In the parameter setting of the Ethernet module in the system definition, set the 'Server FTP command watch timer value' to 0. The port is not closed even if it has not been accessed from the communication partner for a certain period of time and remains open until a port close command is received from the communication partner.
* Default value of this parameter is 3000 (in 0.1-second steps) = 300 seconds.
For details, refer to '4-2 Definition of Module Operations' in the user's manual 'Ethernet Interface Module' (FEH259).