[英]Time consumption of Linux RS485 serial communication
我正在嘗試通過一個USB至波特率38400、1個起始位,8個數據位,無奇偶校驗和1個停止位的USB至RS232至RS485端口與多個Modbus RTU設備進行通信。
與一台Modbus RTU設備的通信過程如下:
根據我的計算和數字示波器,發送8個字節的成本為2.083ms,接收23個字節的成本為5.99ms,Modbus RTU設備的響應時間約為1.3ms。 因此,通信過程的時間總計花費9.373ms。 但是在我的測試程序中,我發現平均通信時間約為15毫秒(平均10000倍)。 我不知道這另外5毫秒是從哪里來的,如何優化我的程序以減少此時間。
提前致謝!
測試程序如下:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <errno.h>
#include <string.h>
#include <signal.h>
void print_hex_buf(unsigned char *buffer, int size)
{
for (int i=0; i<size; i++)
{
printf("%02x ", buffer[i]);
}
printf("\n");
}
void diff_time(struct timeval t1, struct timeval t2, struct timeval *diff)
{
time_t sec;
suseconds_t usec;
//time in two different days
if (t1.tv_sec > t2.tv_sec)
sec = t2.tv_sec + 24*60*60 - t1.tv_sec;
else
sec = t2.tv_sec - t1.tv_sec;
usec = t2.tv_usec - t1.tv_usec;
if (usec < 0)
{
sec -= 1;
usec += 1000000;
}
diff->tv_sec = sec;
diff->tv_usec = usec;
}
int serial_write(int uart_fd, char *buffer, int size)
{
int count = 0;
count = write(uart_fd, buffer, size);
return count;
}
int serial_read(int uart_fd, char *buffer, int size)
{
int count = 0;
int bytes_read = 0;
int read_retry = 0;
fd_set fds_read;
struct timeval timeout;
FD_ZERO(&fds_read);
FD_SET(uart_fd, &fds_read);
timeout.tv_sec = 0;
timeout.tv_usec = 500000; //500ms
int ret = select(uart_fd + 1, &fds_read, NULL, NULL, &timeout);
if (ret > 0 && FD_ISSET(uart_fd, &fds_read))
{
count = read(uart_fd, buffer, size);
bytes_read = (count > 0)?count:0;
while (bytes_read < size && read_retry++ < 500)
{
count = read(uart_fd, buffer+bytes_read, size-bytes_read);
bytes_read += (count > 0)?count:0;
if (bytes_read >= size)
break;
}
}
else
{
printf("Failed to from uart!\n");
return -1;
}
return bytes_read;
}
int main(int argc, char** argv)
{
int fd;
struct termios opt;
int count;
unsigned char send_buf[] = { 0x01, 0x04, 0x00, 0x00, 0x00, 0x09, 0x30, 0x0c};
unsigned char buffer[256];
int iteration = 0;
int delay_ms = 0;
int err_count = 0;
int cycle = 0;
suseconds_t average_time = 0;
setbuf(stdout, NULL);
if (argc != 3)
{
printf("Usage: testuart [uart device] [iteration]\n");
return 0;
}
iteration = atoi(argv[2]);
fd = open(argv[1], O_RDWR | O_NOCTTY | O_NDELAY);
if (fd == -1)
{
printf("Failed to open port: %s\n", argv[1]);
return -1;
}
if (tcgetattr(fd, &opt) != 0)
{
printf("Failed to get uart attribute!\n");
return -1;
}
opt.c_cflag = B38400|CS8|CREAD|CLOCAL;
opt.c_iflag = IGNPAR;
opt.c_cflag &= ~PARENB;
opt.c_cflag &= ~PARODD;
opt.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
opt.c_oflag &= ~OPOST;
opt.c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL|IXON);
tcflush(fd, TCIFLUSH);
if (tcsetattr(fd, TCSANOW, &opt) != 0)
{
printf("Failed to setup serial port!\n");
close(fd);
return -1;
}
while (cycle++ < iteration)
{
printf("Send hex command:\n");
print_hex_buf(send_buf, 8);
struct timeval tm_start;
struct timeval tm_end;
struct timeval tm_diff;
gettimeofday(&tm_start, NULL);
count = serial_write(fd, send_buf, 8);
if (count != 8)
{
printf("Failed to write 8 bytes!\n");
close(fd);
return -1;
}
count = serial_read(fd, buffer, 23);
if (count <= 0)
{
printf("serial read returns %d\n", count);
close(fd);
return -1;
}
gettimeofday(&tm_end, NULL);
diff_time(tm_start, tm_end, &tm_diff);
print_hex_buf(buffer, count);
printf("serial communication costs %ld.%06ld seconds.\n",
tm_diff.tv_sec, tm_diff.tv_usec);
average_time = ((average_time*(cycle-1))+tm_diff.tv_usec)/cycle;
}
printf("%d times, average time in usec is %ld\n", cycle-1, average_time);
close(fd);
return 0;
}
感謝木屑! 以下鏈接有幫助! 平均時間從15ms減少到10ms。 PXA270上的RS232通信中的高延遲
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