RS232 with MATLAB: Doesn't send what I want

Hello everyone! I am using Matlab to communicate with a device on a RS232 connexion. My code is quite simple: I first open the correct COM port: obj=serial('COM3','BaudRate',9600); fopen(obj);

And send I send various ASCII commands. For example: fprintf(obj,'%s\n',[char(hex2dec('A1'))])

I have a sniffer on the RS232 port, so I can make sure I am sending the right command. in this case 'A1'.

However, sometimes when I try to send '8B' it sends '3F' instead (shown by the sniffer): fprintf(obj,'%s\n',[char(hex2dec('8B'))])

This is bizarre, because sometimes it works perfectly fine and for some ASCII characters, it doesn't work!

Do you know why this happens? And what can I do to avoid this? Are there other alternatives to communicate on RS232 serial port with MATLAB?

Thank you in advance for your response and your time!

[Julia]

ok so I understood something. When the characters I send are larger than 127 (so when the 8th bit is a 1: ex. 10000000 and larger), Matlab doesn't send the right number. This is why everything worked well for most of the characters, but as soon as I send a value larger than 127 (80 in hex) it didn't work properly.

Does someone know how I can avoid this? I guess Matlab considers the 8th bit as a sign or something...

Thank you!