cant read the serial port!!!

want to read information from serialport but it says it need fopen, but serialport is the fopen,

also this is a gui

When button pressed

app.sp = serialport('COM3', 9600);

while ~feof(app.sp)
    spLine = fgetL(app.sp);
    spData = sscanf(spLine, '%f V, %i counts, %i ms');

Errors:

Error using feof: Invalid file identifier. Use fopen to generate a valid file identifier.

as well as

Error using serialport (line 116)
Unable to connect to the serialport device at port 'COM3'. Verify that a device 
is connected to the port, the port is not in use, and all serialport input arguments 
and parameter values are supported by the device

[Geoff Hayes]

@SETH GERSBACH - is feof supported for serial ports? You will need to resolve the error for the serial port call. Has this ever worked? Are you passing the correct parameters?

[SETH GERSBACH]

The serial port has worked before it pops up randomly, I think I just need to clean them

I was informed that serialport = read

Beyond that the code won’t work because feof wants a read, but serialport does the reading apparently and when adding a read it still won’t work

[Geoff Hayes]

@SETH GERSBACH - you will need to clarify what you mean by and when adding a read it still won’t work.

Answer 1

[Walter Roberson]

https://www.mathworks.com/help/matlab/ref/serialport.html

feof() is not defined for serialport objects. It was defined for serial() objects.

In order for feof() to be meaningful, serialport objects would need to have an "open" state, with feof set when it is closed. But serialport does not have an open state.

Remember that serial ports are asynchronous, and the serial protocol has no inherent timeout, and no "keepalive", and there is no way to send data that inherently means "end of file" for the serial protocol.

What rs232 does define is the possibility of reading the Carrier Detect pin. However, Carrier Detect dropping is not the same thing as end of file. Carrier Detect was designed around modems, and since the Hayes Smart Modem protocol was created (AT command set), the rule has always been that CD is false in the period before a connection is made, when you are talking to the modem configuring it, and becomes true when the modem frequency / handshakes are completed and the path to transmit data is ready. Lack of CD does not inherently mean that the serial port is EOF.

CD is not pulled true in rs232. When you are working with a non-modem (such as serial over USB), then CD will just be low. It would break the majority of modern uses of COM ports to have low CD automatically mean EOF.

So what can you do? Well, you can https://www.mathworks.com/help/matlab/ref/serialport.getpinstatus.html check the pin status. But unless you are using a modem, expect CD to be false.

rs232 just does not have any meaningful FEOF at the protocol level.

There certainly might be a defined end of file at a higher level. For example if you were running SLIP or PPP then it could make sense for the serial packets to be carrying a RST for a TCP connection being carried. But that involves a software layer receiving the raw bytes and decoding them (and in some cases doing decompression) and making the decoded bytes available... which is not at the serialport level.

Your communications are incorrectly designed if you expect feof on a serialport.