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I'm designing a small board that needs an ethernet connection. However, the RJ45 jack takes considerable space. I would like to reduce the footprint of the connector to safe some space.

Would it be possible to replace it by a (non-standard) smaller connector, like a micro usb connector or a JST PH connector? Which connectors would be suitable and why (or why not)?

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  • \$\begingroup\$ Are you planning to use the magnetics for the PHY interface? \$\endgroup\$ – Sunnyskyguy EE75 Apr 23 at 21:04
  • \$\begingroup\$ How far do you need to send the Ethernet signal? A lot is going to depend on that. If you're going 5 cm to the other side of the board, or 15 cm to the other side of an enclosure, that's very different vs if you want to go 100m to the other end of the lab. If you aren't going very far, you can get away with a lot more spec violation. \$\endgroup\$ – Glenn Willen Apr 24 at 2:47
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    \$\begingroup\$ Yes. I don't think micro-usb will work (because it only has one pair). I would probably look for something designed so support LVDS signals, because it will have the necessary properties for 100 Ohm twisted pair wiring.I also agree with other answers suggesting that short runs will be much more forgiving. It might be good if you could post a system diagram. \$\endgroup\$ – mkeith Apr 28 at 21:22
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You can use any connector or even solder directly wires on the PCB.

The drawback is that you may not be able to be compliant to Ethernet standard, have EMC issue and/or can't reach 100m. But if it's for debug purpose, usually you don't need all the previous.

This little thing (35x25 mm) feature a Gigabit PHY (KSZ9031) and have a soldered RJ45 cable (2m, 5m or 10m). It is used for debug purpose and have been developped due to very tight space constraints and especially the board needed to be as flat as possible. We were able to stream 400Mbps on 5m cable without any issue. (RGMII is provided through a connector on bottom side)

enter image description here

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Yes, there are already many ways this is done, laptop dongles are one way. HDMI has Ethernet routed in it.

Whatever cable\connector you use needs to be able to handle differential pairs with an impedance of 100Ω

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    \$\begingroup\$ No, HDMI does not have Ethernet routed through it. It requires a dedicated PHY chip to combine Ethernet RX pair and TX pair into a single bidirectional pair at both ends. \$\endgroup\$ – Justme Apr 24 at 5:04
  • \$\begingroup\$ I wasn't quite clear, those statements are independent. \$\endgroup\$ – laptop2d Apr 24 at 5:26
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While using Ethernet, then de facto standard is rj45.
The industrial Ethernet specifies some flat connectors as well, but you probably don't want to pay the higher price for the connectors and special cables, also your other end device requires the special connector then as well.
It is not possible to send Ethernet signal over JST, because of interference and electromagnetic behavior of Ethernet (it is really fast), it is just not up to specifications.
USB 2.0 doesn't have enough cables for that purpose, but you could set up Ethernet on USB (the other end would be USB as well). Lucky exploring.

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  • \$\begingroup\$ USB 3.0 definetely has enough pairs for Fast Ethernet. If the cable can handle the signal speed is an other matter. \$\endgroup\$ – Jeroen3 Apr 24 at 5:44
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IF you're not interested in following any particular standard, just achieving a smaller connector footprint within a system you have control over, then I suggest using a USB-C connector.

With careful allocation of the 4 x Superspeed Differential Pairs, combined with Auto MDI/MDIX switching available in many Ethernet Interfaces, it should be possible to use a standard USB-C cable. The two-fold rotationally-symmetry is not then a problem. Impedance is well controlled, and should be fine for short distances.

A further possibility is additionally using the power conductors in the USB-C Cable for power distribution.

Note that the USB-C Cables are not quite rotationally symmetric pinout-wise.

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