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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.

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    \$\begingroup\$ There is not even enough to understand what you mean by Ethernet and what kind of switch IC you have and what interfaces it has left. If you have an IC make/model then say it. \$\endgroup\$
    – Justme
    Nov 14, 2023 at 17:26
  • \$\begingroup\$ i have this ethernet switch which i took from a reference design: KSZ8895RQXI , i receive into this switch 3 ethernet connections, from 3 different cpu's. the output of the switch then converted to usb, which goes to another system. the converter is LAN9500A. my question is, am i missing something? is it all i need? i simply need a switch for 4 ports , and a conversion to usb at the output port. \$\endgroup\$ Nov 14, 2023 at 18:18
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    \$\begingroup\$ Technically, it makes no sense to use copper PHY interface ports between onboard links, which may require one or two transformers anyway, just like with cable. \$\endgroup\$
    – Justme
    Nov 14, 2023 at 18:50
  • \$\begingroup\$ so it seems ok to you use these chips for my requirements ? \$\endgroup\$ Nov 14, 2023 at 20:04
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    \$\begingroup\$ This question would greatly benefit from a block diagram or a schematic, in it's current format it's too vauge to answer. Add other detials like any key IC's you're using. Edit your question to include this and any other information. \$\endgroup\$
    – LordTeddy
    Nov 14, 2023 at 21:56

2 Answers 2

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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.

A section of page 1 of the datasheet linked above, claiming that it has "Five MACs and Five PHYs that are Fully Compliant with the IEEE 802.3u Standard"

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.

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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.

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