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5
FUNCTIONS
5.4 KeepAlive Function
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(1) KeepAlive function
When communication with the target device whose TCP connection is open is
not made for a predetermined period of time, a KeepAlive ACK message is sent
from the QJ71MT91 to the target device. The alive status of the target device can
be checked by whether a response message is returned or not to that ACK
message. ( 1)
1: The connection may be disconnected if the target device does not support
the TCP KeepAlive function (unable to respond to the KeepAlive ACK
message).
(2) To use KeepAlive function
To use the KeepAlive function, set the KeepAlive of the basic parameter to
"Used". (Default: Used)
The following basic parameters are available for the KeepAlive function.
KeepAlive start timer value
KeepAlive interval timer value
KeepAlive resend count
Change the default values set to the basic parameters as necessary.
Refer to Section 7.2 for details of the basic parameters.
(3) Operation of KeepAlive function
(a)
The QJ71MT91 starts the KeepAlive start timer when it receives the last
message from the target device whose TCP connection is open.
(b)
The QJ71MT91 sends the KeepAlive message for alive check to the target
device and starts the KeepAlive interval timer when the KeepAlive start
timer expires.
(c)
The QJ71MT91 resends the KeepAlive message for alive check to the
target device when ACK is not returned from the target device and the
KeepAlive interval timer expires.
The QJ71MT91 sends the KeepAlive message for alive check by the
number of times set as the KeepAlive resend count until it receives ACK
from the target device.
(d)
An alive check error occurs if the QJ71MT91 cannot receive ACK from the
target device after it has sent the KeepAlive message for alive check by the
number of times set as the KeepAlive resend count.
At that time, the QJ71MT91 forcibly closes the TCP connection and issues
an error code.
MELSEC-Q
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