Serial port reading function fread(obj) is giving output empty matrix 1 by 0

i have a query regarding reading of serial port data. i am using a usb rs232 cable and has also been detected by matlab instrhwinfo.

actually, i just need to trigger a matlab program when the pin-2 (rx) of rs232 recieves a high voltage. can i do this by just connecting a battery power supply to pin-2(rx) and ground to pin-5(GND) of rs232 without the use of any microcontroller, pic,etc. can or cannot, i have tried this, and did not get an output as 1 or any other output.

when i typed fread(obj), output is empty matrix 1 by 0 and when other reading functions like fscanf, fgetl, get, i got an empty string.

i dont know where am i going wrong?

[SRI]

Ayush could you send the code so that it is easy to find the Bug inside the code

[Ayush]

on command window, first i got the list of serial ports on my computer

instrhwinfo('serial')

obj=serial('COM18')  

 % in device manager i got to know that rs232 cable is listed as COM18

fopen(obj); 

fread(obj) % or get(obj)  or  fgetl(obj) or  fscanf(obj)

output here is: empty matrix 1-by-0

i dont even know that the matlab is detecting the inputs of the rs232 or not

also i think there is no need to set a new baud rate or any other parameter other than default, because i only need to sense any message other than NULL

Answer 1

[Walter Roberson]

If your baud rate is wrong then the serial port will not be able to properly sense the message.

Have you considered wiring to CD (Carrier Detect) instead of Receive ? Then you can query the pin status; see http://www.mathworks.com/help/matlab/matlab_external/pinstatus.html

[Ayush]

I think i am wrong with the input voltage levels of rs232. It requires -3 to - 25 V for signal 1. It would require max232 ic for changing voltage levels. I have tried communicating with real term terminal software and shorted the rx and tx pin, pin indicator was blinking. I think i should go with the voltage levels first.

I am doing this changing voltage levels stuff. But, do i have to set a baud rate compulsorily.

And also, is there a need to short some pins of rs232.

[Walter Roberson]

I suggest +5V, it should be enough for all modern receivers.

You will not be able to detect characters using the shorting scheme. You might be able to detect BREAK. But Carrier Detect would be a lot easier, and can be done by shorting as well.