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.

Wireless synchronization is use on backpack and proximity readers that are to be used around stationary systems  With wireless synchronization enabled the reader will change the on/off cycle if it hears a charge pulse during the listen time.  Once it cannot hear a charge pulse it assumes that it is in synch.

Wired synchronization is used when stationary readers are placed near each other.  A cable is run between all the readers to send a coordination signal to turn readers on and off simultaneously.

Simple Wired Synch is a cabled version of the wireless synch method.  Readers listen to each other and automatically adjust their times to get in step.  When using Wired Synch mode, one reader should be set to not synch and it will become the master signal for the other readers to match.

Wired Master/Slave synchronization is the most reliable method.  One reader is declared a master while all others are configured as slaves who will only trigger when the master goes on.  The drawback to this method is that when the master is turned off, all readers will stop running too.