The MG90 has 1 RS232 serial port and 2 USB ports. All of the ports can be used for RS232 communications, to a scanner for example, for telemetry purposes, but for GPIO the RS232 port must be used. There are up to 4 GPIO lines that can be configured:

This leaves the other 4 pins for a cut down version of RS232 requiring only the receive, transmit, clear-to-send and ready-to-send lines to operate. A simple Y cable can be built using a Sierra Wireless SKU for a breakout serial cable and 2 female breakout plugs as shown below:

Note that it is VERY important that the ground pin 5 is correctly shared between the 2 female plugs - an extra wire will be needed to go from one female pin 5 to the other. The SKU is 6001095-1 and the details can be found in the following file:

The male end plugs in to the MG90 RS232 serial port while one female end (RS232) goes to the RS232 device (scanner for example) and the other to the links for the systems to be monitored via the GPIO lines.