CLRBIT(OPCSDEFS)	Optical Printer Control System	  CLRBIT(OPCSDEFS)

    NAME
        clrbit - clear bit(s) on an IBMPC port

    USAGE
        clrbit [port] [mask] [softlatch]    # (values hex!)

    EXAMPLES
        clrbit 0378 04 0        # lpt1 port 0378, bit #2 (0x04)
        clrbit 0306 01 1        # kuper logic connector softlatch bit #1

    DESCRIPTION
        This command disables bits on a port based on a bit mask. 
	All bits specified in the mask are cleared. All values are in hex.

	    [port] is the port number in the range 0000-03ff

	    [mask] is a hex byte value indicating the bits
	    to be cleared on that port. 

        SETBIT(OPCSDEFS) and CLRBIT(OPCSDEFS) can be used in OPCSDEFS.OPC
	to initialize port hardware bits to known states on OPCS startup.

    CAVEATS
        o With [softlatch] set to 1, only ports 0x0000 - 0x07ff are allowed.
          Any ports above 0x07ff with [softlatch] enabled causes an error.

        o External programs changing port bits defined to OPCS with [softlatch]
          (e.g. the kuper logic I/O port) should be aware that OPCS is
          maintaining its own internal latch for that port, and that latch
          won't know about hardware changes made by external programs.
        
        o Due to these issues, it's best to avoid using hardware that has to
          be latched. It's usually bad hardware practice to make WRITE ONLY
          ports, since different programs cannot co-communicate with them,
          unless some common data area or driver is arranged.

    SEE ALSO
        DEENERGIZE(OPCSDEFS)  - define port/bit to deenergize motors
        ALLSTOP(OPDSDEFS)     - define port/bit to detect the allstop key
        BUCKLE(OPCSDEFS)      - define port/bit to detect film buckles
        VIEWER(OPCSDEFS)      - define port/bit to detect viewer open
        TRIPSWITCH(OPCSDEFS)  - define port/bit to detect trip switches
        SETBIT(OPCSDEFS)      - set bit(s) on a port
        CLRBIT(OPCSDEFS)      - clear bit(s) on a port
        XORBIT(OPCSDEFS)      - invert bit(s) on a port

    ORIGIN
	Version K1.12d+ Gregory Ercolano, Venice California 03/04/98