My current understanding of how the escp/d4lib communicate with the printer.

Initialy the command to enter ieee 1284.4 mode is sent, which is this:

    unsigned char cmd[] = {
        0x00, 0x00, 0x00, 0x1b, 0x01, '@', 'E', 'J', 'L', ' ',
        '1', '2', '8', '4', '.', '4', 0x0a, '@', 'E', 'J',
        'L', 0x0a, '@', 'E', 'J', 'L', 0x0a
    };

The printer should return acknowledgment of this. Then ieee 1284.4 mode is
initialised, d4lib.c has the Init() function which sends a struct with some
variables filled to the printer over the bulk transfer.

To retrieve the ink level the status command is then sent.