Omron SYSMAC C200HW-PCS01-EV1, Operation Manual

Page: 151 / 261

135
UDP Communications Data is simply sent to the destination. Unlike TCP, the receipt of data is not
checked and data is not re-transmitted. To increase communication reliability,
data resends must be programmed by the user in user applications.
Local node Destination node Transmitted data
ACK (only when processed by application)
10-1-4 Opening TCP Sockets
To achieve highly reliable data communication, TCP establishes a virtual com-
munications circuit between the two nodes before starting data transmissions.
The virtual communications circuit is know as a “connection.”
An open command is executed for a node to establish a connection. The open
method differs depending on whether the node is a client or server. A passive
open method is used to open the node as a server and the active open method is
used to open the node as a client.
Client
TCP
socket
Active
open
Connection
Passive
open
Server
TCP
socket
Client and Server When an application which includes data communications processing between
two nodes is executed, the node which provides the service is known as the
“server” and the node which requests the service is known as the “client.” The
server is booted first and waits for a service request from a client. Data is trans-
mitted only after the client requests the server to establish a connection. If the
TCP protocol is used, this process is carried out automatically at the protocol
level; it is not necessary to run an application program.
Note TCP communications with other TCP sockets will not be possible until the con-
nection for the first socket opened has been closed. For the PC Card Unit, four
sockets are provided for UDP and TCP together. Communications with multiple
UDP sockets is possible, because connections are not set for UDP sockets.
Passive OPEN and Active
OPEN
About Socket Services
Section 10-1