Not sure if it is safe but it is not uncommon. Several Cisco routers and other network equipment for that matter use Rj45 connectors for (serial) console ports. So I guess it is not uncharted territory. Combining them together is not as common though, in fact never seen that before. But I have to wonder if it would work as expected. The cables are twisted for a reason after all. I think it would be very likely that the signals would cause interference on eachother. If it is enough interference to impede the working of the ethernet and/or serial link is hard to say.

I would say if you do it, just use 2 RJ45's, that to me sounds like a better option. There is plenty of faceplates and brackets around that hold 2 of these for network-cabling-infrastructures. You could make it look fairly neat and professional with these.

Cheers,
Hans


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