基于FT232R的OBD2转USB线缆ISO 9141-2协议ECU读取问题求助

Hey Joe, let's work through this frustrating issue you're hitting with ISO 9141-2 and your FT232R OBD2-to-USB adapter. Getting your own sent commands back instead of ECU responses is super annoying—let's break down the most likely culprits and how to fix them.

1. Check for Local Echo or Hardware Loopback #

First off, if your serial port is set to local echo, every byte you send gets immediately sent back to the receive buffer. That's a super common reason for seeing your own commands returned. Double-check your serial init code to turn off echo:

struct termios tty;
if (tcgetattr(fd, &tty) != 0) {
    perror("tcgetattr");
    return -1;
}

// Disable local echo and related flags
tty.c_lflag &= ~ECHO;
tty.c_lflag &= ~ECHOE;
tty.c_lflag &= ~ECHONL;

// Apply changes immediately
tcsetattr(fd, TCSANOW, &tty);

Also, some cheap adapters have a physical loopback mode (check for DIP switches) that routes TX to RX—disconnect the adapter from the car, send a command, and if you still get it back, that's probably the issue.

2. You Might Be Skipping the ISO 9141-2 Handshake #

ISO 9141-2 doesn't let you just send diagnostic commands out of the blue—you need to do a wake-up handshake first:

• Send a 5-baud wake-up signal (a long low pulse, usually ~200ms)
• Wait for the ECU to send back a confirmation byte (often 0x55)
• Only then send your actual diagnostic commands (like 0100)

If you skip this, the ECU won't respond at all, and you'll just read back your own echoed commands. Add the handshake logic before sending your main commands.

3. Verify Serial Port Parameters #

Even if you set the baud rate, make sure all ISO 9141-2 required settings are correct:

• Baud rate: 10400 bps (not 9600, which is common for other OBD protocols)
• Data bits: 8
• Stop bits: 1
• Parity: None
• No hardware flow control (RTS/CTS)

Here's a code snippet to set these properly:

// Configure serial parameters for ISO 9141-2
tty.c_cflag &= ~PARENB; // No parity
tty.c_cflag &= ~CSTOPB; // 1 stop bit
tty.c_cflag &= ~CSIZE;
tty.c_cflag |= CS8; // 8 data bits
tty.c_cflag &= ~CRTSCTS; // Disable hardware flow control

// Set baud rate to 10400
cfsetispeed(&tty, B10400);
cfsetospeed(&tty, B10400);

tcsetattr(fd, TCSANOW, &tty);

4. Fix Timing Issues in Read/Write Logic #

ECUs don't respond instantly—if you read immediately after sending a command, you might be grabbing the send buffer's leftover data before the ECU has a chance to reply. Add a small delay after sending, then read:

// Send your diagnostic command
ssize_t n = write(fd, "0100\r", 5);
if (n < 0) {
    perror("write");
    return -1;
}

// Wait for ECU to respond (adjust delay based on your vehicle)
usleep(150000); // 150ms

// Read the ECU response
char buffer[256];
memset(buffer, 0, sizeof(buffer));
n = read(fd, buffer, sizeof(buffer)-1);
if (n < 0) {
    perror("read");
    return -1;
}

5. Check OBD2 Adapter Wiring #

Make sure your FT232R's TX/RX pins are connected correctly to the OBD2 port:

• ISO 9141-2 uses K-line (pin 7) for communication (some vehicles use L-line pin 15 too)
• If TX/RX are swapped, your commands never reach the ECU, so you get nothing back but your own echo

If you can share your full example code, I can spot more specific issues, but working through these steps should get you past the "reading my own commands" problem.

内容的提问来源于stack exchange，提问作者Joe