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Annotation of xinu/h/pi7300.h, revision 1.1

1.1     ! root        1: /* pi7300.h   header for the ATT7300's parallel interface */
        !             2: 
        !             3: /* The pi is ready to use with no special initialization.  Basically
        !             4:    it has only one mode of operation, so there is nothing to specify.
        !             5:    After writing a data byte to PI_DATA, the LP_STROBE bit must be toggled
        !             6:    from high to low and back to high to "strobe the data to the printer",
        !             7:    i.e., this high-low-high signal is passed out through the port (on pin 1) 
        !             8:    to the printer and tells it that a byte is ready for printing.
        !             9:    Then the printer acknowledges acceptance of the byte by toggling
        !            10:    the Acknowledge signal from high to low and back to high, on pin 10.
        !            11:    Although undocumented in the register description, this signal can
        !            12:    be seen in bit 9 of the GSR (found by reading the circuit diagram,
        !            13:    sheet 6, unit 24D, the latch for the GSR).  More importantly for us,
        !            14:    the low->high transition causes an interrupt, autovectored, level 2.
        !            15:    The interrupt is acknowledged to the hardware by writing to data reg.
        !            16:    (It doesn't have to be the "next byte", since the printer will ignore
        !            17:    data written to the port if there is no strobe signal.) */ 
        !            18: 
        !            19: /* the 8-bit data port, output only-- */
        !            20: #define PI_DATA ((volatile unsigned char *)0x4f0000)
        !            21: /* Note: MCR is also used for clock interrupt control--should have package
        !            22:    controlling updates to it like we have for the console control reg */
        !            23: #define MCR ((volatile unsigned short *)0x4a0000)
        !            24: /* one bit in MCR for lp--data strobe */
        !            25: #define LP_STROBE (1<<13)
        !            26: /* status register--input-only bits */
        !            27: #define PI_STATUS ((volatile unsigned short *)0x470000)
        !            28: /* bits in status reg--not needed to do the basic output operation,
        !            29:    but needed for features such as stopping when the paper runs out */
        !            30: /* from pin 11 of the interface: 1 if busy (printing, set offline, or in error
        !            31:    condition)-- */
        !            32: #define LP_BUSY (1<<7)
        !            33: /* from pin 13 of the interface, 1 if printer online, but may be 1 all the 
        !            34:    time, so not terribly useful--use LP_BUSY or LP_ERROR instead */
        !            35: #define LP_SELECT (1<<6)
        !            36: /* from pin 12 of the interface: 1 if the printer is out of paper */
        !            37: #define LP_PAPEREND (1<<5)
        !            38: /* from pin 32 of the interface: 0 if the printer is out of paper, offline,
        !            39:     or in error state */
        !            40: #define LP_ERROR (1<<4)
        !            41: /* The rest of the bits relate to other devices */
        !            42: 
        !            43: /* The PI interrupts using autovector for level 2 */
        !            44: /* (is this the right type spec??) */
        !            45: #define PI_INTVEC_ADDR ((int (**)())(0x68))
        !            46: 

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