The functionality of the digital ports on Scarlett 18i20 3rd Gen can be changed with the 'Digital I/O Mode' option in the Focusrite Control Settings menu (cog/gear icon).

S/PDIF RCA (Default)

This is the factory default mode, and your Scarlett 18i20 adopts these assignments out of the box.

Use this mode if you need a coaxial S/PDIF audio input, or if you wish to use a coaxial S/PDIF
signal as a clock source. See also: How do I use RCA S/PDIF inputs and outputs?

When the Digital I/O Mode is set to 'S/PDIF RCA', the optical ports will still send and receive ADAT signals (audio and clock). You can use this mode if you are looking to send or receive ADAT signals (audio and clock) at 44.1/48 or 88.2/96 kHz sample rate using a TOSLINK optical cable via the optical ports. See also: Connecting an external preamp to your interface via ADAT

Optical S/PDIF

Select this mode when you are working at 44.1/48 or 88.2/96 kHz sample rates, and you need to send or receive a S/PDIF audio signal via an optical port, using a TOSLINK cable.

You should also use this mode if you want to use an optical S/PDIF signal as a clock source, or send a clock signal to another device via optical S/PDIF.

Dual ADAT

Select this mode when working at a sample rate of 88.2 or 96 kHz (also known as dual-band) and you need more than four ADAT input and/or output channels.

At dual-band sample rates, the maximum number of channels that can be transmitted over a single optical ADAT connection is 4. Dual ADAT mode lets you transmit up to 8 channels in total, i.e. 4 channels for each optical port. See also: Can I connect two ADAT Devices to Scarlett 18i20 3rd Gen?

The functionality of the digital ports on Scarlett 16i16, 18i16, and 18i20 4th Gen can be changed in Focusrite Control 2 (the control app for the Scarlett) under 'Sample rate & clocking'. To do so:

1. In Focusrite Control 2, click the more options … icon (top-right corner).

2. Click on Preferences.

3. Select the Sample rate & clocking tab.

Depending on the interface you have, see the settings in the section below.

On Clarett 4Pre and 8Pre, you can change which digital input port you are using to receive S/PDIF (RCA or Optical) with the “S/PDIF Source” option in the Focusrite Control Settings menu (cog/gear icon).

RCA (Default)

RCA is the default setting. Use this when you need to send an S/PDIF signal to the Clarett using a coaxial (RCA) connection. See this article for reference: How do I use RCA S/PDIF inputs and outputs?

The optical ports will still send and receive ADAT signals (audio and clock).

Optical

You can set the S/PDIF Source to Optical to connect to consumer devices with optical S/PDIF outputs, such as a connection to a games console or TV. See this article for reference: Connecting TVs, Consoles, or other device to your interface.

In this mode, the optical ports will no longer send and receive ADAT signals (audio and clock).