Command Gateway is a very simple software component aimed to support interfacing with the simulator. Here, interfacing means the ability for processes that run outside the simulator to interact with the simulated scene in terms of data manipulation. Command Gateway is the solution used by Open Airbus Cockpit hardware to read and write data from and to the systems being simulated in the main PC.

How it works

We already said that Command Gateway is a very simple component, didn’t we? It’s so simple that it is just a script written in Lua language. If you don’t know anything about Lua, it’s enough to say that it is a programming language aimed to extend the functionality of a software program. And, in this case, that software program is FSUIPC.

FSUIPC is a Flight Simulator/Prepar3D addon mostly known as a extension of the interfacing capabilities of the simulator. It is widely used by addon vendors to integrate their products with the sim. One of the most powerful but barely known features of FSUIPC is its scripting engine based in Lua language. It allows the user to write their own scripts to make use of the FSUIPC interface to implement more sophisticated logic. And that’s exactly what Command Gateway does: it takes advantage of the FSUIPC interfaces by exporting them to the outside world using protocols and channels that are not available in FSUIPC.

The operation mode of Command Gateway is as follows. As mentioned above, it is a script. When executed, it will try to scan all available serial ports of the PC. For each port with a device connected to it, it will try to perform a short handshake to determine whether that’s a Command Gateway compliant device. If so, it will expect to receive any of the following commands from the device:

  • An FSUIPC offset observation request. The device has to indicate the size and the address of the offset it wants to observe. It is used by the device to indicate it wants to observe that offset and be notified every time it changes its value.
  • A LVAR observation request. Similar to the previous one, but for LVARs, i.e. data variables handled by the cockpit gauges of the simulated aircraft.
  • An FSUIPC offset write request. The device indicates the size and the address of the offset it wants to write to, along the new value for that offset.
  • A LVAR write request. Similar to the previous one but for LVARs.

As you may notice, there is no read operation in Command Gateway. This is one of its design principles. The hardware devices are not really interested in reading data but in being notified when such data change. This technique allows your hardware to save computing resources by reducing the number of interactions with the simulator.

After all serial ports are scanned in search for compliant devices, they are able to send one of the command messages described above. In addition, Command Gateway will send messages to report changes in the FSUIPC offsets or LVARs only when they are produced. This process continues until the script is killed or the simulator closes.


In order to install Command Gatway, you will need:

  • MS Flight Simulator X or Prepar3D installed in your system
  • A registered copy of FSUIPC plugin installed and configured for your simulator. You can obtain it from the FSUIPC website.
  • The Command Gateway script. You may find it in GitHub repository.
Important note! FSUIPC scripting is only available for registered versions. You have to adquire a FSUIPC license in order to Command Gateway to work.


As mentioned above, the file OAC_CMDGW.lua contains the Command Gateway script aimed to be run in FSUIPC Lua scripting engine. To do so, just copy the file into Modules/ subfolder of your simulator (e.g., C:Program Files (x86)Microsoft GamesFlight Simulator XModules). That’s all you need to install Command Gatway in your sim.


You have two methods to run the Command Gateway script.

Run on Startup

You can configure FSUIPC to run the script when the simulator starts. To do so, edit the file ModulesFSUIPC4.ini and add the following section at the bottom of the file.


This will make the script to be run when FSUIPC is loaded.

Run on Keystroke

This is the recommented method. Instead of running the script on simulator startup, the script will start when a keystroke is pressed. If there was any running instance of the script, it will be killed and replaced by a new instance. This allow the user to reset the script if a strange behavior occurs.

To configure a keystroke to run the script go to the FSUIPC config window by selecting the Add-ons > FSUIPC option in the top bar. Then the tab Key Presses is the appropriate one to configure keystrokes. Select the key combination of your choice (e.g, CTRL + R) and select LUA OAC CMDGW in the control list. Press the OK botton below and that’s all! The OAC Command Gateway will be launched each time you press the key stroke you have chosen.


Sometimes things go wrong. If you experiment issues with Command Gateway, it would be better off checking out its log file to have more details.

It is highly recommended to configure FSUIPC to log the Lua scripts in separate files. To do so, go to the Logging tab in FSUIPC configuration and mark the option Log Lua plugins separately.

After executing the simulator with separated log files for each script, you’ll find the Command Gateway log file in ModulesOAC_CMDGW.log relative to your Flight Simulator/Prepar3D installation directory. You can open it with any text editor (Windows Notepad will do). There you will find messages lines of the script telling what’s doing. Please read these lines carefully and look for any error message you may find.

If after reading the log file you are not able to determine what’s failing or how to fix it, you can report a bug in GitHub or contact by email.

Command Gateway & Open Airbus Cockpit

As mentioned above, the purpose of Command Gateway in Open Airbus Cockpit is to server as a interface for the hardware devices to the simulator. If you have checked out the hardware section, you’ll know that most of the hardware devices are based on Arduino Platform. Each Arduino board uses its serial port to communicate with Command Gateway and interact with the simulated scene through it.

In the set of Arduino libraries provided by Open Airbus Cockpit you will find one that implements the Command Gateway protocol to make it easier to interface to the Command Gateway. Please have a look to the documentation for further details.

Protocol description

This information is only relevant if you plan to interact with Command Gateway through its protocol from a custom device or addon. This is a specification of the text protocol it uses to communicate with the device.

Command Gateway uses a text protocol to communicate with the devices using the serial port. The text must be encoded in UTF-8. The protocol comprises the following messages.

  • BEGIN {version} {client ID}. This is the handshake message sent by the device once the serial port is open by the server. It indicates the version number of the protocol and an arbitrary token to identify the client session.
  • WRITE_LVAR {lvar name} {value}. A request sent by the client to write to the indicated LVAR with the given numberic value.
  • WRITE_OFFSET {address}:{size} {value}. A request sent by the client to write to the offset indicated by given address and size, with the given value.
  • OBS_LVAR {lvar name}. A request sent by the client to start observing a given LVAR.
  • OBS_OFFSET {address}:{size}. A request sent by the client to start observing a given offset by its address and size.
  • EVENT_LVAR {lvar name} {value}. A event sent by the server to notify a change in the given LVAR value.
  • EVENT_OFFSET {address} {value}. A event sent by the server to notify a change in the offset at given address.

3 thoughts on “Command Gateway

  1. Interesting concept, will give it a try and let know if I was successfull.
    Can you share kind of code for Arduino especially for handshake and examples of exchange of particular variables?

    Many thanks anyway!

      1. Indeed found what I was looking for where you said.

        It’s working fine so far, I’m going to build the concept further on your libraries.

        Will keep you posted on results.


Leave a Reply to Alvaro Polo Cancel reply

Your email address will not be published. Required fields are marked *