From: Dave Angus <dave_angus@hotmail.com>
Subject: APC 940-0020B cable guts

Hello:

I just got my hands on one of these cables, and by nondestructive
testing, I ~think~ I know what's inside.

Definitely:

PC	UPS
4   to   1
5   to   4 and 9
8   to   2

Very very probably:

3 (PC) through 4k7 resistor to pin 1 (PC) which also goes to the
collector of PNP transistor.
7 (PC) to 47k and emitter of transistor.  Other end of resistor to base
of transistor, and from the base a third 47 k resistor goes to pin 5
(UPS)

Please bear in mind that this schematic represents what I think is
inside the cable, but that I haven't actually built up one of these and
tested with a PC and UPS.  But I'd bet a 6-pack that it's correct.  

Here's a text version of the schematic:

  PC                           APC UPS
DB9-F                           DB9-M

pin 7 o--+---------+
         |         |
         |         /
         |         \  47k
         \         /
          \ |      \
        E  >| B    |    47k
            |------+--/\/\/\--o pin 5
        C  /|
          / |  2N2907 or similar
         |
pin 1 o--+        
         |
         \
         /  4k7
         \
         /
         |
pin 3 o--+

pin 4 o--------------------o pin 1

pin 5 o-----------------+--o pin 4
                        |
                        +--o pin 9

pin 8 o--------------------o pin 2

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