DS275
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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 µ
µµ
µF capacitor forms a negative-charge reservoir; consequently, when the TXD line is
spacing (positive), TXOUT still has a negative source available for a time period determined by the
capacitor and the load resistance at the other end (3-7 kΩ
ΩΩ
Ω). This circuit was tested from 150-19,200 bps
with error-free operation using a SN75154 Quad Line Receiver as the receiver for the TX
OUT signal. Note
that the SN75154 can have a marking input threshold below ground; hence there is the need for TX
OUT to
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
data state (a negative voltage that is typically -10 volts). The top DS275’s TXOUT uses this negative
charge reservoir when it is in a marking state. The capacitor will discharge to 0 volts when the TXD line
is spacing (and TX
OUT is still marking) at a time constant determined by its value and the value of the load
resistance reflected back to TX
OUT. However, when TXD is marking the capacitor will quickly charge
back to -10 volts. Note that TXD remains in a marking state when idle, which improves the performance
of this circuit.
ABSOLUTE MAXIMUM RATINGS*
VCC -0.3 to +7.0 volts
V
DRV -0.3 to +13.0 volts
