receive data with specific format via serial communication

i'm controlling a motor working on serial communication. it send the feedback as ( #(motor ID)(command)(value)<cr>) and i need to onle extract the (value)

i used this (feedback = fscanf(s , '*1QD%u<cr>') and feedback = fscanf(s , '*1QD%u') ) to read the value of (motor with ID 1) and for the command of (QD) and this warning always appear :

Warning: Matching failure in format. The format specified for conversion, '*1QD%u', is incorrect.

is there's any other format should i use ?

Answer 1

[Walter Roberson]

In situations where you are reading data to end of line, it is good programming practice to fgets() or fgetl(), and then to manipulate the result, such as by sscanf() or textscan() or regexp() or regexprep() .

Serial ports should be assumed to drop characters or corrupt characters from time to time, and should typically be assumed to be connected to objects outside of your strict control that might enter unexpect states (such as being rebooted or encountering a power glitch).

You should never assume that a serial port is in the proper state: you should receive data and test whether it is acceptable for the current state, and process it if it is.

... And it is easier to debug if you do this.

[khaled saad]

so i should use it as :

s = serial('COM6' , 'BaudRate', 500000 ,'Terminator','CR');
fopen(s);
tline = fgets(s);
feedback = sscanf(tline , '*1QD%u');

is this right ? and how to do a test to check that the received data in the correct state ?

[Walter Roberson]

feedback would be empty if the sscanf() failed because the input did not have the expected format.

To be more correct, if you sscanf() and a format item does not match the input, then all of the format items matched to that point will be returned, giving you a result shorter than you expect. If you only have one format specification then that would be the same as feedback being empty, but if for example you had

S2 = '*1QD723 19';
S1 = '*1QD723 r19';
S0 = '*1QDm23 19';
sscanf(S2, '*1QD%u %u')
sscanf(S1, '*1QD%u %u')
sscanf(S0, '*1QD%u %u')

then you would get back 2, 1, and 0 results -- everything matched before the mismatch occurred.

[khaled saad]

thanks for your responce.

i need to know if i can do this using simulink ?. i used serial configuration block and serial send block but what is the input for the serial send block to send a command like ('#3D1200'). I tried from workspace block but it didn't work

[Walter Roberson]

It appears that in a relatively recent release (R2018a), strings have been implemented in Simulink:

https://www.mathworks.com/help/simulink/ug/simulink-strings.html

[khaled saad]

Now, I used compused string block to create the required format but when i tried to send it using serial send block i got an error that says the string isn't acceptable for serial send block. what can i do to send the exact formate without convert it to double or ASCII ?

[Walter Roberson]

You cannot. The Serial Send Block only accepts bytes. You should use a block to convert to ascii.

https://www.mathworks.com/help/simulink/slref/stringtoascii.html

[khaled saad]

You were right about that the serial communication corrupt characters from time to time. and i sometimes got this message will running the code :

Warning: A timeout occurred before the Terminator was reached.

'serial' unable to read all requested data. For more information on possible reasons, see Serial Read Warnings.

for the control algorithm this isn't allowed so i need to protect the process from such a corruption.For the same format everytime ('*3QD%d'), Is there's any way to be confident that the feedback value is always correct (sscanf() won't fail) ?