About digital sync between audio interface and ext. tools

[Nodular]

Hello,
it might be normal, but I would like to have an opinion.

I'm trying to connect my firewire audio interface ( Edirol FA-101 ) with a digital equalizer ( Behringer DEQ2496 ).
Since DEQ2496's analog in/out aren't top of the art, I would like to use its digital interface.

Both units have got optical in/out that appear to work just fine, but when I make the connection, both units can't sync to each other and they become silent.

In DEQ2496 I/O menu you can choose the source of the signal you want to work with.
Choices are as follows:
- XLR analog IN 1/2
- Pink Noise generator
- XLR AES/EBU digital IN
- TOS digital IN
- XLR MIC preamp

FA-101 has got:
- 8 TRS analog inputs
- 8 TRS analog outputs
- 1 TOS digital input
- 1 TOS digital output

I'm using Ableton LIVE 8 as DAW.

FA-101 has got a selector for internal digital clock rate and must be set properly while the unit is shut off.
A Sync switch on the front panel of the FA-101 must be activated to use the TOS digital input. If the sync is not found the unit becomes silent and causes DAW instability.

DEQ2496 automatically detects digital sync through its AES/EBU and TOS inputs. If sync is not found the units becomes silent.

I've tested many scenarios and made sure that both units can produce and recognize steady digital clock rates that go from 44.1khz to 96 khz.

Ex. :
DEQ2496 output pink noise at all rates -> FA-101 sync is good
FA-101 dig. input switched off -> DEQ2496 sync is good

The only thing that doesn't work is:
FA-101 TOS out -> DEQ2496 TOS in -> DEQ2496 TOS out -> FA-101 TOS in

Could that be that it's simply not the way it should work ?
Or there's a culprit ?
Who is it then ?


Thanks in advance for any feedback.

ciao.