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problem that may occur in this case is if the receiver at the other end requires a negative voltage for

recognizing a mark. In this situation, the full-duplex circuit shown in Figure 3 can be used as

analternative. The 22

µµµµF capacitor forms a negative-charge reservoir; consequently, when the TXD line is

capacitor and the load resistance at the other end (3-7 k

Ω

Ω

Ω

Ω). This circuit was tested from 150-19,200 bps

swing both positive and negative in full-duplex operation with this device.

HANDHELD RS-232-C APPLICATION USING A STEREO MINI-JACK Figure 2

FULL-DUPLEX CIRCUIT USING NEGATIVE-CHARGE STORAGE Figure 3

NOTE:

The capacitor stores negative charge whenever the TXD signal from the PC serial port is in a marking

charge reservoir when it is in a marking state. The capacitor will discharge to 0 volts when the TXD line

back to -10 volts. Note that TXD remains in a marking state when idle, which improves the performance

of this circuit.

ABSOLUTE MAXIMUM RATINGS*

-0.3 to +7.0 volts

-0.3 to +13.0 volts