VIRTUAL SERIAL PORT TECHNOLOGY BASICS

A « virtual serial port » is a software function ( a driver indeed ) emulating a hardware serial port, and used to exchange data in the respect of some serial communication protocol, in order to redirect these data towards a terminal using another data transmission technology. According to the automation field, this functionality can be useful when the virtual serial port redirects the data through an Ethernet port.

A virtual serial port can be used as a communication interface when :

• One of the connected devices ( software such as configuration tool or hardware such as controller or sensor ) can only communicate by using a point to point serial interface such as RS 232 or USB standard.
• And when one of these situations occurs ( this list is not exhaustive ) :
  • The distance between the connected devices is rather large.
  • It is physically hard to access one of the device.
  • The other connected device does not provide a hardware serial port.

To illustrate this, let's take as an example a software used to diagnose a measurement device, which can connect by using a RS232 line only in order to access data into a « time domain reflectometry » ( TDR ) radar level sensor, such as the « Optiflex 1300C » HART sensor from the « Krohne » company ( what imposes to install a HART modem as a communication interface with the sensor ).

Let's take as an hypothesis you are operating at a client's office in Sweden ( Hallå, hallå ! ), and that you wish to establish the diagnostic of a remote sensor located on a site of our friends Maori ( Kia ora, kia ora ! ) in New Zealand, near Auckland.

It is not reasonable to want to interconnect the devices by installing a RS 232 line or an USB line of about 20 000 kms length.

One available solution can be using the services of virtual serial ports :

• Which are installed as client into the device initiating the requests.

  ( In this example, it is the diagnostic software running into the maintenance PC in Sweden )

• Which are installed as server into the remote device answering the requests.

  ( In this example, it is the programmable logic controller including a RS232 hardware port, connected to the sensor )

Each station contains an ADSL modem, a GPRS modem, a satellite communication modem or any other Ethernet compliant modem. The data will be transmitted over an Ethernet network, and in this case probably via Internet.

One of the benefits we can get from the usage of virtual serial port over Ethernet is that since the communication is entirely digital, it is very stable and very robust according to the electromagnetic perturbations.

This virtual port is client according to the data, that is it will initiate requests and it will send commands ( reading requests, writing requests or sending orders for instance ).

Configuration

• A virtual serial port client software is installed, set and running as RS 232 serial port with 'COM y' port assignment, and configured to communicate with the remote virtual serial port server with the IP address 'v.v.v.v:m'.
• The sensor diagnostic software is configured to exchange data through the serial port 'COM y'.
• The transmission parameters of the serial port ( speed, parity, stop bit, handshaking etc ) are identical for both the diagnostic software and the virtual serial port acting as a client.

Operating description

• The diagnostic software send requests toward the 'COM y' serial port.

  From the software point of view, it is just a standard serial port.

• When emitting data towards the sensor, the virtual serial port extracts the available data from the serial flow, puts the data into one or more TCP packets ( according to the amount of data to send ), builds the TCP-IP frame including the IP address ( 'v.v.v.v:m' ) of the remote virtual serial port server, and send this frame through one of its Ethernet port in order to be transmitted via the Ethernet compliant modem.
• When the sensor is answering, operating is reciprocal, from the data flow coming out the sensor connected to the virtual serial port server with the IP address 'v.v.v.v:m', back to the virtual serial port client requester ( with the IP address 'x.x.x.x:n' ) through a compliant TCP/IP modem.

This virtual serial port is server according to the data, that is it will serve the data answering the requests initiated by the client.

Configuration.

• The programmable logic controller provides the virtual serial port server functionality, a hardware serial connection and an Ethernet connection.
• The virtual serial port is configured to be accessed through the Ethernet port Nr 'm', and to exchange the data directly with the hardware serial port assigned as 'COM z' ( the « real » port ).
• The parameters ( speed, parity, handshaking etc ) of the RS232 serial port are identical for the HART modem, the 'COM z' port and the serial interface of the virtual port all together.

Operating description

• When the virtual server receives TCP packets on port Nr 'm' ( coming out the Ethernet router ), it will extract the available data from the packets, put these data serially before transmitting them towards the 'COM z' hardware serial port ( respecting the appropriate serial protocol ), which will transmit these data towards the HART modem.
• Then, the HART modem converts these data into a 4-20 mA signal to send to the HART sensor, in the respect of the HART protocol specifications.
• Then, the sensor answers the requests, and the way the data are coming back is inverse.

Whether none automation application is required ( no more than any other programmed application ), a cheaper solution to install a «virtual serial port server» is to get an Ethernet switch, an Ethernet router or an Ethernet compliant modem ( ADSL, satellite etc ) providing this service and including a hardware serial port ( RS 232 or USB for instance ).

In this case, the device drives the data from the external Ethernet port towards the internal Ethernet port Nr 'm' ( the Ethernet interface of the virtual serial port ) when the Ethernet port Nr 'm' is the required destination, and it drives the data out through a local port whether the required address is not the 'm' port.

Reciprocally, when the virtual serial port receives data on its serial interface, it drives the data through the Ethernet 'm' port, and then to the external Ethernet port.

About the commercial programmable controllers, the eWon PLC provides the virtual serial port server capabilities in association with hardware RS232 port, for instance.

About the open source programmable controllers operating under Linux like « Proview », or any other device using this OS, it is enough to operate a only software solution by using the different tools provided by the operating system.

With Linux, different techniques can be used to implement a virtual serial port server : by using various flow redirecting commands, terminal configuration commands, piped commands, or / and programming in 'C' language, according to what is needed.

Below is a list of some free drivers operating well when virtualizing serial ports for Microsoft operating systems. You will find very few solutions completely free into the market : most of these solutions offer at last one serial port operating without getting a licence, which could increase considerably the costs for a large installation.

• eVcom software - please download eVcom from here - is distributed by the eWon company and it is completely free.

  « eVcom » supports RawTCP protocol, exchanges of data between Modbus RTU and Modbus TCP protocols, and MPI protocol ( Siemens ).

  www.ewon.biz

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• HW VSP3 is free for a single virtual port client.

  www.hw-group.com

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• Com0com is an open source project.

  It is a null-modem emulator which can be associated to the « com port to TCP redirectors » sub-module to operate as a virtual serial port over Ethernet.

  http://com0com.sourceforge.net

Fortunately, there is at least one economical solution with great performance to implement virtual serial ports as clients for any proprietary operating system, without any limitations nor any over cost.

The Linux operating system offers many way in order to manage the data of virtual serial ports, freely, without any limitations and safely, by using robust methods. Here, you will find at least two one's, very quickly.

Redirection of the Ethernet data flow towards / forwards serial data flow by using the redirection and « pipe » Linux commands ( '|' ) between each communication port.

Call of the extremely powerful and high performing « socat » command.

« socat » is a Linux utility software which creates two bidirectional data flows and transfers the data between each other ( under Linux, data flow means data transfer to / from a communication port, a file or any other component ).

Thus, this command allows to communicate with remote devices whatever the data flows taken in consideration.

Last but not least, by using this command, it is easy to manage all that through a virtual private network ( VPN ) and SSH.

http://www.dest-unreach.org/socat/

To avoid the limitations and the costs due to the usage of software ( diagnostics, data treatment or any else ) operating under a proprietary system, one potential solution is to implement the virtual serial ports under a Linux machine where a virtual machine ( Virtual Box, VMware etc … ) has been installed in. This virtual machine will host the proprietary operating system and the associated software.

So, the amount of client serial ports will only be limited by the capabilities of the operating system hosted in the virtual machine, and by the features of the software used to treat the data.

If you opt to use a VPN for security and for safety reasons, you will get a completely virtual system, with a virtual private network, virtual serial ports and virtual machines !

• The virtual serial client port is managed by the Linux hosting machine with the so powerful « socat » command.
• « socat » redirects the data between the Ethernet port and a Linux serial port.
• The virtual machine has been set up so that each of the serial ports used by the diagnostic software is linked to a Linux serial port.