From OpenIPPBX Wiki

RS232 Module

This is the small green module shipped together with IP02, IP04 and IP08. You will need this module to upgrade the kernel of ip02/04/08 or for RS232 connection. It is very useful to developer to debug the firmware for the IP0X.

RS232 Cable

A standard series cable easy to be found in the market. It is used to connect to the IP0x to computer to debug the firmware or upgrade u-boot/kernel.