Omron SYSMAC C200HW-PCS01-EV1, Operation Manual

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5. Another receive request to receive data must be made before the remaining
data, Data B, is sent.
Data B
Transmission
user program PC Card Unit
1. Send request
Source data
1,982 bytes
1,024 bytes
Data A
Data B
958 bytes
Data A
Data B
Data A
2.
3.
Receive node Receiving user program
4. First receive request
1,982 bytes
1,982 bytes
958 bytes
5. Second receive request
1,024 bytes
When using TCP protocol, the fragmented data is passed to the user program.
Therefore, the receiving user program must be able to evaluate the end of the
data transmission, and repeatedly send receive requests until all data has been
received. The receive request is sent twice in the example shown above, but the
data would be even more fragmented if a router were included in the commu-
nications path, and the number of receive requests would need to be increased
accordingly.
When making the receive request, it is not necessary to specify the same data
length as the transmitted data length. For example, if the length setting is shorter
than the actual length of the data, all the data can be received by repeating the
receive requests.
When Using UDP An example of fragmentation and transmission of data using the UDP is shown
in the following illustration.
1, 2, 3... 1. The transmission user program sends a request to send 1,982 bytes of data.
2. The PC Card Unit fragments the transmission data into Data A with 1,472
bytes and Data B with 510 bytes.
3. Data A and Data B are sent consecutively.
4. When the receiving user program sends a request to receive 1,982 bytes of
data, Data A and Data B are linked to restore the original data which is
passed to the user program.
Source data
1,982 bytes
Transmission
user program
1. Send request
Data A
Data B
1,472 bytes
510 bytes
PC Card Unit Receive node Receiving user
program
4. First receive request
1,982 bytes
Data A Data B
3. 2.
As shown above, the UDP protocol handles data communications as data-
grams, so that the transmitted data is restored to the original data before being
passed to the user program. Consequently, if the data length in the receive
request is set to the length of the transmitted data, the entire data can be
received using a single receive data request.
About Socket Services
Section 10-1