When setting up external communication with devices via RS232 on macOS you need to know which COM port to send / receive commands on.

To do this we need to check the embed application settings by opening the settings panel, entering the password and click on 'Settings'.

Then scroll down to the Serial/RS232 devices section and you'll see the available COM ports:

The RS232 device we are looking for is the 'USB-to-Serial' device, so now we know it's on '/dev/tty.usbserial-1410'. You can then use that in your Ext Comms setup and intput it into the Com Port field as:

/dev/tty.usbserial-1410

Further reading: How to set up an RS232 connection with a macOS device and embed signage