APC Serial Cable
The APC AP7921 Rack PDU allows to remotely switch power plugs on and off. It costs roughly 500$. Too bad if the password gets lost. To recover the lost password, one needs the cable with APC part number 940-0144 which has been delivered with the power switch. This is a serial cable whith an RJ14 connector, what is not very common. Too bad if one cannot find this cable anymore. Asking APC support for advice on where to buy this cable from was answered with:
"Unfortunately, we do not sell this communication cable units and there is no other way to reset the password."
Asking them, that given they do not sell the cable, if they could at least give the pinout such that one can build a matching cable by oneself was replied with:
"We do not have the pinout details for our serial cable I am afraid."
Yea, sure. So what is left, is to figure out the pinout by oneself. For everyone with the same problem the details are given here.
The cable 940-0144 has a RJ14 connector on one side and a female DE9 on the other. RJ14 is the same as RJ25 but only uses 4 pins. Wikipedia RJ14 shows the position of Pin1. The pinout of the AP7921 is shown in the following table:
| RJ25 | RJ14 | Function |
|---|---|---|
| Pin1 | ||
| Pin2 | Pin1 | GND |
| Pin3 | Pin2 | TXD |
| Pin4 | Pin3 | RXD |
| Pin5 | Pin4 | GND |
| Pin6 |
The following table shows the pinout of the DE9 connector. Nullmodem DE9 shows the pin enumeration.
| DE9 | Function |
|---|---|
| Pin2 | RXD |
| Pin3 | TXD |
| Pin5 | GND |
Now just connect the receive pin of the RJ14 to the transmit pin of the DE9, the transmit pin of the RJ14 to the receive pin of the DE9 and any of the ground pins of the RJ14 to the ground pin of the DE9. With this configuration you already have a null-modem cable and can use it to directly connect the PDU to your computer.
The procedure how to reset the password is decribed in the device's manual.
Update: This guide has been confirmed to work also with an AP7868, AP7900 and an AP7931.