Mixing : Plug-In and Hardware Inserts : Using Hardware Inserts : Connecting Effects Units Digitally

Connecting Effects Units Digitally
If you want to use the digital inputs and outputs on your Pro Tools system as effects sends and returns to a digital effects device, Pro Tools should be the clock master in most cases. Set your digital effects device to accept an external digital clock so that it synchronizes to Pro Tools.
To set up a digital send to an external device for Avid HDX, Pro Tools|HD, and HD Native systems:
1 Choose Setup > Hardware
2 Select the audio interface from the Peripherals list (HD I/O, HD OMNI, 192 I/O, 192 Digital I/O, 96 I/O, or 96i I/O).
3 Select an audio format and clock format, and set the appropriate channel pair to Digital.
4 Select Internal from the Clock Source pop-up menu.
5 Click OK.
To set up a digital send to an external device from a Pro Tools system with 003, 003 Rack+, 003 Rack, Digi 002, or Digi 002 Rack:
1 Choose Setup > Playback Engine and choose the peripheral.
2 Do one of the following:
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If the external device is connected to the S/PDIF RCA jacks, deselect S/PDIF Mirroring. (When S/PDIF Mirroring is on, digital output at the S/PDIF RCA jacks exactly mirrors the output of analog Channels 1–2.)
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If the external device is connected to the Optical ports, choose a format (most likely S/PDIF for an effects device) from the Optical Format pop-up menu.
3 Choose Internal from the Clock Source pop-up menu.
4 Click OK.
5 Optionally, configure the I/O Setup dialog with new path names for effects routing.
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If you set the Optical Format to S/PDIF, Pro Tools will watch the Optical port for any audio input, and ignore any audio input on the S/PDIF RCA jacks.