Raspberry Pi Thermal Printer Flow Control DTR or XON/XOFF

Question

I use Rpi Zero 2 and Adafruit Nano Printer (link) for Bitmap printing. I ported the C++ Adafruit Thermal Printer library (link) for my Rpi Cpp program, which was pretty straightforward, but I am having difficulty doing DTR flow control.

Currently, I can print images, but I have to generously set a waiting time not to overflow the printer's buffer, which slows down the printing speed too much.

From googling, I realized DTR isn't supported with Rpi (Did I get this right?). So instead, I should use an XON/XOFF control. RX(GPIO 14) and TX(GPIO15) are already taken by the printer. Then, for the XON/XOFF serial interface, which pin should I use, and how should I enable and use it?

1. I am using gpiod.h library, and below is the code to read GPIO25 I tried to get a DTR communication with my printer, which failed.

    //-----------------------------------------------------------------------------
    // gpio
// Open GPIO chip
//const char *chipname = "gpiochip0";
//struct gpiod_chip *chip;
//chip = gpiod_chip_open_by_name(chipname);
chip = gpiod_chip_open("/dev/gpiochip0");

// Open GPIO lines
//struct gpiod_line *line;   
line = gpiod_chip_get_line(chip, 25);

if (!line) {
    std::cout << "failed to get line 25" << std::endl;
    gpiod_chip_close(chip);
    //return -1;
}

gpiod_line_request_input(line, "test1");

// Getting an input from gpio pin 25
int input_val = gpiod_line_get_value(line);

std::cout << "input_val: " << input_val << std::endl;

2. Here, I used serial_write just like it's written in the Adafruit library to enable DTR. Unsure what (1 << 5) means.

    if (dtr != 255)
    {
        write_bytes(ASCII_GS, 'a', (1 << 5));
        dtr_enabled = true;
        std::cout << "dtr_enabled" << std::endl;
    }
    else
    {
        dtr_enabled = false;
    }

3. Now... in the code below, you can see how I tried to enable XON/XOFF.

int Thermal::open_serial()
{
    std::cout << "open_serial called" << std::endl;
termios* tty = new termios;

file_descriptor = open(serial_path, O_RDWR | O_NOCTTY | O_NONBLOCK);

if (file_descriptor &lt; 0) {
    delete tty;
    return -1;
}

// Take over existing settings from open serial port
if (tcgetattr(file_descriptor, tty) != 0) {
    delete tty;
    return -1;
}

// Clear parity bit, disabling parity (most common)
//tty-&gt;c_cflag &amp;= ~PARENB;
// Clear stop field, only one stop bit used in communication (most common)
//tty-&gt;c_cflag &amp;= ~CSTOPB;
// Use 8 bits per byte
//tty-&gt;c_cflag |= CS8;
// Enable RTS/CTS hardware flow control
tty-&gt;c_cflag &amp;= CRTSCTS;
// Enables software flow control
tty-&gt;c_iflag &amp;= (IXON | IXOFF | IXANY);

tty-&gt;c_lflag &amp;= ~ICANON;
cfsetispeed(tty, baud_to_const(baud_rate));

if (tcsetattr(file_descriptor, TCSANOW, tty) != 0) {
    delete tty;
    return -1;
}

delete tty;

wake();
setPrintDensity(print_density, print_break_time); //-&gt; default 10, 2
//setPrintDensity(1, 2); //-&gt; for URL-A3 V3.0 -&gt; 1, 4

return file_descriptor;

}

4. Finally, the printer's manual talks about flow control.

Needing help urgently to complete my prototype. Any guidance would be greatly appreciated!

Answer 1

"DTR isn't supported with Rpi" only BCM2711 (Pi4 et al) supports DTR, although there maybe a software serial which does.

"for the XON/XOFF serial interface, which pin should I use" XON/XOFF is a software protocol - it DOES NOT use any pins.

I looked at the 4 printers mentioned on the Adafruit site and NONE of them use flow control and list connection details. (In typical Adafruit style the "documentation" is poorly organised and somewhat vague.)

To use serial enable in raspi-config and open /dev/serial0 See How do I make serial work on the Raspberry Pi3 or later for more detail.

I suggest you try something simpler e.g. loopback for initial testing.