In this Makers Blog article we discuss the possibilities and advantages which a Com Server provides to the PLCnext platform.
The extensions modules that Phoenix Contact offers for its PLCs can for example be serial adapters like:
All extensions modules have on thing in common: they can be accessed cyclically via processdata. This provides the possibility to directly interact with the serial device from our realtime application.
In some cases this is not what we want. Instead, we want to re-use existing (C/C++) libraries and do not need the realtime context. However, we will find that these libraries usually are programmed to have direct access to a COM or TTY interface.
There are two options to create a TTY interface for a
- Implement some functions (C++ / C#) that create a TTY interface for the AXL or IL serial adapters from inside the
- Use of a COM-Server devices such as the GW Device Server
GW Device Server provides a transparent
TCP to Serial interface. With the help of a tool like
Socat we can forward the Serial/TCP traffic directly to a pseudo TTY interface. Using
GW Device Server also provides the possibility to communicate with the serial device on remote locations via a TLS encrypted channel.
In this tutorial, the PLC performs as client and establishes connection to a
GW Device Server.
(However, it is also possible to switch the PLC
Socat to a listening option and have the GW as client actively connect to the PLC.)
Setting up the GW Device Server
We connect to the
GW Device Server via the Web Based Management. The default IP address of the
GW Device Server is 192.168.254.254.
We use the following settings:
LAN Settings – Security
- Enable SSH Server
- Enable Telnet Server
- Disable Secure Data Mode
- Disable TFTP Server
- Port 1 Configuration (COM1): Enable TCP Connection on Port 8000
- Port 2 Configuration (COM2): Enable TCP Connection on Port 8001
Testing the GW Device Server
To test if we have set up the
GW Device Server correctly, first we connect our PLC to the
GW Device Server via Ethernet. Then we connect the two DSUB adapters of the
GW Device Server with each other.
We can quickly test the setup using the tool
Telnet. With this tool, we can send data from the PLC to the
GW Device Server at Port8000/COM1 through the serial cable to Port8001/COM2 and finally recieve it back at the PLC.
We use the following commands:
sudo passwd root
ip addr add 192.168.254.10 dev eth0
telnet 192.168.254.254 8000
## open another shell session
telnet 192.168.254.254 8001
# Type some messages
Socat easily on a Linux OS, we use the following commands:
git clone git://repo.or.cz/socat.git
## TODO: Modify to your SDK location
./configure $CONFIGURE_FLAGS --prefix=$(pwd)/install --enable-openssl-method
Deploying and installing Socat
To deploy the
Socat installation, we execute the following script:
mkdir -p deploy
mkdir -p usr
cp -r ../../Daemon/* .
cp -r ../socat/install/* usr/
# Create package
tar -cf ../socat-binaries.tar .
scp socat-binaries.tar email@example.com:~/
ssh -ttt firstname.lastname@example.org \
"mkdir -p /opt/plcnext/socat-binaries && \
tar -xf /opt/plcnext/socat-binaries.tar -C /opt/plcnext/socat-binaries"
After deploying, we can now ssh to our PLC. Therefore, we first prepare the PLC by setting up the directories and config files by running the following commands:
deploy$ ssh email@example.com
firstname.lastname@example.org:~$ su root
email@example.com$ mkdir -p /etc/default/socat
firstname.lastname@example.org$ touch /etc/default/socat/socat.conf
email@example.com$ touch /etc/init.d/SocatDaemon
Now, we modify the
socat.conf file to configure the parameters of the serial interface to our needs.
The content of the file looks like this.
If we need to modify further parameters, we will need to also modify the
SocatDaemon file. More information on TTY parameters can be found in the stty manual pages.
Now we create a
SocatDaemon file to automatically connect to the
GW Device Server using a
This SocatDaemon file is an example without error/disconnection management etc. please.
The file has the following content.(click to see/hide code)
Now that all config files are in place, we can proceed by setting up the
Socat binaries and registering the
chmod +x /etc/init.d/SocatDaemon
chmod +x usr/bin/*
cp -r usr /
update-rc.d -s -v SocatDaemon 99
If everything is set up correctly, we can see the running daemon using a command like
ps -e | grep socat. We can also check the
Socat logs located at
Accessing the TTY interface from a terminal
Now we can send messages over the TTY interface.
sudo echo "PLC to GW $var" > /dev/GWDeviceServer
To continuously test the pseudo TTY, we can use a command like
while true; do sleep 5 && sudo echo "PLC to GW $var" >/dev/ttyUSB0 && var=$((var+1)); done
Incoming traffic can be displayed by executing
cat /dev/GWDeviceServer. Now any application can access the Serial device connected to the GW Device Server as if it is directly connected to the PLC.