I recently had the need to send AT commands to a USB modem while it was connected to a Mac using a serial interface. With no ‘terminal’ type application immediately obvious (perhaps I’m wrong?) I found this cunning use of screen to achieve the same results.

In my example, the serial interface is /dev/tty.wwan connecting at 9600 baud.

screen /dev/tty.wwan 9600

To quit screen use Control-A and then Control-\

Other available options:

cs8 or cs7

Specify the transmission of eight (or seven) bits per byte

ixon or -ixon

Enables (or disables) software  flow-control  (CTRL-S/CTRL-Q) for sending data

ixoff or -ixon

Enables  (or  disables)  software  flow-control for receiving data

istrip or -istrip

Clear (or keep) the eight bit in each received byte