Adding the expansion lid to Maxon’s SD-125EL RF link module provides the benefit of installing CTCSS modules, modem boards, or other option boards. Take a look and see how they can extend your communication possibilities Installing the (optional) Maxon CTCSS Encoder / Decoder (SD-010) for the SD-125E Series Increases privacy during audio applications and operates on all 38 EIA standard CTCSS tones.