Omron CJ1W-ETN21 Precautions in Using Socket Services, UDP and TCP Socket Services

Precautions in Using Socket Services Section 6-9

6-9 Precautions in Using Socket Services

6-9-1 UDP and TCP Socket Services

•If a short response monitor time is specified in CMND(490) control data and the Ethernet Unit is operating under a high load, a result may be stored even if the response code indicates a time-out. If this occurs, increase the monitor time specified with CMND(490).

•The socket status area in the CIO Area is zeroed when the PLC’s operat- ing mode is changed (e.g., from PROGRAM to RUN). The actual Ethernet Unit socket status, however, will remain unchanged after the socket status area is zeroed. To avoid this problem, use the IOM Hold setting in the PLC Setup. Refer to the PLC’s operation manuals for details on settings.

•The Results Storage Error Flag will turn ON in the socket status to indi- cate that the specified Results Storage Area does not exist in the PLC. Correct the user program.

•Communications time may increase if multiple Ethernet Unit functions are used simultaneously or due to the contents of the user program.

•Communications efficiency may decrease due to high communications loads on the network.

•All data is flushed from the socket’s communications buffer when a socket is closed with the CLOSE REQUEST command. In some cases, the transmit data for the SEND REQUEST command issued just before the socket was closed may not be sent.

•When sockets are open, the Ethernet Unit provides a 4,096-byte buffer for each TCP socket and 9,016-byte buffer for each UDP socket to allow data to be received at any time. These buffers are shared by all open sockets. Receive data will be discarded for a socket if the buffer becomes full. The user application must therefore issue RECEIVE REQUEST commands frequently enough to prevent the internal buffers from becoming full.

6-9-2 UDP Socket Service

•The UDP socket sets a broadcast address for the remote node address to broadcast data to all nodes of the network simultaneously. The maximum length of broadcast data is 1,472 bytes. Data in multiple fragments (over 1,473 bytes for a UDP socket) cannot be broadcast.

•The UDP socket does not check the transmitted data to ensure communi- cations reliability. To increase communication reliability, communications checks and retries must be included in the user application program.

6-9-3 TCP Socket Service

•If the TCP socket of the remote node closes (the connection is broken) during communications, the TCP socket at the local node must also be closed. The communications Results Storage Area can be used to check if the connection has been broken. Close the local socket immediately after detecting that the remote TCP socket has closed. The following situ- ations indicate that the remote socket has closed.

TCP Receive Results Storage Area:

Response code = 004B (error at remote node)

TCP Send Results Storage Area:

Response code = 0020 (connection broken with remote socket during transmission)

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