The Siemens 6GK1243-0SA20 SINEC CP 2430 is a communications processor that adds two AS-Interface masters to a SIMATIC S5-115U, -135U, or -155U rack. This is the module you need if you're running an older S5 system and want to bring AS-Interface sensors and actuators onto the bus without swapping out the whole PLC. Two masters on a single module means you can double the number of AS-Interface slaves without needing a second slot. That's useful when the S5 rack is already tight.
The module ships with the required connector for the two AS-Interface masters, saving you a separate part order. It mounts directly in the S5 rack and connects via the backplane.