Precautions - Mitsubishi MELSEC L Series User Manual

3.5

Precautions

(1) KeepAlive check
When the protocol is set to TCP, KeepAlive check is performed. (Checking for a response to a KeepAlive ACK
message)
An alive check message is sent five seconds after reception of the last message from the connected device to
check if the device returns a response or not. If no response is received, the alive check message will be resent at
intervals of five seconds. When no response is received for 45 seconds, the connected device is regarded as
non-existent and the connection is disconnected. If the connected device does not support the TCP KeepAlive
function, the connection may be disconnected.
(2) Connections exceeding the setting
Do not exceed the number of connections set for the open settings parameters. Establishing too many TCP
connections from a personal computer may cause the following states, depending on the application.
• Time before timeout error detection is increased.
• An unexpected timeout error occurs in any of the communicating devices.
(3) Retransmission on TCP connection
If no ACK response is returned from the other end of a TCP connection, the ACK will be resent six times, starting
in 0.3 seconds after the first transmission, and then 0.6, 1.2, 2.4, 4.8, and 9.6 seconds. When no TCP ACK
response is returned within 19.2 seconds after the last retransmission, the device is regarded as faulty and the
connection is disconnected. (As a result, the connection is disconnected in total of 38.1 seconds.)
(4) MELSOFT connection over UDP
For UDP communications with multiple MELSOFT devices, set the same number of connections as that of the
connected MELSOFT devices on the screen displayed from PLC parameter.
MELSOFT devices
Set the same number
of protocols as that of
MELSOFT devices.
26
Ethernet
Hub
MELSOFT devices MELSOFT devices