function [t,v,head] = isfread (filename)

% This function loads the binary data from a Tektronix ".ISF"
% file.  The ISF file format is used by newer Tektronix
% TDS-series oscilloscopes.
% 
% USAGE
%
% [x,y] = isfread(filename);
% [x,y,head] = isfread(filename);
%
% INPUT
%
% filename - name of Tektronix ISF file
%
% OUTPUT
%
% x - evenly spaced column-vector of x-values
% y - corresponding column-vector of y-values
% head - (optional) header record of file

FID = fopen(filename,'r');

hdata = fread(FID,511,'char')';			% read first 511 bytes
hdata = min(hdata,126);					% eliminate non-ascii
hdata = max(hdata,9);					% characters from header data
hdata = char(hdata);					% convert to character string

bytenum = getnum(hdata,'BYT_NR');
bitnum = getnum(hdata,'BIT_NR');
encoding = getstr(hdata,'ENCDG');
binformat = getstr(hdata,'BN_FMT');
byteorder = getstr(hdata,'BYT_OR');
wfid = getquotedstr(hdata,'WFID');
pointformat = getstr(hdata,'PT_FMT');
xunit = getquotedstr(hdata,'XUNIT');
yunit = getquotedstr(hdata,'YUNIT');
xzero = getnum(hdata,'XZERO');
xincr = getnum(hdata,'XINCR');
ptoff = getnum(hdata,'PT_OFF');
ymult = getnum(hdata,'YMULT');
yzero = getnum(hdata,'YZERO');
yoff = getnum(hdata,'YOFF');
npts = getnum(hdata,'NR_PT');

if ((bytenum ~= 2) | ...
	(bitnum ~= 16) | ...
	not(strcmp(encoding,'BIN')) | ...
	not(strcmp(binformat,'RI')) | ...
	not(strcmp(pointformat,'Y')))
  fclose(FID);
  error('Unable to process IFS file.');
end

switch byteorder
  case 'MSB'
    machineformat = 'b';
  case 'LSB'
    machineformat = 'l';
  otherwise,
    error('Unrecognized byte order.');
end

ii = strfind(hdata,'#');

fseek(FID,ii,'bof');					% advance to start of data
skip = str2num(char(fread(FID,1,'char')));
fread(FID,skip);
data = fread(FID, npts, 'int16', machineformat);
v = yzero + ymult*(data - yoff);
t = xzero + xincr*(0:npts-1)';

fclose(FID);

if (nargout > 2)
  head.bytenum = bytenum;
  head.bitnum = bitnum;
  head.encoding = encoding;
  head.binformat = binformat;
  head.byteorder = byteorder;
  head.wfid = wfid;
  head.pointformat = pointformat;
  head.xunit = xunit;
  head.yunit = yunit;
  head.xzero = xzero;
  head.xincr = xincr;
  head.ptoff = ptoff;
  head.ymult = ymult;
  head.yzero = yzero;
  head.yoff = yoff;
  head.npts = npts;
end


function z = getnum(str,pattern)
ii = strfind(str,pattern) + length(pattern);
tmp = strtok(str(ii:length(str)),';');
z = str2num(tmp);


function z = getstr(str,pattern)
ii = strfind(str,pattern) + length(pattern) + 1;
z = strtok(str(ii:length(str)),';');


function z = getquotedstr(str,pattern)
ii = strfind(str,pattern) + length(pattern) + 1;
z = strtok(str(ii:length(str)),'"');
z = strtok(str(ii:length(str)),'"');