Writing to a serial port in linux

Question

I am trying to program a robot that has motordrivers on certain serial ports (ie ttyS9)

Through cutecom (as hex input), I can send the following input, which gives me the result I expect:

5aaa0700fffff000

Now I am trying to achieve the same result with a C program, that does the following:

int port9 = open("/dev/ttyS9", O_RDWR | O_NONBLOCK);

char buff[17] = "5aaa0700fffff000";

write(port9, buff, 16);

I've also tried to initialize buff with the hex values seperately:

buff[0] = 0x5;
buff[1] = 0xa;

etc etc.

Both do not work. Is the problem in my code, or in the driver?

I compile using gcc and then run it with sudo. The open function also returns values that are proper (no errors), as well as the write.

Answer 1

Filling a character array with strings will convert characters to their ascii representation not hex as you need.

char buff[17] = "5aaa0700fffff000"; // Incorrect for saving 0x5aaa0700fffff000 to buff

As i see, you want to write 8 byte 0x5aaa0700fffff000 on serial port, a char is 8bit (1 byte) and you have to send 8 bytes not 16, so the code should be something like this

buff[0] = 0x5a;
buff[1] = 0xaa;
...
write(port9, buff, 8);

Answer 2

but maybe it's a number so you'll need unsigned long long int t= 0x5aaa0700fffff000; write(.., (char *)&t, 8); unsigned long long int t= 0x5aaa0700fffff000; write(.., (char *)&t, 8);

that means byte order in memory is completely different to one you'll gain with just assigning bytes as you read it from screen.