About DDE Communication with Dymosim

This section describes how to establish a DDE connection between a client and the Dymosim application using the WM_DDE_INITIATE message.

Important: Dymosim can be compiled as a Windows application with built-in DDE server with realtime capability. To be able to perform such a compilation, DDE must be selected as an embedded server. See Interacting with the Dymosim DDE Server Using the Internal Graphical User Interface. To be able to use Dymosim as DDE Server, you must have the Systems FMU Export license. For further information on licensing, see the Dassault Systèmes License Server Essentials Guide.

The WM_DDE_INITIATE message must be sent from the DDE client with the application name dymosim plus any topic.

After the simulation finishes or is stopped, Dymosim sends a WM_DDE_TERMINATE message to all connected clients.

A client can terminate the connection by sending the same message.

This section lists the commands that can be sent to Dymosim using WM_DDE_EXECUTE messages.

Command	Action
run	Starts simulation (if simulation is not started automatically), or resume simulation after a pause command.
stop	Stops simulation. Has no effect if simulation not yet started.
pause	Pauses simulation. The simulation is temporarily halted until a run command is given. Note that DDE requests are handled while pausing.
logon	Enables logging to file.
logoff	Disables logging to file.

This section lists the parameters that can be specified in Dymosim using WM_DDE_POKE messages.

The WM_DDE_POKE message includes the name of the parameter and its new value (the string representation of a number).

There are four special variables:

Variable	Values
realtime_	Set to 1 to enable realtime mode, or to 0 to disable realtime mode.
tscale_	The scale factor between simulated time and real time. In realtime mode, the simulator maintains the relationship: real-time = tscale_ * simulated-time:
abstol_	Absolute tolerance for hot-linked variables (default 0), see later.
reltol_	Relative tolerance for hot-linked variables (default 0), see later.

When a simulation is started, simulation parameters, auxiliary variables, and states may also be specified. However, these are not set immediately, but only at certain points in the simulation that allow for updates. Hence, when a client makes a request (for example for a variable), an update from another or the same client may be pending. Such pending update values are reported in the response.

This section lists the parameters that can be requested from Dymosim using WM_DDE_REQUEST messages.

When a WM_DDE_REQUEST message with the name of a parameter is received by Dymosim it responds by returning the current value of the variable (the string representation of a number), or zero if no such variable exists.

In addition to the four special variables listed in the section above, there are another three variables that can be requested (see table below).

When a simulation is started, simulation parameters, auxiliary variables, and states may also be requested.

The value returned is the current value at the last accepted output point.

Variable	Values
delayed_	Number of seconds the simulation was delayed at the last accepted output point.
status_	State of the DDE server. The state is composed as the Boolean OR of the following parts: 1 simulation started (running) 10 simulation paused 100 current simulation time = 0 As an example: 11 indicates that the simulation is started but paused at a simulation time greater than 0.
time_	Current simulation time in seconds.

This section describes how variables can be hot-linked using WM_DDE_ADVISE messages.

When a variable is hot-linked, it is sent to the client at output points when a significant change its value has occurred.

A significant change of a variable is determined from absolute and relative tolerances (which can be set by the DDE client) as follows:

where x₀ = value last sent to client and x = current value.

The variable is sent to the client when changed is true at an output point. The variable is also sent at the first output point following the hot-link operation.

Dymosim counts the number of links to a variable, and any corresponding "unlink" messages. Updates for the variable are sent while the link count is greater than zero.

MATLAB example: ddeadv(channel, 'time_', 't=[t x];', 'x')

A graphical user interface is available for the Dymosim DDE application.

See Interacting with the Dymosim DDE Server Using the Internal Graphical User Interface.

This section describes the limitations of the Dymola Behavior Modeling app with regard to DDE.

• Dymosim cannot be created as a DLL when DDE is selected.
• Currently only the solvers Lsodar, Dassl, Euler, Rkfix2, Rkfix3, and Rkfix4 are supported. Other solvers (Goddess solvers) are not supported with DDE.
• When a client requests data from the DDE server, it specifies the required data format. Dymosim only supports the following clipboard formats:
  • Plain text (used typically by MATLAB).
  • XITable (used typically by Microsoft Office Excel).