--- a/scripts/control/abcddim.m
+++ b/scripts/control/abcddim.m
@@ -1,71 +1,121 @@
-## Copyright (C) 1996, 1997 John W. Eaton
-##
-## This file is part of Octave.
-##
-## Octave is free software; you can redistribute it and/or modify it
-## under the terms of the GNU General Public License as published by
-## the Free Software Foundation; either version 2, or (at your option)
-## any later version.
-##
-## Octave is distributed in the hope that it will be useful, but
-## WITHOUT ANY WARRANTY; without even the implied warranty of
-## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-## General Public License for more details.
-##
-## You should have received a copy of the GNU General Public License
-## along with Octave; see the file COPYING.  If not, write to the Free
-## Software Foundation, 59 Temple Place - Suite 330, Boston, MA
-## 02111-1307, USA.
-
-## Usage: [n, m, p] = abcddim (a, b, c, d)
-##
-## Check for compatibility of the dimensions of the matrices defining
-## the linear system (a, b, c, d).
-##
-## Returns n = number of system states,
-##         m = number of system inputs,
-##         p = number of system outputs.
-##
-## Returns n = m = p = -1 if the system is not compatible.
-
-## Author: A. S. Hodel <scotte@eng.auburn.edu>
-## Created: August 1993
-## Adapted-By: jwe
+# Copyright (C) 1993, 1994, 1995 John W. Eaton
+# 
+# This file is part of Octave.
+# 
+# Octave is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by the
+# Free Software Foundation; either version 2, or (at your option) any
+# later version.
+# 
+# Octave is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+# for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with Octave; see the file COPYING.  If not, write to the Free
+# Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
 
 function [n, m, p] = abcddim (a, b, c, d)
 
+# Usage: [n, m, p] = abcddim (a, b, c, d)
+#
+# Check for compatibility of the dimensions of the matrices defining
+# the linear system (a, b, c, d).
+#
+# Returns n = number of system states,
+#         m = number of system inputs,
+#         p = number of system outputs.
+#
+# Note: n = 0 (pure gain block) is returned without warning.
+#
+# Returns n = m = p = -1 if the system is not compatible.
+#
+# See also: is_abcd
+
+# Written by A. S. Hodel (scotte@eng.auburn.edu) August 1993.
+# a s hodel: modified to accept pure-gain systems aug 1996
+# $Revision: 1.15 $
+# $Log: abcddim.m,v $
+# Revision 1.15  1998-11-06 16:15:36  jwe
+# *** empty log message ***
+#
+# Revision 1.1.1.1  1998/05/19 20:24:05  jwe
+#
+# Revision 1.4  1997/12/01 16:44:22  scotte
+# *** empty log message ***
+#
+# Revision 1.3  1997/02/12 15:38:14  hodel
+# *** empty log message ***
+#
+#
+# fixed typo
+#
+# Revision 1.1  1997/02/12 11:34:53  hodel
+# Initial revision
+#
+# Revision 1.7  1997/02/07 15:29:56  scotte
+# fixed is_square check to allow for empty a matrix
+# (this allows for pure gain blocks)
+#
+
   if (nargin != 4)
-    error ("usage: abcddim (a, b, c, d)");
+    error ("abcddim: four arguments required");
   endif
 
   n = m = p = -1;
 
-  [an, am] = size(a);
-  if (an != am)
-    error ("abcddim: a is not square");
+  [a,an,am] = abcddims(a);
+  [b,bn,bm] = abcddims(b);
+  [c,cn,cm] = abcddims(c);
+  [d,dn,dm] = abcddims(d);
+
+  if ( (!is_square(a)) & (!isempty(a)) )
+    warning (["abcddim: a is not square (",num2str(an),"x",num2str(am),")"]);
+    return
   endif
 
-  [bn, bm] = size(b);
-  if (bn != am)
-    error ("abcddim: a and b are not compatible");
+  if( (bm == 0) & (dm == 0) )
+    warning("abcddim: no inputs");
+  elseif (bn != am)
+    warning (["abcddim: a(",num2str(an),"x",num2str(am), ...
+      " and b(",num2str(bn),"x",num2str(bm),") are not compatible"]);
+    return
   endif
 
-  [cn, cm] = size(c);
-  if (cm != an)
-    error ("abcddim: a and c are not compatible");
+  if( (cn == 0) & (dn == 0 ) )
+    warning("abcddim: no outputs");
+  elseif (cm != an)
+    warning (["abcddim: a(",num2str(an),"x",num2str(am), ...
+	" and c(",num2str(cn),"x",num2str(cm),") are not compatible"]);
+    return
   endif
 
-  [dn, dm] = size(d);
-  if (cn != dn)
-    error ("abcddim: c and d are not compatible");
+  have_connections = (bn*cn != 0);
+
+  if( (dn == 0) & have_connections)
+    warning("abcddim: empty d matrix passed; setting compatibly with b, c");
+    [d,dn,dm] = abcddims(zeros(cn,bm));
   endif
 
-  if (bm != dm)
-    error ("abcddim: b and d are not compatible");
+  if(an > 0)
+    [dn, dm] = size(d);
+    if ( (cn != dn) & have_connections )
+      warning (["abcddim: c(",num2str(cn),"x",num2str(cm), ...
+	" and d(",num2str(dn),"x",num2str(dm),") are not compatible"]);
+      return
+    endif
+
+    if ( (bm != dm) & have_connections )
+      warning (["abcddim: b(",num2str(bn),"x",num2str(bm), ...
+	  " and d(",num2str(dn),"x",num2str(dm),") are not compatible"]);
+      return
+    endif
+
+    m = bm;
+    p = cn;
+  else
+    [p,m] = size(d);
   endif
-
   n = an;
-  m = bm;
-  p = cn;
-
 endfunction