RS-232-TTL converter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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When developing various kinds of electronic devices using microcontrollers, it is very often useful to be able to connect them to a personal computer via a serial port. However, this cannot be done directly, since according to the RS-232 standard, the signal is transmitted at levels of -3..-15V (logical <1>) and +3..+15V (logical <0>). To convert RS-232 levels to standard TTL logic levels, special converter chips are usually used. However, it does not always make sense to include a level converter in the circuit of the device being designed, since it often happens that communication with a computer is needed only at the stage of manufacturing and debugging the device, and there is no need for it in the final product. The logical way out in this situation can be the manufacture of a separate RS-232 to TTL level converter, a diagram of one of the possible options for which is shown in Figure 1.

Fig. 1

The proposed converter is based on the widespread MAX232A level converter chip from Maxim (U1), which also has many analogues from other manufacturers (Analog Devices, LG, etc.). This microcircuit is designed for a 5V supply voltage and has a built-in voltage doubler and inverter on switched capacitors to obtain the +10V voltages required to work with RS-232 signals. The microcircuit requires 4 external capacitors (C1, C2, C3, C4) with a capacity of 0.1 uF, which are used in the voltage converter. In addition, to simplify the use of this converter, it provides power supply directly from the serial port, eliminating the need for external power supplies. The supply voltage of 5 V is created by a low-power linear voltage regulator LM78L05 (U2), the input of which is connected to the storage capacitor C6. Capacitor C6 is charged via a diode from the Data Terminal Ready signal (DTR, fourth pin of the 9-pin RS-232 connector). Diode D1 can be of any type (the author used a diode in a surface mount package, soldered from a burnt motherboard). Normal operation of such a power converter requires the DTR signal to be logic zero most of the time. This must be provided by the terminal program or user program being used.

The use of the converter described above turns out to be convenient in those cases when, during the operation of the device, communication with a computer is not required, but it is needed at the stage of debugging or manufacturing the device. A typical example of this would be, for example, a device with flash or EEPROM memory that requires initial initialization. In addition, it is often very convenient during the development process to output various kinds of debugging information to the serial port, which sometimes makes it possible to do without hardware emulators.

Publication: cxem.net

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