ICP DAS USA M2M-720-A Faq page 17

Step 3: Confirm that the Com Port device is connected with Server
and Client has the same communication settings with
Server's "Operation Mode" page. If it is different, please
break the connection and change the settings then reconnect
the client.
Server's "Operation Mode" page
Step 4: Confirm the Cable connected Server/Client to Com Port
device is wired correctly. If the connection is RS485, the user
can refer to user manual section 2.2.2. If the connection is
RS-232 and the Com Port device is a data circuit-terminating
equipment (DCE), the user just needs to match the signal
names to connect Server/Client to Com Port device, else the
user needs to use a crossover cable to connect.
Some Com Port devices will wait for one of the handshaking
lines to go to the correct level forever. Depending on the
signal state it might sometimes work, other time it might not.
Here, we connect the M2M-720-A and the Com Port device
via handshake looped to avoid the Com Port device waits
handshake line signals, as shown in below.
M2M-720-A FAQ (Version 1.20, Oct/2012) PAGE：17