On your MAX233 schematic have you verified that you have +5V on pin 7?

Also you show on the schematic that pin 4 of the MAX233 is connected as a receiver off the serial port connector on "Pin 2 DB9 or DB25".

Likewise you show on the schematic that pin 5 of the MAX233 is connected as a driver to the serial port connector on "Pin 3 DB9 or DB25".

(Note first of all that your use of the term DB9 for a nine pin serial port connector is not technically correct. The DB sized connector is a shell/body size for a D-Sub 25 pin connector size. The correct designator for the D-Sub 9 pin connector shell/body size is the DE9).

Your pinnings would be OK for a cable of a DE9 cable routed to a device such as a modem. If you are using a DB25 type cable routed to a modem device you would need to swap the pin 2 & 3 functions.

If you are trying to connect the TxD out of the shown circuit on the MAX233 on pin 5 into a PC then you would need to connect that pin to the RxD pin of the PC's DE9 pin 2 (RxD). Likewise if you are attempting to connect the RxD into of the shown circuit on the MAX233 pin 4 over to a PC then you would need to connect that pin to the TxD pin of the PC's DE9 pin 3 (TxD).

This chart shows the typical pinning for PC type serial ports on the standard DE9 and DB25 connectors. Note that the signals on the two connectors pins 2 and 3 have swapped functions.

    DE9 DB25     PC  
    pin pin sig dir
    --- --- --- ----
     nc  1  GND <->
     3   2  TXD -->
     2   3  RXD <--
     7   4  RTS -->
     8   5  CTS <--
     6   6  DSR <--
     5   7  GND <->
     4   20 DTR -->

Michael Karas