MATLAB: Does MATLAB print a special character ‘?’ when I read from the hardware device

arduinoh/wInstrument Control Toolboxpackageunique

I am connecting to a hardware device via a serial interface, writing and reading data to and from that device. Reading '80' from the device indicates successful communication.
Why does MATLAB print a special character with the data read from my device?
s = serial('COM4');
set(s,'Baudrate',19200,'Terminator','CR/LF');
fopen(s);
fprintf(s,'cmd401');
fscanf(s)
ans =
80

Best Answer

Using the 'unicode2native' function in MATLAB reveals that this special character has a decimal value 6, which corresponds to the Unicode 'Acknowledgement' control character.
The character that is being returned before '80' is the Acknowledgement Character(ACK), which is a commonly used signal in communication protocols that indicates successful receipt of the message.
While the '80' may be a user-configured success signal, the 'ACK' character is likely being generated by the device to indicate a successful action or the receipt of a command. The device documentation may be able to confirm this.
Since the 'ACK' is generated by the device, it may not be possible to suppress it but there are a number of ways to ignore it.
To receive the '80' without the 'ACK' character, try the following workarounds:
1) Use 'fscanf' to read just a single character in order to clear the 'ACK', and then read the remaining characters, separately. For example:
fscanf(s, '%c', 1); % read in the first character in the device's output buffer
fscanf(s) % read the remaining characters. This should return '80'.
2) Use indexing to trim the leading character from the character array you read from the device. For example:
cmd_success = fscanf(s);
cmd_success = cmd_success(2:end)
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