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6.2 EN311 Socket Interface Usage Notes
6
Figure 6-6 Handling of Transmitted/Received Data in the TCP Socket
(8) When the TCP socket interface is used, transmitted data is divided (fragmented) into
units of a size determined when the connection between the two sockets was
established. The upper limit of the fragment size is 1460 bytes. This is the limit of the
length of packets (data units flowing on the network) transmitted over the
transmission line (10BASE-5/2). See Figure 6-8.
For example, when transmitting 2000-byte data, the event shown in Figure 6-7 can
occur. The largest segment size that can be received in this case is 1460 bytes.
(a) The sending S controller user program requests a transmission of 2000 bytes.
(b) The transmitted data is divided into two units called segments, A (with 1460
bytes) and B (with 540 bytes) by the sending EN311.
(c) A and B are put onto the transmission line in order.
(d) The data that arrived at the receiving EN311 is acquired by a receive request for
2000 bytes from the S controller, and stored in the user specified register area
(see section 6.4.7, "TCP receive request (using the URCV_T instruction)").
The amount of data that can be acquired by a TCP receive request differs
depending on the timing with which the S controller issues the receive request
and the timing with which the data arrives at the EN311.
• If data has not yet arrived at the receiving side EN311 when the receive
request is issued:
Æ When segment A arrives at the EN311, only segment A will be passed to
the requester.
• If only segment A has arrived at the receive side EN311 when the receive
request is issued:
Æ Only segment A will be passed to the requester.
• If segments A and B have arrived at the receive side EN311 when the receive
request is issued:
Æ A data item consisting of A and B will be passed to the requester.
(e) If B was not received, another receive request can be issued from the S controller
to acquire B.
EN311 TCP socket S controller
Consecutive data (up to 5840 bytes)
Receive request
Data received so far
Max. 2000 bytes
Transmitted
data
Data received by the S controller
Receive response
Transferred data
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