If RC-03 Classics, set the port speed to 10mb / half-duplex, disable autonegotiate.
If RC-03 Coldfire, set the port speed to 10mb / full-duplex, disable autonegotiate.
Also. these devices are susceptible to interference from broadcast traffic. If there are devices on the same LAN that tend to generate high amounts of broadcast packets on the same subnet as the readers (such as IP-Cameras), consider isolating the readers to their own subnet to mitigate this.
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