PC Serial Peripheral Design (4)

So far we have studied the characteristics of the RS-232 outputs in detail. Now it is the turn of the inputs. The standard demands that an input voltage of more than +3 V is considered as ‘high’, while a voltage lower than –3 V is considered as ‘low’. Between these voltages the level is not defined.Typical RS-232 interface cards use a type 1489 receiver (Figure 1). This IC requires a single 5 V power supply. The schematic shows a simple switching stage consisting of three transistors. As can be seen, the threshold voltage will not be very different from the base-emitter threshold of about 0.6 V. Taking into account the voltage divider at the input (consisting of a 3.8 k? and a 10 k? resistor) we arrive at a figure of about 0.8 V. A further resistor RF links the output of the second transistor stage back to the base of the first, providing feedback.