See also: http://www.maxim-ic.com/ MAX232 TTL to/from RS232 level converter.

• TTL to RS232
  • Steve Walz of armory.com says "Use a TTL HI output for the RS232 *GROUND*!! Then you get +/-5VDC levels to fake out the RS232. When TTL-GND goes HI, then a TTL-XMT LO looks like -5VDC and a "mark". Even sign problems go away in hardware! When TTL-GND goes LO, and TTL-XMT goes HI, it looks like +5VDC and a "space"! You can derive the two TTL signals from one off an inverter or inverter pair! Then magically, you have direct TTL to RS232 conversion!!
  • IDEA: Pin 2 on a peripheral connector (pin 3 on a host connector) will typically rest at -12 as driven by the external device. My Fluke 77/AN reads about -10 on pin 3 of the DB9 connected to my PC. Some people (Tom Engdahl at http://www.hut.fi/Misc/Electronics/circuits/rspower.html) have suggested using this fact to derive -12 for TX power. This makes sense as we normally are not RXing as we TX. How about adding a diode, resistor and big fat cap to provide a more or less steady source of -12v to drive pin 3 below zero when the NPN is biased off? Scenix did this on a demo modem board, Nationals app note for a mouse actually skipped the cap and Engdahl did something similar in a line buffer circuit http://www.hut.fi/Misc/Electronics/circuits/rsbuffer.html but he was using an external power source for extra juice so the amplifier type would have to be changed from emiter follower to a voltage gain amp. They did not use RTS as a source of +12 volts, connecting the transistor to +Vcc (local +5 volts) instead. My thinking is that if RTS isn't at +12, the peripheral shouldn't be sending anyway. In fact, if I bias this correctly, setting up the TX pin as an input would allow the periperal to read the RTS status without needing another IO pin. Here is a circuit and board in Eagle and a picture of the circuit
• RS232 to TTL
  • Tjaart van der Walt says "I've done this with a pair of simple transistor invertors for years without any problems. Use the RTS line for the supply for the two transistors, and a zener to limit the voltage going to the [processor]." Get 12V from the RS232 port on your [host] [ed: RTS, pin 20, is normally high (+12volts)], feed it through a 10k resistor to the collector of *any* NPN transistor [ed: 2N222]. Ground the emittor. The input is via a 10k resistor onto the base, and the output is on the collector" http://www.wasp.co.za/~tjaart/images/ttl2rs232.jpg
  • Gerald Coe of Devantech says "Feed the RS232 into a 6k8 resistor. Connect the cathodes of two 1n4148's to the other end of the resistor. Connect the anode of one diode to the PIC. Connect the anode of the other diode to ground. Connect another 6k8 pull-up resistor from the PIC pin to 5v. The data enters the pic upside down, but as you're doing the Rx in software, that doesn't matter.
  • Ties Bows says "I use just a single 100K series resistor  [and a 5V1 zener] to get 9600 Baud rs232 into a pic" http://www.geocities.com/SiliconValley/Sector/3863/uir/Images/ir12b_schematics.gif