选择合适的RS - 232收发器

　　Increased Battery Life through Autoshutdown Techniques

　　Many RS-232 transceivers provide a simple shutdown feature that is activated by applying a logic level to the shutdown pin. Since most RS-232 devices are only used for a short period of time， the shutdown feature can save energy and increase the run-time of portable equipment batteries.

　　Other RS-232 devices incorporate a more advanced feature called Autoshutdown， which typically limits the shutdown supply current to 1uA. These devices can determine whether a transmission cable is attached to the RS-232 transceiver. If a cable is not attached to the RS-232 transceiver， the unit will go into shutdown until the presence of a transmission cable is detected. A transceiver equipped with the Autoshutdown feature monitors the RS-232 signals at its receiver inputs to determine whether a cable is connected. When a cable is not present， the signals at the RS-232 receiver inputs are approximately 0V. If the transceiver detects that all the receiver inputs are between -0.3V and +0.3V for greater than 30µS， it assumes that a valid transmitter cable is not connected and will automatically put itself into a low-power mode （shutdown）。 The part will automatically take itself out of shutdown if any one of the receiver‘s inputs exceeds +2.7V （i.e.， whenever the cable is reconnected）。 This feature can be overridden with signals that force the device on or off. See Figure 1. The MAX3228E， MAX3229E and the MAX3246E are three examples of transceivers that incorporate the Autoshutdown feature.