Well, I have this WAP54G V31, and only 3.3V level out on serial.
Asked for a MAX233 in the shop.. needed to be ordered, was expensive...
Told them: Forget it, I will use transistors.

But the second thing, No supply should be used, not even the 3.3V

My solution? I control the terminal (Linux minicom) and the RS232 port too (outb() ).
So here it is folks:



                                                                      Embedded connector

9 pol D connector                                                --------0  >
                                                                |
                                                                c 
TXD 3 -------------------------- 5k6 ------------------------ b   NPN
                                       |                        e
                                       k                        |
                                      Diode 1N4148              |
                                       a                        |  
                                       |                        |  
GND 5 -------------------------------------------------------------------0 GND
                                              |
                                              |
       +12V                                   |              
DTR 4 ----------------------                  |
                  |         |                 |
                  |        5k6                |
                  c         |                 |
                    b ------|                 |
                  e NPN     |                 |
                  |         |                 | 
RXD 2 ------------|         |                 |
                  |         |                 |
                  e         |                 b
                    b ------|         ----- c   e----- 3k3 --------------0 <
                  c         |        |         
                  | PNP     c        |        PNP
                  |           b------|       
                  |         e NPN    |
                  |         |       3k3 
       - 12V      |         |        |
RTS 7  ------------------------------
                 

All NPN transistors BC547, all PNP transistors BC 557.
Could be almost any universal NPN and PNP, not critical.


As you can see the top is classical, but the reverse path uses a common base for level shifter :-)
I start minicom, hardware flow control off, and then after that a small program that sets 
DTR to +12 V and RTS to -12 V, there is my supply :-)

Maxim defeated :-)