Should be input. The comms are both bi-directional. The master speaker talks 5C comms on the input and speaker comms on the output. Slave speakers talk speaker comms on both input and output.

Lounge: G61RSL, DSP3200, DSP3300, DSW, HD621+Vertex2, SBT, Kef Ci160.
Office: DSP33, D1500, SBT
Archive: 568.2m, 562v3, 562v, DSP3100