All those PowerPC Macs that came with built-in AAUI and RJ-45 ports were explicitly stated by Apple to be able to use only one port at a time. And 'use', in this context, means have a connection to another device.

The same principle applies to plug-in cards with two or more ports, be they PDS, CS, CS II, or NuBus. There were many LC PDS cards with two ports, many Compact AIOs with PDS cards with two ports, and many NuBus cards with three ports. Often the RJ-45 port wasn't present at all, leaving the field clear for AUI and BNC.

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