www.pudn.com > SerialGPS.zip > ReadData.m

function [ value, bytes ] = ReadData(gps, id, count, type) 
% ReadData Read formatted data from the GPS, managing DLE stuffing 
 
global pid_dle_byte pid_nak_byte 
 
sz = sizeof(type); % Number of bytes 
bytes = zeros(1, sz * count); 
n = 0; 
for i=1:count 
    value(i) = 0; 
    for j=1:sz 
        % Read a byte 
        byte = fread(gps, 1); 
        n = n + 1; 
        bytes(n) = byte; 
         
        % If we've read a DLE, there should be another one immediately 
        % following. If not, this data is corrupt. 
        if (pid_dle_byte == byte) 
            b = fread(gps, 1); 
            if (pid_dle_byte ~= b) 
                WritePacket(gps, pid_nak_byte, id); 
                error('Missing DLE stuffing byte'); 
            end 
        end 
         
        % Convert (if necessary) a multi-byte numeric value from the stream 
        % of bytes to the correct value. The GPS uses little-endian 
        % storage. 
        value(i) = bitor(value(i), bitshift(byte, 8*(j-1), sz*8));     
    end 
     
    % Correct for 2s complement negative numbers, if necessary. Only do 
    % this for integer types with the high order bit set. 
    if ((type(1) == 'i') && (byte >= 128) ) 
        value(i) = bitset(value(i), 32, 0); 
        value(i) = value(i) - (2^((sz*8)-1)); 
    end 
     
    % Interpret the bits as an IEEE floating point number (1 sign bit, 8 
    % exponent bits, 23 mantissa bits) 
    % 
    % If E=255 and F is nonzero, then V=NaN ("Not a number")  
    % If E=255 and F is zero and S is 1, then V=-Infinity  
    % If E=255 and F is zero and S is 0, then V=Infinity  
    % If 0 0) 
            f = f / (2^23) + 1; 
            value(i) = (2^(e-127)) * f; 
            if (s), value(i) = value(i) * -1;, end 
     elseif (e == 0 && f ~= 0) 
            f = f / (2^23); 
            value(i) = (2^-126) * f; 
            if (s), value(i) = value(i) * -1;, end 
        elseif (e == 0 && f == 0) 
            value(i) = 0; 
        end 
         
        % Convert to single precision 
        value(i) = single(value(i)); 
       
    end 
end