RS232 Basic Circuits

Minimal components and it can compete with the well known Maxim devices. Tested upto 115_200 Baud. With a few more components, you can even derive the power supply for your PIC and electronics from the RS232: circuit below.

The electrical specifications of the serial port is contained in the EIA (Electronics Industry Association) RS232C standard.
It states many parameters such as -
1. A "Space" (logic 0) will be between +3 and +25 Volts.
2. A "Mark" (Logic 1) will be between -3 and -25 Volts.
3. The region between +3 and -3 volts is undefined.
4. The inputs are loaded to ground with a load of 3kOhm .. 7kOhm

D-Type-25 Pin No.  D-Type-9 Pin No.  Abbreviation Full Name 
Pin 2  Pin 3  TD  Transmit Data 
Pin 3  Pin 2  RD  Receive Data 
Pin 4  Pin 7  RTS  Request To Send 
Pin 5  Pin 8  CTS  Clear To Send 
Pin 6  Pin 6  DSR  Data Set Ready 
Pin 7  Pin 5  SG  Signal Ground 
Pin 8  Pin 1  CD  Carrier Detect 
Pin 20  Pin 4  DTR  Data Terminal Ready 
Pin 22  Pin 9  RI  Ring Indicator 

Basic Receive Circuits

for Baudrates uptill 19200, R3 can be increased to 100 kOhm

Equal to the circuit above, except the connection of the RS232 GND.

Basic Transmit circuits

For Baudrates upto 19200, R4 can be increased to 10 kOhm. RTS should be set high by the PC. Instead of RTS, also DTR can be used.

The same as above but with a transistor (note the extra resistor R5).

NOT WORKING Transmit circuits, although some claim succes with one of these

The voltage on the RxD pin varies only from +5Volt to what ?? to nothing !!

The voltage on the RxD pin varies only from 0 to +5Volt.

And some more examples from the net.

This article comes from Robotik & Elektronik

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