      SUBROUTINE YZERO (CHAN, IERR)
C-----------------------------------------------------------------------
C! fill a TV memory plane with zeros
C# Y1 TV-util
C-----------------------------------------------------------------------
C;  Copyright (C) 1995
C;  Associated Universities, Inc. Washington DC, USA.
C;
C;  This program is free software; you can redistribute it and/or
C;  modify it under the terms of the GNU General Public License as
C;  published by the Free Software Foundation; either version 2 of
C;  the License, or (at your option) any later version.
C;
C;  This program is distributed in the hope that it will be useful,
C;  but WITHOUT ANY WARRANTY; without even the implied warranty of
C;  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
C;  GNU General Public License for more details.
C;
C;  You should have received a copy of the GNU General Public
C;  License along with this program; if not, write to the Free
C;  Software Foundation, Inc., 675 Massachusetts Ave, Cambridge,
C;  MA 02139, USA.
C;
C;  Correspondence concerning AIPS should be addressed as follows:
C;         Internet email: aipsmail@nrao.edu.
C;         Postal address: AIPS Project Office
C;                         National Radio Astronomy Observatory
C;                         520 Edgemont Road
C;                         Charlottesville, VA 22903-2475 USA
C-----------------------------------------------------------------------
C-----------------------------------------------------------------------
C   YZERO fills an TV TV memory plane with zeros the fast way.
C   Inputs:
C      CHAN   I     channel # ( 1 - NGRAY+NGRAPH), 0 => all
C   Outputs:
C      IERR   I     error code of Z...XF: 0 - ok, 2 - input error
C   Virtual TV version
C-----------------------------------------------------------------------
      INTEGER   CHAN, IERR
C
      INTEGER   IC
      INCLUDE 'INCS:DTVC.INC'
      INCLUDE 'INCS:DVTV.INC'
C-----------------------------------------------------------------------
      IERR = 2
      IF ((CHAN.LT.0) .OR. (CHAN.GT.NGRAY+NGRAPH)) GO TO 999
C                                       fix common
      IF (CHAN.LE.NGRAY) THEN
         IC = CHAN - 1
         IF (IC.GE.0) IC = 2**IC
         CALL MOVIST ('OFFF', IC, 0, 0, 0, IERR)
         END IF
C                                       set up comm buffer
      BUFSW = 1
      BUFSR = 0
      CALL MKYBUF ('WRIT', 'YZERO', BUFSW, BUFSR, CHAN, HEADER)
      CALL ZVTVXF (BUFSW, BUFSR, HEADER, IERR)
C
 999  RETURN
      END
