From what documentation I could find, the UPS should be (is!) supported by Linux and ought to work with apcupsd. I looked for specific problems such as the infamous Microlink protocol, and found none.

I have found a feedback from a guy in UK that reports using this very model on a not-too-different OS version (his OpenSuSE 12.1, mine 12.3 x86_64).

The USB port is detected, lsusb reports

Bus 002 Device 003: ID 051d:0002 American Power Conversion Uninterruptible Power Supply

and lsusb -v -s002:003 confirms and expands:

Bus 002 Device 003: ID 051d:0002 American Power Conversion Uninterruptible Power Supply
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0
  bDeviceProtocol         0
  bMaxPacketSize0        64
  idVendor           0x051d American Power Conversion
  idProduct          0x0002 Uninterruptible Power Supply
  bcdDevice            0.90
  iManufacturer           1 American Power Conversion
  iProduct                2 Back-UPS RS 900G FW:879.L4 .I USB FW:L4
  bNumConfigurations      1
  Configuration Descriptor:
    [...]
    Interface Descriptor:
      [...]
      bInterfaceClass         3 Human Interface Device
      bInterfaceSubClass      0 No Subclass
      bInterfaceProtocol      0 None
      iInterface              0
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.00
          bCountryCode           33 US
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength    1134
         Report Descriptors:
           ** UNAVAILABLE **
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval             100
Device Status:     0x0000
  (Bus Powered)

The kernel recognizes this and duly sets up

crw------- 1 root root 180, 96 Nov  4 16:11 /dev/usb/hiddev0

As far as I know, everything is as it should be.

I have put the standard configuration in /etc/apcupsd/apcupsd.conf (which is Unix-terminated, ASCII-only, no BOM (just in case))

UPSCABLE usb
UPSTYPE usb
DEVICE

(I have also tried commenting out DEVICE, and setting a device of /dev/puppa results in an access attempt to /dev/puppa, not some /var/lib/dev/puppa or /dev/puppa\r\n).

Yet, what apcaccess tells me is

VERSION  : 3.14.10 (13 September 2011) suse
CABLE    : USB Cable
DRIVER   : USB UPS Driver
UPSMODE  : Stand Alone
STARTTIME: 2013-11-04 16:24:22 +0100
MODEL    :
STATUS   : NOBATT
LINEV    : 000.0 Volts
LOADPCT  :   0.0 Percent Load Capacity
BCHARGE  : 000.0 Percent
TIMELEFT :   0.0 Minutes
MBATTCHG : 5 Percent
MINTIMEL : 3 Minutes
MAXTIME  : 0 Seconds
SENSE    : Low
LOTRANS  : 000.0 Volts
HITRANS  : 000.0 Volts

It doesn't recognize the model, and reports no battery (and no voltage). This confirms that it's not the Microlink problem, or it would report the battery status, if precious little else.

If I disconnect the USB cable, I get an apcupsd message to the effect that communications have been lost; and I get the "communication restored" broadcast too, if I reconnect the cable. apcupsd is monitoring.

So everything tells me that it should work -- only it doesn't. Does anyone spot what I'm missing?