[Tfug] Harmless musing

[George Cohn]

Bexley Hall wrote:
> Hi,
> 
> --- George Cohn <gwcohn at simplybits.net> wrote:
> 
>> Bexley Hall wrote:
>>> So, what "little detail" am I missing that
>> explains
>>> why The Powers That Be opted to wire hubs/switches
>>> one way and "computers" another?

OK, remember, the RJ11 phone connector came first.  It uses the two 
center pins for tip and ring. Came in three sizes, one pair in a four 
pin plug, two pair in a 4 pin plug, and three pair in a 6 pin plug.

Then when they decided to make T1 connectors based on the eight pin 
connector, they used pins 1&2 for one side and 4&5 for the other.  By 
coincidence 4&5 are the center two pins.

RJ45 Data connectors also use a pair for the center pins, 1&2, 3&7, 4&5, 
7&8.

See anything common here?  All four types use the center two pins as a 
pair.

I wonder if it was done this way on purpose for data cable because you 
don't normally use the center pair unless it is POE.  I suspect it had 
something to do with the fact that analog telephony used nominally 48 
volts on that pair and they didn't want a miss plugged cable blowing up 
the data port on a piece of gear.

BTW, remember the DEC MMJ connector?  Modified three pair connector like 
the phone connector except the plastic tab was offset to one side so 
they couldn't be plugged into a phone jack.  Used mostly for DEC serial 
devices, and rarely seen today unless it's some 20 year old legacy gear.

Over the years I've crimped more of all types of these than I want to 
admit!  ;-)

George Cohn