On 23/03/2012 5:03 PM, Dan Williams wrote:
On Fri, 2012-03-23 at 21:57 +0100, Søren holm wrote:
Hi
On Wintendo it is possible to specify the state of DTR/RTS in conjuction with
opening the port. How about using O_DIRECT or add a new openmode to tell the
driver to leave DTR/RTS as they are. That way it would not imply any change.
Actually when I first started, I assumed was doing it wrong, there must
be a way to open the port without altering it, I thought. Google-ling
around trying to find just the right parameter to pass along with open()
In the end I used:
fd = open(comPort,O_RDWR|O_NOCTTY|O_NDELAY)
Which unfortunately prevents me from knowing the original state of the
DTR/RTS.
I'd rather see sysfs attributes for the pin states in the tty's sysfs
dir instead of munging open() to do it.
Dan
Simply put I see 3 ways to resolve this:
1. Add a new parameter to the open() call so you can open the device
file like:
fd = open(comPort,O_NO_ALTER_DTR_RTS)
Pros: Would open access to elaborate programs written in C
Cons: Using TIOCMGET and TIOCMSET is outside the reach of a simple shell
script. I guess I could write a small wrapper in C to make it available
to script writers but this is not in keeping with the Unix/Linux philosophy.
2. Add a single file in sysfs something like:
/sys/class/tty/ttyUSB0/device/TIOCM
Pros: Access to all pins with a single read from a shell script.
Cons: The script needs to parse out the individual bits from the 1 byte
read. You would also need to read the state of DTR/RTS before you can
write a new value to /sys/class/tty/ttyUSB0/device/TIOCM if you want to
change only one of the two. Actualy this is currently the case with
TIOCMGET and TIOCMSET.
3. Add multiple files to sysfs something like this:
/sys/class/tty/ttyUSB0/device/RTS (rw, Request To Send)
/sys/class/tty/ttyUSB0/device/CTS (r, Clear To Send)
/sys/class/tty/ttyUSB0/device/DSR (r, Data Set Ready)
/sys/class/tty/ttyUSB0/device/DCD (r, Data Carrier Detect)
/sys/class/tty/ttyUSB0/device/DTR (rw, Data Terminal Ready)
/sys/class/tty/ttyUSB0/device/RI (r, Ring Indicator)
Pros: Very easy to script. Ability to set DTR directly without any
prior read.
Cons: Could have race conditions when needing to read multiple pins.
Being greedy, I would like to see all three solution implemented, but I
could live with just one.
Gérald
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