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Old 09-19-2006, 11:44 AM   #1
Registered: Feb 2006
Location: Chile
Distribution: Slackware 10.2
Posts: 53

Rep: Reputation: 15
managing termios for ser ports

I am trying to follow an example for simple serial port IO. ttyS0 for example.

I have a problem with the termios values particularly c_cflag and or cfsetispeed / cfsetospeed.

For the funtion cfsetispeed for example, it refers to a value like B38400, which they say is defined as 38400, or, #define B38400 38400. But using the other method to set termios.c_cflag it's supposed to be an OR of varios controls, like ( CS8 | B38400 | CLOCAL | CREAD ) for instance. Here obvious B38400 = 38400 is obviously not compatible to this flag setting. The bits would conflict with others.

In my termbits.h it says the following
#define B38400 00000017 !!!!
NOTE it does NOT say
#define B38400 0x00000017

If I use cfsetispeed (with B38400) c_cflag is not changed ( using 38400 or 0x00000017 or B_38400 ).

I cannot make the serial port work, even though the "open" returns a good fd.

What is going on??

Old 09-20-2006, 04:30 AM   #2
Wim Sturkenboom
Senior Member
Registered: Jan 2005
Location: Roodepoort, South Africa
Distribution: Slackware 10.1/10.2/12, Ubuntu 12.04, Crunchbang Statler
Posts: 3,786

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B38400 not defined as 38400. It's 'something' that reflects the baudrate 38400. The value is 15 which matches your termbits.h (00000017 is an octal value, so 1*8 + 7 = 15). You can print the value with a simple printf.

I've posted an init routine that I usually use in this thread
Old 09-21-2006, 07:37 AM   #3
Registered: Feb 2006
Location: Chile
Distribution: Slackware 10.2
Posts: 53

Original Poster
Rep: Reputation: 15

Thanks for the clarification. It hadn't occured to me that it was octal, and some other references that I had seen indicated that B38400 defined directly to 38400(??) so I was left with a confusion. Hex ( or Octal ) made more sense to me. I'm not at my office today, but when I get back I'll take another look.

Thanks for the in depth response.



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