Hi,
Suppose we intend to convert only TxD pin of PC COM port TTL compatible (i.e. what MAX232 and DS275 do). Now, who says it's impossible or even not suitable to use a reverse biased fast zener diode connected to GND and of course a series 1k resistor connected between TxD and zener? Why? What modifications do you sujjest? The reason for this arrangement is saving power: virtually no extra power supply is needed in comparison to 8mA or more (based on some rumors) for MAX232 or it's Texan counterpart DS275. How many milliampers does this configuration manage to deliver to the next stage? What about the waveform? Will it be "square enough" or the zener deteriorates it?
Best.