Need to have the serial port configured correctly and all before plugging in that cable -- eg. by having apcupsd all set up and running on the computer. Not a bad thing to do regardless, as it'll enable controlled shutdown/restart on battery exhaustion during power failures too.
But... But... How do you ensure apcupsd is running when you turn on the UPS - if the computer on which apcupsd is running is powered by the UPS?
My (now fuzzy) recollection is that the 'serial port' on older APC units accommodated both serial communications and direct wire voltage level signaling.
Some pins were used for simple RS-232 data (send and receive), other pins were used with DC voltages to signal 'on battery power' and 'turn the UPS off', and perhaps some other functions. I seem to recall that there was no hardware flow control or handshaking for the serial data.
The special APC cable separated the functions so the 'serial data' mode would work with a normal PC serial port. I do not recall if there was a different APC cable for using the voltage signal method.
APC tweaked the port configuration over the years, so not all models had the exact same port configuration. And I think there were different versions of the serial cable too.
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