From tomj@fido.wps.com Fri Feb 11 10:38:14 1994
Received: from fido.wps.com by fnord.tlg.org (8.3/wps.com-hackery)
	id KAA13418; Fri, 11 Feb 1994 10:38:09 -0800
Received: by fido.wps.com (5.67/wps.com-hackery)
	id AA02048; Fri, 11 Feb 94 10:41:29 -0800
From: tomj@wps.com (Tom Jennings)
Message-Id: <9402111841.AA02048@wps.com>
Subject: 56K info!
To: pozar@kumr.lns.com (Tim Pozar)
Date: Fri, 11 Feb 1994 10:41:28 -0800 (PST)
Cc: jh@nbn.com, randy@psg.com (Randy Bush), gnu@cygnus.com (John Gilmore),
        grossman@cygnus.com (Stu Grossman), tomj@tlg.org,
        tomj@fido.wps.com (Tom Jennings)
X-Mailer: ELM [version 2.4 PL23]
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Content-Length: 1537      
Status: OR

Got the following info from the PacBell installer guy. Turns out, my
line had "the latch" flipped. Duh. I guess it's some sort of test-board
latch, which -- get thi s-- reverses the Rx/Tx clock feed, and closes
("latches") a "relay" within the DSU to force Link Loop.

Fact #1:
	Reversing the Rx/Tx pairs causes the LL light to light.

This guy was really helpful. I told him I had no wayt o sanity-check a
56K line. I drew out what he described for myself, and he even helped
annotate it. This is what I got:


					MAY HAVE THESE SWAPPED -- TEST
                      Zr
	Rx	0---/////---0		to DSU Tx

		^	    ^
		|(-)	    |(-)
		|	    |
	       48v         48v
		|	    |
		|	    |
		v(+)	    v(+)

	Tx	0---/////---0		to DSU Rx
                      Zt		(has 56K clock at Nyquist f)
		|	    |
		V	    V
	     ground      ground


Characterising:

Zr and Zt depend on line length; I don't know what nominal is. Just
measure from Tip to Ring on a pair. [We need to characterize ours for
future reference.] There is no battery Tip/Ring on pairs.

There is 48v battery from Rx to Tx pairs. Negative is on the Rx (telco
view). Check tip to tip, and also tip to ring -- thereby checking for
one open conductor.

The clock is a 56K Nyquist frequency -- what'sat, 2X? So no wonder it
can't be heard on my 'phone. I need to get a real test phone. 

My remaining confusion is POV -- ie. "Rx" to telco means "Tx" to us -- I
think. Easily checked on a good line.


-- 
 Tom Jennings -- tomj@wps.com -- World Power Systems --  San Francisco, Calif.