When communicating with various micro processors one needs to convert the RS232 levels down to lower levels, typically 3.3 or 5.0 Volts.
Here is a cheap and simple way to do that.
Serial RS-232 (V.24) communication works with voltages -15V to +15V for high and low. On the other hand, TTL logic operates between 0V and +5V . Modern low power consumption logic operates in the range of 0V and +3.3V or even lower.
| RS-232 | TTL | Logic |
| -15V … -3V | +2V … +5V | High |
| +3V … +15V | 0V … +0.8V | Low |
Thus the RS-232 signal levels are far too high TTL electronics, and the negative RS-232 voltage for high can’t be handled at all by computer logic. To receive serial data from an RS-232 interface the voltage has to be reduced. Also the low and high voltage level has to be inverted.
This level converter uses a Max232 and fivecapacitors. The max232 is quite cheap (less than 5 dollars) or if youre lucky you can get a free sample from Maxim.
The MAX232 from Maxim was the first IC which in one package contains the necessary drivers and receivers to adapt the RS-232 signal voltage levels to TTL logic. It became popular, because it just needs one voltage (+5V or +3.3V) and generates the necessary RS-232 voltage levels.
The requred parts:
- 1 x female serial port connector
- 1 x max 232
- 4 x 1uF capacitor
- 1 x 10uF capacitor
- Soldering iron, wires, breadboard etc.
Schematic
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