Receive-only ethernet?

Hi, all,

Will an ethernet connection work if it is wired as "receive only"? Or are the transmit signals essential even for one-way communications? By comparison, with an RS-232 port you can unhook the tx line and the port will receive data just fine. Can the same be said for broadcast UDP/IP over an ethernet link if the tx pins are disconnected?


Neil Ferguson

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Neil Ferguson
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UTP ethernet needs to see a device, but it doesn't need to be the one at the other end. With many RS232 devices you need signals like CTS and DCD though, or it won't send. It is common to wire those active.

A powered transceiver or hub will generate link pulses on the transmit lines which can be connected to the receive lines on the transmit only port. Receive only is fine, but transmit only isn't.

-- glen

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glen herrmannsfeldt

A normal UTP Ethernet connection needs both TX and RX pairs to be connected for the data flow to work. To get data from switch it needs to see that there is an Ethernet card on the other end of the link. Wires for both TX and RX are needed to establish the link between the devices.

If you can set the in some way the Ethernet signal source to send data out without it getting link pulses in from other end of link, then receiving should work with many Ethernet cards with just RX pair connected....

There are for example Ethernet signal sniffing adapters (Ethernet TAP) that allow you to monitor both incoming and outgoing data traffic on line. The signals on the existing Ethernet connections is split to the RX inputs of two Ethernet card used for sniffing. You will need to use two Ethernet interfaces to examine both halves of the full-duplex signal.

Generally there is no problem is receing Ethernet signals with only RX pair connected when the signal source sends the packets to this wire pair. The problem is to get that other end to send out that data to line when it does not detect the receiver to be there... The Ethernet devices like hubs and swiches are designed in suwch way that data is sent out only to those ports where there is some active receiver connected (where the hub/switch receives link pulses and/or data packets from).

Here are some plans that could be useful:

How to make a sniffing (receive only) UTP cable

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Construction and Use of a Passive Ethernet Tap
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Tomi Holger Engdahl Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.