UPS devices  > Best Power pinouts
APCUPSD Voltage Signalling Simple-Custom Cable for Best Power Fortress Rackmount 750 VA, 1050 VA, 1425 VA, 1800 VA, and 2250 VA

You do not have this cable unless you built it yourself. The Simple-Custom cable is not an Best Power product.

For smart UPSes (such as Best Power Fortress Rackmount) using voltage signalling, if you are going to build your own cable, we recommend to make the cable designed by the apcupsd team as follows:

Pin Number
Pin Name
Direction Computer
Pin Number
Pin Name
n/a Shield -- n/a Shield Frame/Case Ground
4 Signal Ground (Common) -- 5 GND Ground
5 Normally Open Low-Battery-Alarm Contact --> 8 CTS Low Battery
7 Remote Shutdown <--   ***  
8 Normally Closed On-Battery Contact --> 1 DCD On Battery
  *** <-- 7 RTS Kill UPS Power
  **** <-- 4 DTR DTR set to +5V by Apcupsd

* -- look please Best Power Fortress Rackmount pinouts.

** -- look please APCUPSD User Manual | Cables | Simple-Custom Voltage-Signalling Cable for dumb UPSes at

*** -- computer pin 7 (RTS) is connected to base NPN transistor 2N2222 via the resistor 4k7...10k (look please below).

**** -- computer pin 4 (DTR) is connected to computer pin 1 (DCD) via the resistor 4k7...10k, and also it is connected to computer pin 8 (CTS) via the resistor 4k7...10k.

UPS (DB9M)                                       IBM PC (DB9F)
-----------                                      -------------
PWR_FAIL 8 <-------------------------------------+-----< 1 DCD
                                             R4 [#] R1
Shutdown 7 <------------------------------+-[##]-+-----< 4 DTR
                                          |     [#] R2
LOW_BATT 5 <------------------------------|------+-----< 8 CTS
                        R1-R4 4k7...10k   k\ b    R3
                        VT1 2N2222 (NPN)    -----[##]--< 7 RTS
Common   4 <------------------------------+------------< 5 GND
Shield     x-------------------------------------------x Shield

When using this cable with apcupsd specify the following in apcupsd.conf:

DEVICE /dev/ttyS0 (or whatever your serial port is)


9 pin D-SUB male connector layout
9 pin D-SUB male connector
at the UPS towards
9 pin D-SUB female connector layout
9 pin D-SUB female connector
at the Computer towards
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Best Power Fortress Rackmount APCUPSD Simple-Custom Cable pinout diagram
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