How do I write a C-code for establishing serial communication in loopback mode.

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I want to write a c code to transfer data serially out from my laptop and receive it back on the same laptop.

I eventually want to read from a file (.dat file) and write it back to a new file (.dat file) .

And later make it work in real time that is : Through some external source i get my data (which is stored in a dat file previously) and directly stream my data to loopback without having the need to store it in a file and read from it.

What I have tried:

I have a usb to serial cable and I've connected the transmitter and receiver pins together for loopback. Do i need any more connection?

My code seems to transmit data fine (how can i be sure of that? ).
But is not reading it.

The flow of my program :
1) open port (read write): fd = open("/dev/ttyUSB0",O_RDWR|O_NOCTTY|O_NDELAY)
2) Set attributes of termios
3) Write data to Tx . (Data predefined in the code itself)
4) Read data :
while(1)
{ bytes_read =read(fd,&read_buffer,10);
if (bytes_read == -1)
switch(errno) {
case EAGAIN : continue;
default : goto quit;
}
}
The program is stuck in this loop.

Thank You.

Posted 8-Jun-16 19:29pm

Archit Mathur

Updated 8-Jun-16 19:54pm

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Sergey Alexandrovich Kryukov · Answer 1 · 2016-06-08T19:39:00

Solution 1

Please see, for example, this manual: Serial Programming Guide for POSIX Operating Systems.

—SA

Posted 8-Jun-16 19:39pm

Sergey Alexandrovich Kryukov

OriginalGriff · Answer 2 · 2016-06-08T19:54:00

"Do i need any more connection?"
Depends on how you have configured the port (and sometimes the hardware): for a "proper" loopback, it's a good idea to connect RTS to CTS (7 to 8 on 9pin serial connector), and DTR to DSR and DCD (1, 4, 6 on 9pin) to bypass any hardware handshaking, and to set RTS and DTR high.
"The program is stuck in this loop"
That may be because you are insisting it reads 10 bytes at a time - so if you only send 9, it will never get past the Read call.
I'd read byte-by-byte (especially in the beginning) and buffer up responses into a complete "command" rather than assuming it will be complete and correct at all times.
But...since you specify O_NDELAY when you open the port, if sufficient data isn't available (i.e all ten bytes) it will return EAGAIN immediately. Since Serial is generally slow (9600 baud is around 960 characters per second) and processors are fast, it's unlikely that all (or even any) of the data has looped back immediately after the write operation.

Start by checking your loopback works with a terminal program, then check your code by sending and receiving character by character.