I'm designing a PCB and it requires me to receive three Ethernet ports, and output one of them to the forth port. Then it goes from Ethernet to a USB converter, and goes to the system.
My question is, do I need a PHY? The systems entering into the three ports have PHY on them. Going to magnetics, and then to the switch. But, when one port goes the the output port, do I need to do anything else? I'm no expert in Ethernet, this is the first time I've dealt with a switch for Ethernet.
It looks to me like you have a PHY whether you need one or not: according to its datasheet, the KSZ8895_QX has an integrated 10/100 PHY on each port.
Note that an Ethernet switch is not just a normal electromechanical-like switch like you seem to be thinking. It doesn't just directly connect one input to one output; it reads the input packet and reproduces it on the output. This means a PHY on each port is necessary. If it did just directly connect one input to one output (as you could do, for example, with a relay matrix--though it'd be hard to maintain signal integrity, especially at 100 Mbps, through a bunch of relays), you wouldn't necessarily need a PHY, but this would sacrifice any ability to route packets in real-time to the appropriate output.
You can always put a pulse transformer down and just connect the two 10/100 interfaces to each other.
Some devices allow you to connect 10/100 ports together in an embedded system using dc blocking caps (instead of a transformer). You would need to research this to make sure both devices would be happy with this, possibly you could ask Microchip about it.
