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Old 03-03-2018, 09:15 AM   #1
crabbit
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Registered: Feb 2018
Posts: 13

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Problem testing serial program I am writing in C


I have a small program that opens and listens to serial ports. I developed it on a virtual machine and to test it I opened terminal windows using \\.\pipe\com1 (/dev/ttyS0) etc. All this works flawlessly in my VM but when I moved my code over to raspian (Stretch) I am unable to send data using
Code:
screen /dev/ttyUSB1 19200.
My program sets /dev/ttyUSB1 to 19200 but stty reports it as 9600 even when my program is running.

I am not getting any errors when calling

Code:
infd = open (intty, O_RDWR | O_NOCTTY);
or
Code:
tcsetattr (infd, TCSANOW, &newtio)
with
Code:
newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;
(BAUDRATE = B19200)

Am I missing something that has to do with USB serial ports?

Last edited by crabbit; 03-03-2018 at 09:42 AM.
 
Old 03-03-2018, 02:24 PM   #2
crabbit
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Registered: Feb 2018
Posts: 13

Original Poster
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One more piece of information. I am able to write but unable to read from the port,

Code:
int main() {
  char byte;
  char buf[100];
  buf[0] = 0;
  int fd = open("/dev/ttyUSB1", O_RDWR);
  if (!fd) printf("Error: %d\n", errno);
  int ret = write(fd, "Test", 4);
  printf("%d\n", ret);
  ret = read(fd, buf, 4);
  printf("error %d\n", errno);
  printf("%d\n", ret);
  buf[5] = 0;
  printf("%s\n", buf);
  return 0;
}
Code:
4
error 0
0
Here is my stty
Code:
pi@raspberrypidev:~/src $ sudo stty -F /dev/ttyUSB1 -a
speed 19200 baud; rows 0; columns 0; line = 0;
intr = <undef>; quit = <undef>; erase = <undef>; kill = <undef>; eof = <undef>;
eol = <undef>; eol2 = <undef>; swtch = <undef>; start = <undef>; stop = <undef>;
susp = <undef>; rprnt = <undef>; werase = <undef>; lnext = <undef>;
discard = <undef>; min = 0; time = 5;
-parenb -parodd -cmspar cs8 -hupcl -cstopb cread clocal crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff
-iuclc -ixany -imaxbel -iutf8
-opost -olcuc -ocrnl -onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt
-echoctl -echoke -flusho -extproc

Last edited by crabbit; 03-03-2018 at 02:26 PM.
 
Old 03-04-2018, 12:34 AM   #3
NevemTeve
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Registered: Oct 2011
Location: Budapest
Distribution: Debian/GNU/Linux, AIX
Posts: 3,641

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Your error-checks are broken: open(2) returns -1 on error; read(2) returns -1 on error, 0 on end-of-file, positive value means length of the data read in.
Also you left out the switch to raw mode https://www.systutorials.com/docs/li...n/3-cfmakeraw/
Nonethess, you still read 4 bytes, but they might be non-printable characters. Use printf's %x in a loop.

Last edited by NevemTeve; 03-04-2018 at 12:39 AM.
 
Old 03-04-2018, 01:14 PM   #4
crabbit
LQ Newbie
 
Registered: Feb 2018
Posts: 13

Original Poster
Rep: Reputation: Disabled
Quote:
Originally Posted by NevemTeve View Post
Your error-checks are broken: open(2) returns -1 on error; read(2) returns -1 on error, 0 on end-of-file, positive value means length of the data read in.
Also you left out the switch to raw mode https://www.systutorials.com/docs/li...n/3-cfmakeraw/
Nonethess, you still read 4 bytes, but they might be non-printable characters. Use printf's %x in a loop.
I corrected the error check and there is no error opening /dev/ttyUSB1.
the write returned that 4 bytes where written.
the read returned 0 bytes where read.
no error with either write or read.

the stty is for /dev/ttyUSB1 after cfmakeraw()

I am getting some more serial cables and null modem adapters to test this using a seperate system rather than screen() or a test program running on the same system.
 
1 members found this post helpful.
Old 03-06-2018, 01:36 PM   #5
crabbit
LQ Newbie
 
Registered: Feb 2018
Posts: 13

Original Poster
Rep: Reputation: Disabled
Quote:
Originally Posted by crabbit View Post
I corrected the error check and there is no error opening /dev/ttyUSB1.
the write returned that 4 bytes where written.
the read returned 0 bytes where read.
no error with either write or read.

the stty is for /dev/ttyUSB1 after cfmakeraw()

I am getting some more serial cables and null modem adapters to test this using a seperate system rather than screen() or a test program running on the same system.
Once I connected my laptop to the PI I was able to sucessfully test my program. I suspect that however you write to /dev/ttyUSBx you cannot read "that" data back unless the port is set up for loop back because all programs are running on the host side of the UART.
 
  


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