Newsgroups: comp.sys.ibm.pc.hardware.networking,comp.os.linux.networking
Subject: Re: One-way (unidirectional) ethernet cable
References: 
"Emilio A. Lazo Z."  writes:

> HI!
> 
> Please, anyone knows how to build an one-way straight ethernet cable?,
> i.e. a CAT5 RJ-45 cable with only one direction of flow of data,
> the receive direction.
>
> Frank Sweetser, in comp.os.linux.networking suggest me to search for this
> info in google but I can't find it out...Please Frank, do you know how to
> do this?
> 
> May be a cable with only pins 3 and 6? I've tested it and doesn't works...
> 
> **Note: I know that a communication needs Tx/Rc, but I need the cable**
> 
> Please!,


I do not have any specific plans to do this kind of cable.
If you want this as a really working solution, buy a special
network sniffing adapter (some network anakyzer makes sell those
as accessories, they have four RJ-45 connectors: two for data
going through, and tho to send out the data moving through
adapter on both directions). This will work, but is much
more complicated than just simple cable.

Basically you can thinks that just cut the TX wire could do what
you want. It could work in some very fortunate cases, but
generally it does not work. Ethernet systems need both Tx and Rx
pairs for the sytem to work because of various low level handshakings 
involved in Ethernet communcations. Simplest handshaking is
called "hartbeat", which is used in 10 Mbps Ethernet for the 
HUB to know that there is Ethernet card connected (HUB
does not send anythign to port unless it gets signal from NIC in
other end of cable). 10 Mbps Ethernet card will generally 
receive enything going to Rx pins no matter if Tx pair is connected ot not.
In 100 Mbps is Ethernet more complicated
handshaking is needed (10/100 Mbps speed auto-nagotiation and
half duplex / full duplex auto-nagotiation). 
If auto-nagotiation is disabled something could work or not.
But there is still that HUB/Switch problem (does not
send anythign to port unless detects signal from NIC).

By thw way what is the need fo rthis cable ?
What is intended application ?

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