this hardware interrupt means that a program or operating system can be informed of a change in state of the RI pin, without requiring the software to constantly "poll" the state of the pin. RI does not correspond to another signal that carries similar information the opposite way. On an external modem the status of the Ring Indicator pin is often coupled to the "AA" (auto answer) light, which flashes if the RI signal has detected a ring. The asserted RI signal follows the ringing pattern closely, which can permit software to detect distinctive ring patterns. The Ring Indicator signal is used by some older uninterruptible power supplies (UPSs) to signal a power failure state to the computer. Certain personal computers can be configured for wake-on-ring, allowing a computer that is suspended to answer a phone call. RTS, CTS, and RTR The Request to Send (RTS) and Clear to Send (CTS) signals were originally defined for use with half-duplex (one direction at a time) modems such as the Bell 202. These modems disable their transmitters when not required and must transmit a synchronization preamble to the receiver when they are re-enabled. The DTE asserts RTS to indicate a desire to transmit to the DCE, and in response the DCE asserts CTS to grant permission, once synchronization with the DCE at the far end is achieved. Such modems are no longer in common use. There is no corresponding signal that the DTE could use to temporarily halt incoming data from the DCE. Thus RS-232's use of the RTS and CTS signals, per the older versions of the standard, is asymmetric. This scheme is also employed in present-day RS-232 to RS-485 converters. RS-485 is a multiple-access bus on which only one device can transmit at a time, a concept that is not provided for in RS-232. The RS-232 device asserts RTS to tell the converter to take control of the RS-485 bus so that the converter, and thus the RS-232 device, can send data onto the bus. Modern communications environments use full-duplex (both directions simultaneously) modems. In that environment, DTEs have no reason to deassert RTS. However, due to the possibility of changing line quality, delays in processing of data, etc., there is a need for symmetric, bidirectional flow control. A symmetric alternative providing flow control in both directions was developed and marketed in the late 1980s by various equipment manufacturers. It