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MELSEC-F
COMMUNICATION PROCEDURE5
5 - 9
1) Checking by KeepAlive
This method is used for a connection opened via the TCP/IP protocol.
The Ethernet module performs an existence check by sending an
existence check ACK message to a remote device with which
communication has not been performed for a certain period of time and
waiting to see whether or not a response is received.
(*1)
*1 The connection is cut off if the remote device does not support the
TCP KeepAlive function (response to KeepAlive ACK messages).
2) Checking by PING
This method is used for a connection opened via the TCP/IP or UDP/IP
protocol.
The Ethernet module performs an existence check by sending a PING
command (using the ICMP echo request/response function) to a remote
device with which communication has not been performed for a certain
period of time and waiting to see whether or not a response is received.
(*2)
*2 Note that the Ethernet module automatically returns an echo
response packet when it receives a PING echo request command.
(It sends a response to the received PING command even if the
connection used in the data communication with the remote device
is closed.)
(b) The following actions are taken if a response message cannot be received
(an error is detected) from the remote device.
• The corresponding connection will be forcibly closed (the line is
disconnected).
(*3)
*3 It is possible to reopen it with a user program.
• The open completion signal (the corresponding bit of BFM#20480) is
turned off and, the error code (C035
H) is stored in places such as the open
error code storage area.
Also, the open/close status (BFM #1602 to 1609) value becomes "0000
H".
Example: Assuming the values are set under the condition that the number of
retries is three, the Ethernet module performs target existence check at
the timing shown in the figure below.
(An example of existence check by PING)
Ethernet module
PING
command
PING
command
(Retransmitted)
PING
command
(Retransmitted)
Open completion signal
BFM#20480
Last
communication
Target
existence
check starting
interval timer
value
Target
existence
check interval
timer value
Target
existence
check interval
timer value