TTL to RS232 logic translation

Question

Preface: I don't have any MAX232 ICs handy, and only need unidirectional communication (from TTL to RS232). It's just a quick test to see if an old GPS OEM board still works. I don't have any MOSFETs on hand either - just some basic BJTs, as shown in the schematic. I'm using a dual supply instead of parasitic voltages from the serial port.

Here's the translation I was after:
TTL_Tx --> RS232_Rx
    0V         ~ +12V
    5V         ~ -12V

^{simulate this circuit – Schematic created using CircuitLab}

I thought this would work (and it does in LTSpice).
Q₁ - Turns Q₂ on when the TTL input is high, providing a path for current to flow
Q₂ - Provides a translation of 5V->12V and 0V->-12V at its collector
Q₃ - Inverts Q₂'s collector signal for the final output

It works perfectly in LTSpice, but on my oscilloscope I see that the output is not being pulled down to the negative rail:

Of course, this actually works fine. The serial port is happy and the values are in spec. But I'm still trying to figure out why the circuit doesn't behave as I expected.

Answer 1

You probably have some load connected to the output when you measured it with the 'scope.

If the equivalent resistance of the load is less than maybe 50 kohms, and it's pulling toward ground, your 10 k pull-down (R3) isn't going to be able pull the output close to -12.

Answer 2

You can't drive the Q2 base as you have shown. There is no current limit (other than that provided by Q1 Beta). ...the output impedance of an RS232 transmitter is normally about 300 Ohms ...so 10 k pulldown won't work. Try something like this:

^{simulate this circuit – Schematic created using CircuitLab}