Serial communication : data missing with readasync

I'm writing data to serial port with fwrite function and would like to read the answer with readasync function. My code is :

wanted_position1 = 240;
fwrite(s, [170;0;wanted_position1],'async');   
pause(1)
readasync(s)
while strcmpi(get(s, 'TransferStatus'), 'read') % wait till the read asynchronous operation is finished
    pause(0.1)
end

while s.BytesAvailable>0 % read the data coming from serial port
    read_position = fscanf(s)
end

The problem is that the value of s.ByteAvailable is lower than expected and I don't receive all the data sent by the device connected to the serial port. For example instead of having : read_position = '100,110,120,130,140,150,160,170,180,190,200,210,220,230,240,' I get : read_position ='190,200,210,'

I've also noticed that the value of s.ByteAvailable is changing from an execution to another : sometimes it is equal to zero and I get no data.

[Michael McKinney]

I'm having this same issue. Did you ever find a solution

Answer 1

[Walter Roberson]

I do not recommend readasync() because the transfer is not started until the readasync() call -- so anything that was sent after the prior read might or might not happen to still remain within the internal buffers.

The hardware buffer capacity of cheap serial ports used to be as little as 1 byte, with up to 16 bytes for more expensive (but not unreasonable) boards -- and considerably larger buffers for fairly expensive boards.

When you use readasync() whatever was in transit will have been discarded, except for whatever is left in the hardware buffers.

You are better off using bytes received function callback: when you use that, then the model is that MATLAB itself will take care of collecting the data in the buffer in the background, and will invoke the callback when the buffer is full or the terminator character is reached or there is a timeout.