The S/PDIF connection on the back of your Saffire or Scarlett is a connection that allows two channels of digital audio to be passed along a single unbalanced co-axial connection. This can be used to connect a device with a digital output or input to avoid unnecessary conversion from analogue to digital and vice versa
To send audio to another external device
If you are sending audio from your Saffire or Scarlett to another device you should first connect the S/PDIF output of the interface to the S/PDIF input of the external device - such as a CD recorder.
You should then launch Saffire or Scarlett MixControl in order to route audio to the S/PDIF outputs. To do this, select the desired S/PDIF output (pictured below) and choose what you wish to route to this output using the drop down menu. You can choose one of the six analogue or digital inputs, the twelve DAW outputs, or one of the six Saffire MixControl custom mixes blending a combination of these. Make sure that if the S/PDIF input is connect to the same external device then this is muted to prevent feedback.
You should also select the 'Sync Source' to 'Internal' using Saffire or Scarlett MixControl and refer to the user guide of the external device to set its sync source to external S/PDIF. You should also make sure the sample rate for both the Saffire Pro and the external device are the same. This will set the Saffire or Scarlett as the master clock and the external device as the slave with the clock embedded in the channel code being sent from the Saffire or Scarlett to the external device.
To receive audio from another external device with S/PDIF output
If you are receiving audio from an external device (such as a CD player) you should first connect the S/PDIF output of the external device to the S/PDIF input of the Saffire or Scarlett.
You should then launch Saffire or Scarlett MixControl and set the sync source to SPDIF (shown below). You should also refer to the user guide of your external device to make sure that it is set to its internal clock. Finally, make sure that the sample rate of the two devices are matched. The output from the external device should now appear as the SPDIF Inputs within your DAW. (The Input number will vary depending on which Saffire or Scarlett you have.)