B.4 Uninterruptible Power Supplies

Some computer systems need to run reliably, unattended, even in the presense of transient AC power outages; network servers are an obvious example. Many of these are connected to Uninterruptible Power Supplies (UPS).

For control reasons, some of these UPS devices are connected to the dependent PC through a serial port. Unfortunately, it is common to connect these devices to the standard serial control leads in non-standard ways. The effect is that in some cases, the Serial enumerator could shut down the power in the UPS under unusual circumstances.

Since the control ports on these UPS devices are not standard serial, the operation of the Enumerator might not be benign, in which case some precaution needs to be taken. There are several examples; here is a non-exclusive list:

the user should not cold-start the system while dependent on UPS battery power.
to use means embedded in Windows 95 to lock out the Enumerator from the port.
to use an adaptor cable wired differently (e.g. don't connect DTR to a shut off relay).
to use an adaptor cable that filters DTR (e.g. short pulses on DTR are ignored).
to use a new adaptor cable with embedded Plug and Play response logic embedded in it.