Connect to a remote USB IO Module with Windows
The network connectivity of LucidControl USB IO Modules opens them to a much wider field of applications. Together with the Raspberry Pi, the USB IO Modules are e.g. able to measure or control temperatures over the Internet.
In earlier articles I described how the LucidControl USB IO Modules can be accessed over a network by employing a Raspberry Pi. There are solutions available that make the USB IO Modules ready for network communication.
Network Device Server and Client Computer
First of all, and that’s true for all described solutions, a TCP communication is established by running ser2net on the network device server. The ser2net application is routing a data stream from a TCP socket to a local device like a serial port. And since our LucidControl USB IO modules use the standard USB CDM profile, they behave like a standard serial port. The LucidIoCtrl command line tool as well as the API support direct socket communication.
USB IO Module Network Device Server with socat and ser2net
I also described how to create a virtual serial device with socat on a Linux client computer. Virtual devices are routed to a TCP socket of a network device server ser2net is listening to. The virtual devices are transparent and accessible on the client computer the same way as a local device is.
In this article I show now a method how to create virtual devices on Windows operating systems that are routed to a USB IO Module connected to a network device server.
But why should you use this method, when the LucidControl tools support direct TCP connections?
Beside of the tools that are provided by us it is also possible to access the USB IO modules directly. Since the protocol of the USB IO modules is fully documented and easy to understand, a developer can implement the USB IO module from scratch sending data frames to them.
For the USB IO Modules is an implementation for ProfiLab-Expert 4.0 available. ProfiLab-Expert is a program for visualization and control of analog and digital signals. It allows you to use the USB IO Modules in order to control e.g. a temperature by changing the state of a digital output module which controls a heater by switching it on and off.
ProfiLab-Expert accesses the USB IO Modules natively without using an API by sending data frames directly via the serial port to the IO Module.
Creation of a virtual Device on a Windows Client
LucidControl USB IO Modules can be used with Windows without the need of a driver. The IO modules implement the standard CDM profile that is natively supported by most Windows versions.
Linux has with ser2net and socat all tools available which are helpful in order to create a network device server as well as a virtual device on the client computer connected to the network device server.
For Windows the situation is different as third party tools are necessary for this task. A comport interface can only be installed as a new device what needs a kernel mode driver. Such software is different compared to application software and needs special knowledge of the Windows Driver Development Kit (WDK).
If an application which is running in user mode has a bug it should not crash the whole system. In worst case the application terminates, but the system should remain responsive and other applications are not supposed to be affected by this and should continue running.
In kernel mode things are different and everyone who is involved into driver programming knows that the famous “blue screen” still exists and is not a thing of the past.
We did some driver project in the past for our RFID Universal Reader Module what is a reader for contactless cards that can e.g. access contactless passports and can also be used in banking applications. In this field PC/SC is a standardized interface and a reader should support the Windows Smart Card API. Having this experience, I think I can estimate how complicated it is to make a new device operating.
Fortunately, there are serial port redirectors available that install a new serial port which is forwarded to a TCP socket. We successfully tested our USB IO Modules e.g. with COM2TCP and the VSP driver of HW Group.
Finally, we put special attention on the open source project com0com. It is not a port redirector as e.g. the VSP driver is, but a null-modem emulator that installs a pair of virtual serial ports connected to each other.
Connecting to a Network Device Server with com0com Null-Modem Driver
The picture explains the concept of connecting to a network device server via a null-modem emulator driver. At a first glance it seems to be difficult using two virtual serial ports instead of one. The gray highlighted part shows the function of the kernel mode driver, the yellow highlighted part shows applications running in user mode.
com0com creates a pair of serial port devices (here COMa and COMb) that are connected to each other. Data sent to COMa is received by COMb and vice versa.
This concept has a big advantage because it allows having an application running in user mode between the serial ports that accesses the serial port COMa and connects it with a remote TCP socket. The program com2tcp (not to mix up with COM2TCP) connects a serial port with a remote TCP socket. It belongs to the com0com project but it is not automatically installed by the setup and must be downloaded separately.
Setup com0com for TCP redirection
The com0com project can be downloaded as a Windows installable file. When the driver is supposed to be installed on a recent Windows operating system a problem will arise because of the driver not being signed with a valid signature but with a test signature only.
Nowadays, Windows needs drivers being signed with a valid signature. While this was earlier only true for the 64 bit versions of Windows 7 it became more difficult in newer Windows versions to run unsigned kernel mode software. The validation of driver signatures can be disabled in general (e.g. when the system is started) but the complete deactivation should only be a temporary solution for a development computer. We cannot recommend this for a productive system.
On our download page we provide a signed driver of com0com that can be used in conjunction with our USB IO Modules. The installation is straight forward and it is explained in this document.
Note:We are not the developer of com0com, we only signed the existing kernel modules and we appreciate the developers work very much. The com0com project can be found on Sourceforge.
Validation of the com0com Driver Signature
Continue the installation as explained and at some point Windows will ask you to confirm the installation of the signed software.
Click Install in order to agree that you want to install the kernel mode driver signed with our signature. You might be asked several times to confirm the driver installation.
Windows Driver Update Installation of com0com Null-Modem Driver
For some reason Windows might be searching Windows Update for a more recent driver what does not make sense for me and can be skipped by clicking Skip obtaining driver software from Windows Update. But even then it might take a few minutes (at least on my system) to complete the installation.
Device Manager with com0com Devices
After the installation has completed, com0com devices appear on the Ports section. In this example COM6 and COM7 serial ports have been installed.
The settings of the com0com ports can be changed and also new ports can be created by using Setup for com0com. This application is copied onto the system during the com0com setup.
Setup for com0com Application
This screen shot shows the current configuration on our machine. A new pair of linked serial ports can be created by clicking Add Pair. The installation of a new pair of serial ports took some time on our machine because Windows Update was looking for some more recent driver versions.
Pending Installation of com0com
The setup program recognizes the slow installation and shows a dialog that can be confirmed in order to complete.
Setup for com0com – new serial Port
The fresh created ports have the names CNCA2 and CNCB2.
For compatibility reasons we recommend using standard names for serial ports. Some applications may not be able to work with a serial port name like CNCA0 but needs standard names like COM6. We also recommend activating use Ports class what installs a serial port in the Ports section of the device manager. Otherwise the port pair appears in the com0com section only.
Setup for com0com “use Ports class”
When use Ports class is enabled for CNCA2 and CNCB2 ports Windows assigns new serial port names to the ports. Here the first available free ports were COM11 and COM12.
Connecting Serial Ports to a TCP Socket
One virtual comport of the pair can now be connected with the TCP socket of a network device server running ser2net.
Connecting to a Network Device Server with com2tcp
com2tcp –ignore-dsr \\.\COM6 RPI-AZ-2 4001
This command connects the virtual serial port COM6 (that is connected to COM7) with the TCP port 4001 of the computer named RPI-AZ-2.
Accessing USB IO Modules with com0com
This command connects to COM7 that is connected to COM6. COM6 is opened by com2tcp and redirected. LucidIoCtrl asks the USB IO Module for device information. The Analog Input Module answers and returns that it is able to measure 4 voltages in the range of 0 to 10V.
This article explained how a virtual serial port that is connected to a TCP socket can be installed on a Windows operating system and how a connection to a network device server can be established by using the LucidIoCtrl tool.
Although our tools and API support direct TCP socket communication, a virtual comport that is redirected to a TCP socket is helpful for compatibility reasons e.g. using the USB IO Modules together with ProfiLab-Expert.
By using com0com there is no software change necessary when switching from a local connected IO Module to a remote device. By changing the comport number the measurement object can be routed to an IO module connected to a remote network device server.