[Tfug] Harmless musing

[Bexley Hall]

Greetings!

--- George Cohn <gwcohn at simplybits.net> wrote:

> Bexley Hall wrote:
> > --- 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.

OK, that makes sense.  *But*, there is nothing that
prevented The Powers That Be from declaring "All
Network Devices Shall Have IDENTICAL Pinouts and
Interconnection Of Any Two Shall Be By Way Of A
CrossOver Cable"...  (?)

> 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.

Yup.  I still have a few cables from the Alpha I had.

And, since the connector *relies* on that little
"tit" to hold the connector *in* the mating
receptacle, you can't file one off of a "wrong"
style connector and use it in its place (in a jam)
 
> Over the years I've crimped more of all types of
> these than I want to admit!  ;-)

For me, the bane has been IDC ribbon cables  :<

--don


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