reading with serial port in matlab

Hello, I want to start the serial comport communication so as to read the continuous incoming data from serial comport which will be transmitting the a single sample in every 5ms uisng baudrate of 9600.The single samle will consists of 6ASCII characters(4 ASCII charcters+CR+CL) which will be transmitted in 5ms. I know there are so many available threads about this from which I found some related as here

http://www.mathworks.se/matlabcentral/answers/97373-is-it-possible-to-read-from-or-write-to-a-serial-port-with-matlab

http://www.mathworks.se/matlabcentral/answers/72392-serial-only-reads-one-string I used

s1 = serial('COM2', 'Baudrate', 9600, 'Terminator', 'LF');
fopen(s1);

But I don't understand how to move forward after this if I want to collect the 200 samples(1 sample in 5ms 200 makes 1 sec) and display the plot of the collected data after every 1 sec using the continuous incoming serial values.

Thanks.

Answer 1

[Walter Roberson]

You should probably be using 'CR/LF' as the terminator.

Do you want to collect 200 samples exactly, or do you want to collect 1 second's worth?

If you want 200 samples, then see http://www.mathworks.com/help/instrument/bytesavailablefcn.html -- set the bytesavailablefcncount to 200*6 and the mode to byte and set bytesavailablefcn to a routine that will fread() the 200*6 bytes, decode the sequences, store the data into a cumulative buffer if required, and plot the new data.

If you want 1 second's worth, set the bytesavailablefcncount to infinity, the mode to byte, and no bytesavailablefcn callback. Then create a timer() object that fires every 1 seconds. The timer routine should grab the bytesavailablecount (no 'fcn' in that name), and fread() that many bytes, decode the sequences, store the data into a cumulative buffer if required, and plot the new data.

Either way you should be sure to set the InputBufferSize to several times one second's-worth of data, so that data can be accumulating in the buffer while the callback is processing and plotting.

Note: it will be notably more overhead to attempt to read and process each packet as it comes in. Unless you need to control something in real time, try to read several samples at once.

Note: if your serial port is "serial over usb" then you are going to encounter additional timing constraints and might not be able to sample more than 50 Hz unless the driver on the far end is smart about sending the bytes over the USB.

[Stefan]

Thanks Walter but ended here

s = serial('COM1', 'Baudrate', 9600, 'Terminator', 'CR/LF');
s.BytesAvailableFcnCount =1200 ;
s.BytesAvailableFcnMode = 'byte';
s.BytesAvailableFcn = "  "; 
    plot(data);

I was unable to trsanfer this into matlab code "bytesavailablefcn to a routine that will fread() the 200*6 bytes, decode the sequences, store the data into a cumulative buffer if required, and plot the new data".

Coming to the "serial over usb" I am using UART transmission and it is working fine uto the UART speed of 115200 as checked it by displaying on the hyperterminal.

[Walter Roberson]

You have not indicated anything about the format or interpretation of the bytes of data, so I can't tell you yet how to convert the characters into plottable values.

[Stefan]

I think you are asking about the way I need to collect and display.If so,I want to collect 200 samples and display them. Thankyou.

[Walter Roberson]

Could you give a couple of lines of samples, and indicate for each line what numeric value the text should be interpreted as?

For example, the device might be transmitting '8F3C' as the text for a sample, that that might represent -(15 + 60/256)

[Stefan]

yes Walter,

The sample obtained will be converted to ASCII and transmitted through the UART so that the original value will be printed on the hyperterminal

For example if obtained data is 3421 then it will send in this flow 3 + 0x30(for ASCII conversion) then 4+0x30 then 2+0x30 and atlast 1+0x30.OS that the hyeperterminal will show the value 3421 then adding two ADCII characters CR+LF to print next coming in newline and form starting position. The data on the hyperterminal will appear like this 3421 3989 4321 etc...

[Walter Roberson]

Okay, and value to be submitted to plotting would be 3421 decimal ?

If so then

   new_data_decimal = sscanf( fread(s, s.BytesAvailable), '%f' );

you can then plot new_data_decimal. I will switch to your interface question to give some more hints on that.

[Stefan]

Walter,I tried to plot continuously after getting 500samples like this using a baudrate of 115200 for 500 samples(500*6 ASCII)=3000.

s = serial('COM1', 'Baudrate', 115200, 'Terminator', 'CR/LF');
s.BytesAvailableFcnCount =3000 ;
s.BytesAvailableFcnMode = 'byte';
while(1)
new_data_decimal = sscanf( fread(s, s.BytesAvailable), '%f' );
    plot(new_data_decimal);
end;

But it is showing some error like this

"Error using serial/fread (line 131) SIZE must be greater than 0.

Error in serial_example (line 5) new_data_decimal = sscanf( fread(s, s.BytesAvailable), '%f' );"

I want to update the plot continuously after collecting 500samples. Thanks.

[Walter Roberson]

s.BytesAvailableFcn = @(evt, src) plot( sscanf( fread(s, s.BytesAvailable), '%f' ) );

then remove the while(1) loop completely, so that the drawing is triggered only by the bytes available fcn callback.

Notice you did not give any drawnow() or pause(), so you did not give any opportunity for the screen to be updated. The easiest way to add that in is to make the callback function a full function rather than an anonymous function, and pass the axes in

function update_plot(S, evt, thisax)
  plot( thisax, sscanf( fread(s, s.BytesAvailable), '%f' ) );
  drawnow();

And then the callback assignment would become

s.BytesAavailableFcn = @(src, evt) update_plot(src, evt, thisax);

where thisax is the axes earlier created to plot into, as show in your other thread.

You can mostly get away with relying on the default axes instead of passing it around, but your plotting will go wrong if the user clicks on any window other than the one being plotted into, so it is safer to pass the axes.

[Stefan]

Hello walter,

I made below code while uisng thisax form the other thread

fignum = figure('Units', 'norm', 'Position', [.2 .2 .7 .7]); 
thisax = axes('Units', 'norm', 'Position',[0.1 0.2 0.8 0.5]);
box1 = uicontrol('Style', 'text', 'Units', 'norm', 'Position', [.3 .8 .15 .05], 'String', 'Peaks');
box2 = uicontrol('Style', 'text', 'Units', 'norm', 'Position', [.45 .8 .15 .05]);
s = serial('COM8', 'Baudrate', 115200, 'Terminator', 'CR/LF');
s.BytesAvailableFcnCount =3000 ;
s.BytesAvailableFcnMode = 'byte';
s.BytesAvailableFcn = @(src, evt) update_plot(src, evt, thisax);

and function

function update_plot(s, evt, thisax)
  plot( thisax, sscanf( fread(s, s.BytesAvailable), '%f' ) );
  drawnow();
end

But this is only showing the window with a empty box for the plot and peaks(anyway this will be updated later) but it is not plotting the values from serial in the plot box.But the serial port is continuously sending data. Also how to save the sample number and its corresponding sample value in a array because I want to check a condition once the 500 samples are available like how many values(this gives the peaks) are > a threshold in 500 samples. Thanks.

[Walter Roberson]

First thing would be to check whether the update_plot function is being called at all. Put a breakpoint there, or put a disp() statement saying that it is called. You might want to break the function up into more statements so you can debug further, such as saving the result of fread() into a variable, displaying the length of the variable, and so on.

[Stefan]

I guess the code is not reading the comport or something because even when the serial cable is not pluggedin(connected) it is displaying the figure with empty plot and peaks block. And it is not calling the update_plot function as verified this adding a disp('Called') in this fucntion. Is there anything that I am missing with serial reading part.

[Walter Roberson]

I do not recall asking before: have you verified the communications using something like hyperterm ?

For testing purposes have you tried switching to byte mode with a count of 1 and terminator set off, just to see if you can get bytes through?

Does the device need hardware flow control?

[Stefan]

I just checked with the hypertermianl and it was displaying the ADC values. Walter I don't know how to implement this "switching to byte mode with a count of 1 and terminator set off, just to see if you can get bytes through". What is hardware flowcontrol of device? Can you explain me the things. Thanks.

[Walter Roberson]

Set up a little test code that opens the device with Terminator '' and BytesAvailableFcnMode = 'byte' and BytesAvailableFcnCount 1 and a small callback such as

@(src, evt) fprintf(2, '%02x ', fread(src, 1) );

and see if you get data through. This would be for testing if the problem is the terminator.

Hardware flow control involves additional pins on the RS232 cables, with the sender not being allowed to send data until the receive signals (on pins) that it is read to receive information. The default is usually for the signal to be off (do not send data). To experiment with that set 'FlowControl', 'hardware' when creating the connection.

[Stefan]

I used like this

s = serial('COM1', 'Baudrate', 115200,'Terminator', '');
s.BytesAvailableFcnCount =1;
s.BytesAvailableFcnMode = 'byte';
s.BytesAvailableFcn = @(src, evt)fprintf(2, '%02x ', fread(src, 1) );

But I was unable to see anything as the output. I am not using RS232 instead using USB to Mini usb from microntroller to the PC and communication is done through UART.I am receving evrything perfectly on hyperterminal.Can you provide any solutions for this.

Thanks

[Stefan]

Hello Walter, Can you help me out with this issue of collecting data through serial communication into matlab

[Walter Roberson]

Please try with FlowControl set to Hardware.

[Stefan]

Hi Walter, I tried like this by using hardware flowcontrol

clear all;
  clc;
  s = serial('COM8','FlowControl','hardware', 'Baudrate', 115200,'Terminator', '');
  s.BytesAvailableFcnCount =1;
  s.BytesAvailableFcnMode = 'byte';
  s.BytesAvailableFcn = @(src, evt)fprintf(2, '%02x ', fread(src, 1) );

I am unable to see anything.But when I checked with the termianl part it displaying as shown Here Could please just check what's wrong with it.

[Walter Roberson]

I do not see anything obviously wrong. My reaction at the moment (and I haven't had any coffee yet this month) is to check the baudrate against both ends.

[Stefan]

I am using the same baudrate on the both ends.For the hyperterminal printing I am using 115200 baudarate as shown in the image here.