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I'm having a custom cable made that will only use 2 of the 4 pairs for Ethernet. I'm connecting a 10/100 Mbps device to a network with other GigE devices.

Pinout is clear: enter image description here

However, I have seen one similar adapter online that places a resistor across the unused pairs - (See second device).

Are those resistors necessary? If so, recommend resistance? Any further advice making 4-pair GigE and 2-pair 10/100 devices play nice?

Thanks

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  • \$\begingroup\$ I am having a hard time seeing why there would be resistors on the unused pairs. Maybe for radiated emission purposes? In any event 100 Ohms should be about right if you did use the resistors. You may also be able to use cable with only two pairs. As far as 100 Mbit and 1 Gbit devices playing nice, this depends on the switch or router you use. The switch or router must be able to separately negotiate different speeds on every port. Some switches will drop down to 100 Mbit if there is even one device on the switch that is low speed. \$\endgroup\$ – mkeith Jan 19 at 17:38
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The resistors are there to terminate the unused pairs absorbing any signals they may have picked up.

They're usually added to improve EMI, and prevent signals that have crosstalked from the other pairs bouncing back and forth in the un-useds pairs potentially degrading the signals in the pairs that you are using.

You can happily live without them, especially if your cable run (including any structured wiring you use) is short.

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No, those resistors are not needed. 2 pairs is all that is needed for 100Base-TX.

If you look at Siemens Profinet Cable, which is basically Ethernet, you'll see 2-pair cable used. It works with regular ethernet devices as well.

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