USB port to test AT commands


I am testing the AT commands on EC25 modem through SecureCRT software using telnet protocol.
In SecureCRT software there are 4 USB ports are showing ttyUSB0, ttyUSB1, ttyUSB2 and ttyUSB3.
Which port I should use to test AT commands please help.

/dev/ttyUSB2 is the AT command port.

/dev/ttyUSB3 is the data port. This will also respond to AT commands, but is usually used for establishing a PPP data session using the ATD command.

OK then I should use /dev/ttyUSB2 port to test AT commands. Right?

Correct. But /dev/ttyUSB3 should also respond correctly to some (perhaps all) AT commands.

OK thanks.

I have one more issue observed while testing AT commands.

While testing AT commands when I send any command sometimes I have not received its response. I need to disconnect the device and again need to re-connect the device then I get the response.

Is this happening with any particular AT commands? Or only after running a particular command?

The simplest AT command is AT, and it does nothing except elicit an “OK” response.

Its happening with any random AT commands.

For example I sent “AT+QSTK?” its response should be “+QSTK: 1,1,300” if STK is enabled.

But in this case if I sent above AT command its response is not printing on screen when I disconnect the device and connect it again and sent the same command the correct response gets print on screen.