function result=watlow96(command,value)

global  w96_s;

%This function controls the watlow 96 controller on the Cincinnati Sub-zero
%temperature chamber.  One must configure the following lines:

serial_id='COM1';
serial_baud=19200;

%This devices speaks something called MODBUS. Modbus has the following format:
% <slave address> <function field> <register field> <data field> <CRC>
% The funtion and data fields can vary in length.  The slave address is an
% 8 bit number of value 1-247; however, our Watlow 96 has an address of 1.
%
% from the document: Modbus_Application_Protocol_V1_1b.pdf if found that
% the function fields are as follow:
%   0x01 read coils
%   0x02 read discrete inputs
%   0x03 read holding register
%   0x04 read input register
%   0x05 write single coil
%   0x06 write single register
%   0x07 serial exception status
%   0x08 serial diagnostics 
% In matlab, the format for this becomes quirky, for instance:
% [1 3 00 100 0 1] would mean: to address 1, read, register number 100 (as
% 0+100), 1 (this seems to denote 1 decimal place, I sent 0 but it balked,
% but 1 works <shrug>).  The controller will then return the temperature.



if (nargin < 2) || isempty(value)
    value='none';
end
if (nargin < 1) || isempty(command)
    command='none';
end

switch command
    case 'init'
        
        switch value
            case 'close'
                if ~isempty(w96_s)
                    fprintf('free serial port\n');
                    fclose(w96_s);
                    w96_s=[];
                    result=0;
                else
                    fprintf('I could not free the serial resource because it was not allocated.\n');
                    result=-1;
                end
                
                
            otherwise  %connect if we shouldn't discoonect    
            if isempty(w96_s)   
               w96_s=serial(serial_id, 'BaudRate', serial_baud);
               %w96_s=serial('COM1', 'BaudRate', 19200); 
               fopen(w96_s);
            else
                fprintf('The serial device was already initialized...  Type watlow96(''init'',''close'') to disconnect.\n');
            end
                result = 0;
        end
    case 'read'
        if(isempty(w96_s))
           fprintf('Device is not open.  Type watlow96(''init'',''open'') to create the serial device. \n'); 
        end
        
        message=[1 3 00 100 0 1];
        crc=generateCRC(message);
        fwrite(w96_s,[message crc]);
        result = fread(w96_s,7);
        result=convert2dec(result(4:5))/10;
        fprintf('current temperature: %.2f C\n',result);

    case 'set'
        if strcmpi(value,'none')
            result = -1;
            fprintf('no temperature value given?\n');
        else
            setpoint=round(value*10);
            setpoint=convert2bytes(setpoint);
             message = [1 6 1 44 setpoint];  %register 300
             crc=generateCRC(message);
             fwrite(w96_s,[message crc]);
             %now that we wrote, we need to see what the response was.
             result=fread(w96_s,8);
             if(result(2)==6)
                % result
                 result=convert2dec(result(5:6))/10;
             
                 fprintf('setpoint read back as: %.2f C\n',result);
             else
                fprintf('unexpected result in sending command, packet:');
                result
             end
            
        end
    otherwise
        fprintf('unknown command, valid commands are:\nwatlow96(''init'',''open'') -- open the serial port\nwatlow96(''init'',''close'') -- close the serial port\nwatlow96(''read'') -- read current chamber temperature\nwatlow96(''set'',<temp>) -- set temperature\n')
        fprintf('an example would be:\nwatlow96(''set'',34.1) to set the chamber to 34.1C\n');
end

return



%utility functions.
function retval = convert2bytes(value)
    if value < 0
        ndnum=bitcmp(abs(value),16);
        value=ndnum+1;
    end

    bytes=dec2hex(value);

    if length(bytes) > 2
        byte1=bytes(1:length(bytes)-2);
        byte2=bytes(length(bytes)-1:length(bytes));
    else
    byte1=0;
    byte2=bytes;
end
retval=[hex2dec(byte1) hex2dec(byte2)];

return

function retval = convert2dec(value)
%make the return decimals.  Make a signed 16-bit number and return it.
cmd=[];
sp=dec2bin(value,8); %convert the arrya
numreg=size(sp);
for i=1:numreg(1)
    cmd=[cmd sprintf('sp(%d,1:8) ',i)];
end
bnum=eval(['[' cmd ']']);
dnum=bin2dec(bnum);

% negative?
if bitget(dnum,8*numreg(1))==1
    ndnum=bitcmp(dnum,8*numreg(1));
    dnum=-1*(ndnum+1);
end
    
retval=dnum;

return

function crc = generateCRC(message)
%this generates the checksum as described by the MODBUS docuemnt
if ~isnumeric(message)
    message=hex2dec(message);
end

pad=hex2dec('A001'); 
xd=hex2dec('FFFF');

for i=1:length(message)  
    xd=bitxor(message(i),xd);   
    for j=1:8 
        if bitand(xd,1) 
            xd=bitshift(xd,-1);
            xd=bitxor(xd,pad);
        else
            xd=bitshift(xd,-1);
        end
    end
end

xdb=dec2bin(xd,16);
crc(1)=bin2dec(xdb(9:16));
crc(2)=bin2dec(xdb(1:8));

return