CODEWITHSUNDEEP
clinuxserial-port
arminb
arminb

Reputation: 2093

RS232 Serial Pin Read in C in Linux

Ist there any possibility to read values from the pins of the COM Port? Any solution in C under Linux is appreciated!

Upvotes: 0

Views: 2376

Answers (3)

unwind
unwind

Reputation: 399871

Yes, see for instance this guide.

You use the ioctl() function, to read the various control pins. Data is, of course, best read through the normal read() handling, you don't want to be polling asynchronuous serial data.

I don't think your assumption (expressed in a comment) that the driver must check the pin-states to handle data is correct, normally a serial port is "backed" by an UART and that typically handles the RX/TX pins in hardware.

Upvotes: 3

Mats Petersson
Mats Petersson

Reputation: 129374

The exact answer to this question depends on the precise hardware in question. I know of a piece of code where I worked, based on receiving the letter 'a' as the indication of bitrate, and it would poll the RX pin to detect the transitions between 0 and 1 to detect the "width" of the bits, and it would then calculate the correct clock-rate for the serial port and configure the serial port to match the bitrate of the other end.

A "PC" type hardware solution will not be able to read the RX/TX pins. In other hardware, it may be possible to do so. Many embedded systems allow various pins to be configured as inputs, outputs or "have a function" (in our case, RX, TX, CTS, RTS, etc) - so for example, you could configure the RX pin to be a input, and thus read the state of it. Of course, the normal serial port drivers will probably set these pins to "have a function" [or expect the boot code running before the kernel is started to have configure it this way]. So you would have to reconfigure the pins in some kernel code of your own, most likely. Beware that this may cause unexpected side-effects with the driver for the actual serial port - it may "get upset" when it tries to do things to the serial port and it's "not working as expected" because it's been "misconfigured".

You can almost certainly read (and/or write) the state of the control pins, such as CTS, RTS via IOCTL calls.

Upvotes: 1

Barath Ravikumar
Barath Ravikumar

Reputation: 5836

Am pretty sure , you can't read/write pins of UART. Even at the hardware level , you have to read/write an entire byte.There is no bit access or read/write pin access. The Byte is read/written in the Receive/Transmit UART buffer .

In either ways you can't just access the buffer directly , on your behalf the linux driver will do the job. You just have to make use of the driver in your application , to work with the UART , the linux driver for UART provides , standard API's like open(),read(),write(),ioctl() through which you interact the UART device.

If you want to work with drivers , and new to this field , the best place to start will be this book.

Upvotes: 0

Related Questions