RS-232 is one of the most common communication interfaces found in personal computers. It is an asynchronous serial communication standard developed by the Electronic Industries Association (EIA). The RS-232 interface typically comes in two forms: a 9-pin connector (DB-9) or a 25-pin connector (DB-25). Most PCs are equipped with at least two RS-232 ports, commonly referred to as COM1 and COM2.
In serial communication, both devices must use a standardized interface to ensure compatibility and smooth data exchange. The RS-232-C standard, also known as EIA-RS-232-C, is widely used for serial communication. The "-C" in its name refers to the version of the standard, meaning it is essentially the same as "RS-232."
Developed in 1970 by the EIA in collaboration with Bell Systems, modem manufacturers, and computer terminal producers, the RS-232 standard defines the electrical and mechanical characteristics of the interface. Its full title is "Standard for the Interconnection of Data Terminal Equipment (DTE) and Data Communication Equipment (DCE) for Serial Binary Data Interchange." The original specification used a 25-pin DB-25 connector, but IBM later simplified it to a 9-pin DB-9, which became the de facto standard for personal computers.
In industrial applications, only three lines are usually used: RXD (receive data), TXD (transmit data), and GND (ground). This simplification makes the interface more practical for control systems and other embedded applications.
**RS-232 Serial Communication Program**
Here is a simple example of a C program that demonstrates basic RS-232 serial communication using low-level I/O functions:
```c
#include
#include "stdio.h"
#include
void main() {
char ch;
// Initialize serial port
outportb(0x3fb, 0x80); // Set LCR, enable access to DLL/DLM, disable interrupts
outportb(0x3f8, 0x0C); // Set baud rate low byte (DLL)
outportb(0x3f9, 0x00); // Set baud rate high byte (DLM)
outportb(0x3fb, 0x03); // Set LCR, disable access to DLL/DLM
outportb(0x3fc, 0x03); // Set MCR, DTR and RTS enabled
while (1) {
// Send data
if (bioskey(1)) {
ch = bioskey(0) & 0x0ff;
if (ch == 27) exit(0);
outportb(0x3f8, ch);
}
// Receive data
ch = inportb(0x3fd);
if (ch & 0x01) {
ch = inportb(0x3f8);
printf("%c", ch);
}
}
}
```
Note: The base address for COM1 is 0x3F8, and for COM2 it is 0x2F8. Depending on your setup, you may need to adjust the port accordingly. This example uses BIOS functions for direct hardware access, which is suitable for older systems or embedded environments. For modern applications, using higher-level libraries or drivers is recommended for better portability and reliability.
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