Re: Wiring a "read only" cable

[Matt Kettler <mkettler () evi-inc com>]

I think the 14 pin connector you are looking at is AUI, not a 100baseT 
ethernet PHY connection. Perhaps you can point out where this diagram is on 
silicondefense.com?

Also with 100baseT it is impossible to create a read only cable by just 
cutting one or more pins of a twisted pair cable. The PHY layer sends short 
bursts of test signal that are expected to be echoed back in order to 
establish link.

Your best bet is to use an AUI adapter and cut pins on the AUI side. Of 
course, this implies having a 100mbit ethernet card with AUI input (uncommon).

Baring that you can create a "denatured" ethernet cable where the TX+ and 
TX- signals are not on the same twisted pair. This ruins the controlled 
impedance of the pair, and introduces bit errors into the data. If the 
denatured cable is the right length the short test bursts with make it 
through fine but data packets will have errors in them and be dropped. This 
process is not an exact science, but there is a website somewhere detailing 
what can be detailed. I do not have the URL offhand.


-----Original Message-----
> From: Joe Pampel [mailto:joe () ardsley com]
>
> What am I missing here?
>
> Trying to make a read only 100Base-T cable for a sensor and it has 8  pins -
> 4 pairs. So far so good.
> www.silicondefense.com has a schematic showing 14 pins and cutting pins 3
> and 10...
>
> Can you see my confusion?  My understanding of this kind of connector is
> like this:
> from : http://yoda.uvi.edu/InfoTech/rj45.htm
>
> -----------------------------------------------------------------------
> Pin Number Designations
>
>    Color Codes for T568B
> Pin     color  pair  name
> ---     -----  ---- ---------
> 1       wh/or   2   TxData +
> 2       or      2   TxData -
> 3       wh/grn  3   RecvData+
> 4       blu     1
> 5       wh/blu  1
> 6       grn     3   RecvData-
> 7       wh/brn  4
> 8       brn     4
>
> ------------------------------------------------
>
> This would indicate not crimping the Orange pair to pins 1 & 2. And of
> course if you're a wise-guy you put a splitter on the jack and plug an RJ-11
> in and use the middle pair for a POTS line.. but anyhow... ;-)
> Anyone else run into this?
>
> ps: wiring sucks when you're color blind. :-)
>
> - Joe


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