modular serial hardware (was Re: a printer oddity)

Pete Turnbull pete at dunnington.plus.com
Sat Apr 12 19:15:44 CDT 2008


On 12/04/2008 14:51, Roy J. Tellason wrote:
 > Someone else wrote:
>> For RJ-45s, the most common arrangement I've seen is the one presently used
>> by Cisco - the center two conductors on the cable are both ground, then
>> flanked by TxD and RxD, then flanked by handshaking lines.  Most of the gear
>> I've seen lately that uses RJ-45s for serial, though, stops with the inner 4
>> pins. 
>>
>> One handy thing about this interconnection scheme is that you don't
>> _need_ to change the shells for null modems (though you can).  You
>> can flip one end of an RJ-45 cable (making it unsuitable for Ethernet
>> use) to make the cable a null modem - the signals are arranged in a
>> "paired" fashion to make that work.
[ ... ]
>> In practice, the flat cables that Cisco ships are assembled inverted
>> compared to "Ethernet" cables, so it matters what you grab when you
>> link up two devices.

Ask Google about "YOST", which is where Cisco and Sun copied the scheme. 
  But be aware that almost every way you could think of to use an RJ45 
for serial has been tried by someone -- and if you want a laugh of the 
"I don't believe it" sort, look up EIA/TIA-561.

> Is that the same as a "crossover" cable for LAN use or different?  I've not 
> really looked at the wiring of those things much.

No, LAN cables are wired with twisted pairs for 1+2, 7+8, 3+6 and 4+5. 
To make an Ethernet crossover, you swap 1+2 with 3+6 but you leave the 
others as-is.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York



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