Why does MATLAB print a special character '?' when I read from my hardware device?

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

Answer 1

[MathWorks Support Team]

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)