary data
between a PLC and a host computer or between two PLCs. The Ethernet
supports two socket services: a UDP socket service and a TCP socket
service.
■ Using Sockets with the Ethernet Unit
The Ethernet Unit supports up to 16 simultaneous socket connections for the
socket services, 8 each for UDP and TCP sockets.
Socket numbers 1 to 8 are assigned to sockets for both UDP and TCP
sockets. Sockets are managed from the ladder-diagram program by assigning
a socket port for each socket number. The socket port number is assigned
when the socket is opened.
6-1-2 Socket Port Numbers
Port numbers up to 1023 on a UNIX workstation can be used by the
superuser only. Port numbers 0 to 255 are reserved for well-known ports.
Consequently, port numbers 1024 and above should be used for socket
services. The Ethernet Unit does not support port #0.
Some port numbers over 1024 may be reserved on some workstations (for
example, the X-window server is port #6000). Do not use port numbers that
are already reserved for other processes.
The setting status of the UNIX workstation port numbers can be checked in /
etc/services.
UDP socket
port 1
TCP socket
port 65535
Sockets
Socket ports are
assigned to socket
numbers.
UDP socket 1
TCP socket 8
Socket
services
UDP open request