RS232 communication with a PC

AN1753

6/22

DocID9986 Rev 2

2

RS232 communication with a PC

2.1

Main features

Electrical and protocol characteristics of RS232 are different from those used by the UART.

In RS232 communication, high level is typically +7V and low level is typically -7V, while the

ST7LITE0 I/Os work at CMOS levels (0, +5V).

Furthermore, the polarities are different. A ‘1‘ bit coming from the UART corresponds to a ‘0‘

bit in RS232, and a ‘0‘ bit to a ‘1‘ bit. It is true for all bits including the start and stop bits. So

it is necessary to implement a conversion between the PC and the ST7LITE0. In the

application, a MAX232 is used for this purpose.

2.2

PC configuration

The PC will be used as a terminal interface. “Hyperterminal”, the terminal application

software, is used as an interfacing software to test the function [.zip file attached]. The test

environment is Windows.

Under Windows, open the Hyperterminal application. To configure it, go to the port settings

and set the parameters to your application requirements. The options in this window must

be the same as the ones defined for your ST7FLITE0 communication device, except the

port.

After selecting the right serial communication port, select the same baud rate as the one set

for the ST7LITE0. As the PC accepts only one baud rate, transmission and reception baud

rates will have the same value. Data word can be 8/7 bits, but you can choose to use 1 or 2

stop bits.

«Flow control» can be either Xon/Xoff or none. The PC is then correctly configured.