Sync channel doesn't work.Make sure that both MMV muxes have firmware version 4.06 or greater, which is the first firmware revision to support this feature. This feature is just for point to point applications.
Other possible causes:
1. Composite link cables are improperly pinned for this application.
2. The V.35 or V.24 shunt is set improperly for the peripheral device.
3. The clocking isn't set correctly.