HDX readers need to synchronize their charge/listen cycles if the antennas are in close proximity (within read range) to each other. The best indication that synchronization is needed is the yellow noise light that starts flashing when another reader in the vicinity is turned on.
The ORSR and ORMR readers are automatically in synchronization if they all have a GNSS signal (Blue LED ON). The precise time reference from the satellites is used to control the charge/listen pulse timings. If the GNSS signal fades, a temperature compensated oscillator will keep accurate time for up to 24 hours after.
Where a GNSS signal is not available, the readers can be connected by CAT5 cable with RJ45 connectors. If the host reader has GNSS, the reference will be shared by others and their blue LED will be on even if their GNSS receivers are not detecting a signal.
If the host reader does not have a GNSS signal, it will still send the unreferenced time (indicated by 'U' after the time in reports) so all readers will be in synchronization.