0 ms: TCP connection will not be closed if the
serial line becomes idle.
1-65535 ms: The NPort 6000 series device server
automatically closes the TCP connection if there is
no serial data activity for the given time. After
the connection is closed, the NPort 6000 series
device server starts listening for another
TCP connection.
Note: NPort 6150, 6250, 6250-S-SC, 6250-M-SC
do not support write access.