Newsgroups: comp.dcom.lans.ethernet
Subject: Re: Receive-only ethernet?
References: 
"Neil Ferguson"  writes:

> 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?

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
http://www.geocities.com/samngms/sniffing_cable/

Construction and Use of a Passive Ethernet Tap
http://www.snort.org/docs/tap/


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/