Connectors - Couple Port in Summing Mixer ToolMix16
Analog Summing Gear ToolMix
Detailed view of the Couple Port, used for linking with other ToolMix / ToolMst devices and of the Master Switch that selects between master and slave operation of the device.
Summing Mixer ToolMix16 XLR Pinning Couple Port  Total View Rear Panel 

ToolMix16 Couple Port