Unit_GPIBDir (ul_pbar_gpib)

 *  Unit GPIBDir intent:
 *    The module GPIB functions in this unit support a GPIB instrument directory
 *    facility.  The GPIB instrument directory is contained in a fileshare file
 *    named "PBAR_RF:PBARGPIB.DAT" which contains identification and location
 *    information for GPIB instruments residing in the Pbar source.  Each
 *    directory entry contains 64 ASCII characters with the following format:
 *                       11111111112222222222333
 *              12345678901234567890123456789012
 *              ^  ^                    ^
 *              tp name                 model   
 *              xx xxxxxxxxxxxxxxxxxxxx xxxxxxxx
 *
 *              33333334444444444555555555566666
 *              34567890123456789012345678901234
 *              ^       ^       ^       ^
 *              house   module  node    address
 *              xx      xxxx    xxxx    xxx
 *    Where:
 *              tp - 2 character instrument type (e.g., NA, SA, OS, VM)
 *              name - 20 character identifying name of instrument
 *              model - 8 character model ID of instrument
 *              house - 2 character ACNET house ID
 *              module - 4 character ACNET module ID
 *              node - 4 digit integer ACNET node ID
 *              address - 3 digit integer GPIB instrument address
 *
 *    The 64 characters are broken into two groups of 32 characters, above, to
 *    indicate how the file looks when edited with the fileshare file dump
 *    program on D50.  It is intended that entries to the directory be made
 *    using that program.  An example entry following the above format would
 *   look like:
 *    D50 column:        11111111112222222222333
 *    		12345678901234567890123456789012
 *    		NA AP10 HF NA           HP8720
 *    		2D      GPI1    0027    012
 *
 *    Users should first call GPIB__InitInstrumentDir() to open the instrument
 *    directory file and initialize the directory system.  Subsequent calls to
 *    GPIB__GetNextInstrument(), passing the desired instrument type code, will
 *    return a 20 character instrument name string and an instrument index for
 *   instruments of that type found in the directory.  Finally a call to
 *    GPIB__GetInstrumentInfo() passing an instrument index as described above
 *    will return the address information for the individual instrument.
 *    GPIB__CloseInstrumentDir() is provided to close the instrument directory
 *    file and release the directory information resources when they are no
 *    longer required.  GPIB__InitInstrumentDir() may be called again to reset
 *    the directory search so that subsequent GPIB__GetNextInstrument() calls
 *    search for another instrument type.
 *    
 *    Functions in this unit include:
 *
 *	GPIB__InitInstrumentDir()	Open and Initialize Instrument Directory
 *      GPIB__GetInstrumentCount()	Return Count of Given Instrument Type
 *	GPIB__GetNextInstrument()	Return Next Instrument of Given Type 
 *	GPIB__GetInstrumentInfo()	Return Instrument Address Data by Index
 *	GPIB__CloseInstrumentDir()	Close Instrument Directory
 *      GPIB__InstrumentMenuSelect()	Select Instrument From Pop-up Menu