It has been common for the "nominal" input threshold voltage of an RS232 receiver to be at around 1.5 volts. The parts also have a typical input hysteresis of about 0.5 volts. Finally an input impedance of 2K to 5K is typically seen. The MAX202 data sheet linked by Kai confirms all these typical values.

Now let me comment about the transistor circuit for receiving an RS232 signal. It can be done..... gosh knows I have made more than a few test jigs and software development boards that used a transistor circuit to receive RS232 onto a circuit board. However I always add an additional 1N41418 type silicon diode in series with the transistor base to easily raise the switching threshold to two Vbe drops above ground. This solves the problem I have seen countless times where the driver from some non-compliant RS232 equipment is using a GND reference for its TxD pin. In the case of a GND reference the signal into the simple transistor receiver circuit has little or no decent low level noise margin.

If it were still common to have +12V and -12V volt supplies around on every board I would still probably use the old '1488 and '1489 parts for RS232 interfacing. However the cost of providing the necessary supplies for the RS232 driver has become expensive in practice and inconvenient in the design. As such the famous MAX232 and all its spinoff brothers and sisters came into being so as to provide the RS232 driver voltages from just a single +5V or +3.3V supply.

Also keep in mind that if one sets out to design a transistor circuit RS232 receiver that has characteristics similar to the typical values I stated above then the circuit gets too complicated with too many components to justify itself over the use of say a MAX232 or ADM211 type integrated chip RS232 transceiver. The simplest transistor circuits have these main issues...

1) The input threshold voltage level is quite often around about 0.7 volts or so thus the signal has little or no low level noise margin.

2) The input impedance is not maintained over the full range of input signal swing. The simplest circuits (like shown at the start of this thread) exhibit a reasonable input impedance when the signal is at a voltage above the threshold but then go to typically a much much higher input impedance when the signal goes below the threshold level.

3) Finally the absence of hysteresis in the switching voltage threshold of the simplest transistor circuits makes these circuits cause additional errors due to noise sensitivity and to the resultant signal distortion that occurs due to the controlled slew rates of the signle from a compliant RS232 driver.

Michael Karas