Virtual Serial Port for Linux

Question

I need to test a serial port application on Linux  however  my test machine only has one serial port    Is there a way to add a virtual serial port to Linux and test my application by emulating a device through a shell or script   Note  I cannot remap the port  it hard coded on ttys2 and I need to test the application as it is written

User · Accepted Answer

You can use a pty   pseudo-teletype   where a serial port is a  real teletype   for this   From one end  open  dev ptyp5  and then attach your program to  dev ttyp5  ttyp5 will act just like a serial port  but will send receive everything it does via  dev ptyp5   If you really need it to talk to a file called  dev ttys2  then simply move your old  dev ttys2 out of the way and make a symlink from ptyp5 to ttys2   Of course you can use some number other than ptyp5   Perhaps pick one with a high number to avoid duplicates  since all your login terminals will also be using ptys   Wikipedia has more about ptys  http   en wikipedia org wiki Pseudo terminal

User · Answer

I can think of three options   Implement RFC 2217  RFC 2217 covers a com port to TCP IP standard that allows a client on one system to emulate a serial port to the local programs  while transparently sending and receiving data and control signals to a server on another system which actually has the serial port   Here s a high-level overview   What you would do is find or implement a client com port driver that would implement the client side of the system on your PC - appearing to be a real serial port but in reality shuttling everything to a server   You might be able to get this driver for free from Digi  Lantronix  etc in support of their real standalone serial port servers   You would then implement the server side of the connection locally in another program - allowing the client to connect and issuing the data and control commands as needed   It s probably non trivial  but the RFC is out there  and you might be able to find an open source project that implements one or both sides of the connection   Modify the linux serial port driver  Alternately  the serial port driver source for Linux is readily available   Take that  gut the hardware control pieces  and have that one driver run two  dev ttySx ports  as a simple loopback   Then connect your real program to the ttyS2 and your simulator to the other ttySx   Use two USB lt -- Serial cables in a loopback  But the easiest thing to do right now   Spend  40 on two serial port USB devices  wire them together  null modem  and actually have two real serial ports - one for the program you re testing  one for your simulator   -Adam

User · Answer

Would you be able to use a USB- RS232 adapter  I have a few  and they just use the FTDI driver  Then  you should be able to rename  dev ttyUSB0  or whatever gets created  as  dev ttyS2

User · Answer

Use socat for this   For example   socat PTY link  dev ttyS10 PTY link  dev ttyS11

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You may want to look at Tibbo VSPDL for creating a linux virtual serial port using a Kernel driver -- it seems pretty new  and is available for download right now  beta version   Not sure about the license at this point  or whether they want to make it available commercially only in the future   There are other commercial alternatives  such as http   www ttyredirector com    In Open Source  Remserial  GPL  may also do what you want  using Unix PTY s  It transmits the serial data in  raw form  to a network socket  STTY-like setup of terminal parameters must be done when creating the port  changing them later like described in RFC 2217 does not seem to be supported  You should be able to run two remserial instances to create a virtual nullmodem like com0com  except that you ll need to set up port speed etc in advance   Socat  also GPL  is like an extended variant of Remserial with many many more options  including a  PTY  method for redirecting the PTY to something else  which can be another instance of Socat  For Unit tets  socat is likely nicer than remserial because you can directly cat files into the PTY  See the PTY example on the manpage  A patch exists under  contrib  to provide RFC2217 support for negotiating serial line settings

User · Answer

There is also tty0tty http   sourceforge net projects tty0tty  which is a real null modem emulator for linux  It is a simple kernel module - a small source file  I don t know why it only got thumbs down on sourceforge  but it works well for me  The best thing about it is that is also emulates the hardware pins  RTC CTS DSR DTR   It even implements TIOCMGET TIOCMSET and TIOCMIWAIT iotcl commands  On a recent kernel you may get compilation errors  This is easy to fix  Just insert a few lines at the top of the module tty0tty c source  after the includes    ifndef init MUTEX  define init MUTEX x  sema init  x  1   endif  When the module is loaded  it creates 4 pairs of serial ports  The devices are  dev tnt0 to  dev tnt7 where tnt0 is connected to tnt1  tnt2 is connected to tnt3  etc  You may need to fix the file permissions to be able to use the devices  edit  I guess I was a little quick with my enthusiasm  While the driver looks promising  it seems unstable  I don t know for sure but I think it crashed a machine in the office I was working on from home  I can t check until I m back in the office on monday  The second thing is that TIOCMIWAIT does not work  The code seems to be copied from some  quot tiny tty quot  example code  The handling of TIOCMIWAIT seems in place  but it never wakes up because the corresponding call to wake up interruptible   is missing  edit  The crash in the office really was the driver s fault  There was an initialization missing  and the completely untested TIOCMIWAIT code caused a crash of the machine  I spent yesterday and today rewriting the driver  There were a lot of issues  but now it works well for me  There s still code missing for hardware flow control managed by the driver  but I don t need it because I ll be managing the pins myself using TIOCMGET TIOCMSET TIOCMIWAIT from user mode code  If anyone is interested in my version of the code  send me a message and I ll send it to you

User · Answer

Using the links posted in the previous answers  I coded a little example in C   using a Virtual Serial Port  I pushed the code into GitHub  https   github com cymait virtual-serial-port-example    The code is pretty self explanatory  First  you create the master process by running   main master and it will print to stderr the device is using  After that  you invoke   main slave device  where device is the device printed in the first command   And that s it  You have a bidirectional link between the two process   Using this example you can test you the application by sending all kind of data  and see if it works correctly   Also  you can always symlink the device  so you don t need to re-compile the application you are testing

User · Answer

Complementing the  slonik s answer   You can test socat to create Virtual Serial Port doing the following procedure  tested on Ubuntu 12 04    Open a terminal  let s call it Terminal 0  and execute it   socat -d -d pty raw echo 0 pty raw echo 0   The code above returns   2013 11 01 13 47 27 socat 2506  N PTY is  dev pts 2 2013 11 01 13 47 27 socat 2506  N PTY is  dev pts 3 2013 11 01 13 47 27 socat 2506  N starting data transfer loop with FDs  3 3  and  5 5    Open another terminal and write  Terminal 1    cat  lt   dev pts 2   this command s port name can be changed according to the pc  it s depends on the previous output   2013 11 01 13 47 27 socat 2506  N PTY is  dev pts   2   2013 11 01 13 47 27 socat 2506  N PTY is  dev pts   3   2013 11 01 13 47 27 socat 2506  N starting data transfer loop with FDs    you should use the number available on highlighted area   Open another terminal and write  Terminal 2    echo  Test   gt   dev pts 3   Now back to Terminal 1 and you ll see the string  Test

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