If you have to upload firmware to a router or a switch with xmodem, get the package ⇒lrzsz-0.12.20.tar.gz.

First, install Xcode from the App Store.

Then compile the package and install it.

tar xvzf lrzsz-0.12.20.tar.gz
cd lrzsz-0.12.20
./configure --disable-nls && sudo make install

Next, start a console session with screen

screen /dev/tty.Keyserial1 9600

When you are asked from the program to upload the firmware using xmodem, do:

Press ctrl-a
:exec !! lsx -b -X /path/srw2016-24-10086.ros

The upload will start.

Don’t be surprised. This software is very old and the upload takes forever.