With many other audio interfaces, you can just use internal clock and plug in your outboard FX through the S/PDIF port. However, in order to use a digital FX box with Mbox 2 Pro, you have to ensure that the FX box is the S/PDIF clock master.

With the Mbox 2 Pro, you must use S/PDIF sync mode in order to hear audio on the S/PDIF input. So if your digital FX box is slaved to Mbox 2 Pro57;s S/PDIF output, and Mbox 2 Pro is slaved to the FX57;s S/PDIF output, you57;ve just created a feedback loop in the clock signal, and Mbox 2 Pro will fail to lock to sync.

The only way to break this feedback loop is to set up the the digital FX box as the clock master, so it is not deriving its clock from the Mbox 2 Pro.

**** NOTE : Because the MBox2Pro cannot clock to external devices at higher sample rates (88.2k or 96k), using S/PDIF for an external effects loop can only performed at lower sample rates (44.1k, 48k). ****