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If i wanted to do a motherboard that held 16 or 24 raspberry-pi compute modules, could i connect them together using the onboard gig ethernet without using any magnetics? Just by using the gige output from the module going to a commercial switch chip.

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In general, that should work. Here's an example board with 4x CM4 modules and a RTL8367N switching chip: https://github.com/will127534/Miniature-CM4-Cluster

You're going to at least want to AC couple through some capacitors to avoid problems with common mode / RX bias issues between the different PHYs. But if it's on-mobo you don't need to have as much of a fault voltage rating compared to standard Ethernet which IIRC specs 1 kV isolation on the transformers.

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