The RS-485 bus interface is a widely used serial communication standard known for its ease of network integration, strong resistance to interference, and long transmission capabilities. It utilizes balanced transmission and differential reception, which allows it to effectively reject common-mode noise. Combined with the high sensitivity of the transceiver, it can detect voltages as low as 200mV, enabling reliable communication over several kilometers. By using the RS-485 bus, a single pair of twisted wires can connect multiple systems, forming a distributed network. This approach offers simple setup, cost-effective equipment, and extended communication distances.
Here’s an example of an RS-485 communication program written for a 51-series single-chip microcontroller:
```c
#ifndef __485_C__
#define __485_C__
#include
#include
#define unsigned char uchar
#define unsigned int uint
/* Communication Commands */
#define __ACTIVE_ 0x01 // Host asks if the slave exists
#define __GETDATA_ 0x02 // Host sends a read request
#define __OK_ 0x03 // Slave response
#define __STATUS_ 0x04 // Slave sends device status
#define __MAXSIZE 0x08 // Buffer length
#define __ERRLEN 12 // Any frame longer than this is considered invalid
uchar dbuf[__MAXSIZE]; // Buffer for device status information
uchar dev; // Local device number
sbit M_DE = P1^0; // Driver enable (active high)
sbit M_RE = P1^1; // Receiver enable (active low)
void get_status(); // Function to retrieve device status
void send_data(uchar type, uchar len, uchar *buf); // Send data frame
bit recv_cmd(uchar *type); // Receive command from host
void send_byte(uchar da); // Send a single byte
void main() {
uchar type, len;
/* System initialization */
P1 = 0xFF; // Read local device number
dev = (P1 >> 2); // Extract device address
TMOD = 0x20; // Timer 1 in mode 2
TH1 = 250; // Set baud rate
TL1 = 250;
TR1 = 1; // Start timer
PCON = 0x80; // SMOD = 1
SCON = 0x50; // Mode 1, 9600 bps, receive enabled
ES = 0; // Disable serial interrupt
IT0 = 0; // Level-triggered external interrupt 0
EX0 = 1; // Enable external interrupt 0
EA = 1; // Global interrupt enable
while(1) {
if(recv_cmd(&type) == 0) continue;
switch(type) {
case __ACTIVE_:
send_data(__OK_, 0, dbuf); // Send OK response
break;
case __GETDATA_:
len = strlen(dbuf);
send_data(__STATUS_, len, dbuf); // Send status
break;
default:
break; // Invalid command
}
}
}
void readstatus() interrupt 0 using 1 {
get_status(); // Update device status
}
/* This function receives a data frame and checks it.
The function returns 1 if the frame is valid, otherwise 0. */
bit recv_cmd(uchar *type) {
bit db = 0; // Set when last byte is 0xDB
bit c0 = 0; // Set when last byte is 0xC0
uchar data_buf[__ERRLEN]; // Store received frame
// Code for receiving and processing the frame would go here
return 1; // Placeholder
}
#endif
```
This code provides a basic structure for implementing RS-485 communication on a 51 microcontroller. It includes functions for sending and receiving data, handling commands, and managing the device's status. The system is designed to be efficient and scalable, making it suitable for various industrial and automation applications.
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