	(REGULAR VERB)

	Syntax:    CHNGONLINE on-line_file_type on-line_file_version

	CHNGONLINE is for 12-m on-line spectral-line data only.  It
	allows the user to change the assumption about unspecified
	feed or subscan number as well as the actual on-line file
        currently being accessed for that file type.

	The 12-m has two types of spectral line data, filter
	bank and hybrid spectrometer.  The current numbering scheme is
	that filter bank scans have subscan numbers .01 through .04.
        Hybrid spectrometer subscan numbers start at .11.  Continuum data
 	is also written to the same file as the filter bank data.

        There are a number of situations where UniPOPS must make an
	assumption about the subscan number or location of the on-line
	data.  If you "GET" a scan without specifying the subscan number,
	UniPOPS assumes either .01 or .11 depending on the most
	recent CHNGONLINE use (.01 being the default at startup).
	The same is true of the related verbs (GET0, GET1, OFF, ON,
	GGET, etc), but is not true of CGET and CGGET which must
        always be supplied with a valid subscan number.

        In addition, SELECT, SUMMARY, and TELL currently only select from
        or report on one type of data file.  CHNGONLINE determines
	which type is used (the default being filter bank data at
	startup).

        During a single 12-m observing run, several on-line data files
        may be required to hold all of the data.  CHNGONLINE allows the 
	user to select from which of these several on-line files to retrieve 
	data.  The default on-line_file_version at startup is the highest 
        version for each on-line file type found in the on-line data area 
	(indicated by the environment variable TOPDATA) for each on-line 
	file type.

        Valid 12-m on-line_file_type values are:

	    HCTYPE - hybrid spectrometer 
	    FBTYPE - filter bank
            CONTYPE - continuum

        CONTYPE is allowed only while runing condar, HCTYPE and FBTYPE
        are allowed only while running line.

        The following rules are used in interpreting on-line_file_version
            0: use the highest (most recent) version
            n: use version n (n between 1 and 999 inclusive)
           -n: use version (current - n), i.e. -1 would mean use the version
               preceding the version currently in use.

        Three pointer adverbs indicate the currently opened version number
        of each of the 3 file types:

            FB_VER - the current filter bank file version number
            HC_VER - the current hybrid spectrometer file version number.
            CON_TYPE - the current continuum file version number

        If the requested version number does not exist, a Severe error
        occurs (error code 375).  ERROFF can therefor be used in
        conjunction with CHNGONLINE.  If an error in version number is
        noticed, the current status of version number does not change
        but the default file type WILL change.

        An attempt is always made to open a gains file of the same version
        number as its corresponding pdfl file (i.e. if pdfl_hc.xyz_004 is
        opened for access, gzfl_hc.xyz_004 should also be opened), there
        is no error if the associated gains file is unavailable.  There is
        no way to select a gains file having a different version number from
        the pdfl file of the same type.
        
	The FILES verb indicates the current file type being used :
	FB means that filter bank or continuum assumptions are in effect, 
        HC means hybrid spectrometer assumptions are in effect.

        Use OLFILES to obtain a listing of the currently opened on-line files.

        Use CHNGVER to obtain a listing of available versions and to
        select one of those versions.

        HC_VER, FB_VER, and CON_TYPE may be altered by CHNGONLINE.

	EXAMPLES:

	The user is reducing filter bank data and wants to examine the
	the hybrid spectrometer data:

	   > CHNGONLINE(HCTYPE, HC_VER)
           > GET 1234

		This would return scan 1234.11
                By using HC_VER, the current version number is not changed.

		Assuming the above CHNGONLINE has not been superseeded by
		a subsequent CHNGONLINE the following would select scan
		from the hybrid spectrometer data file:

	   > SELECT ONDATA


         The user wants to examing filter bank data from file 003:

           > CHNGONLINE(FBTYPE,3)

                FB_VER would change to version 3, assuming version 3 exists.

         The user wants to examine the most recent continuum data (this
         user must be running condar):

           > CHNGONLINE(CONTYPE, 0)

                CON_VER would be set to the highest version number found.

         HC_VER is currently set at 5, the user wants to access version 3:

           > CHNGONLINE(HCTYPE,-2)

                HC_VER would change to 3, assuming it exists.

         Alternatively,
    
           > CHNGOLINE(HCTYPE,(HC_VER-2))

         would have the same effect.

         If the user types the following, and version 7 does not exist:

           > CHNGONLINE(FBTYPE, 7)

         The message for error code 375 would be displayed and the
         default file type would be changed to filter bank.  The
         value of FB_VER would remain the same, however.

