Stream Connect

Connects to a remote host.

Library

Description

The Stream Connect block establishes a connection to a remote host. The connection is made each time the block executes, not at model start. Streams have two modes: blocking (the default) and non-blocking.

In blocking mode, this block does not return until either the connection is established or the underlying protocol times out. Blocking mode is more suitable for use in an asynchronous thread rather than a periodic task in the model so that the sampling rate of the model is not affected. Asynchronous threads may be created using the Asynchronous Thread block.

In non-blocking mode, this block returns immediately. If the connection has been established, then the err output will be zero. If the connection is pending then the error code -QERR_WOULD_BLOCK is returned. If the connection failed then another negative error code is returned. Furthermore, all Stream blocks using the stream also become non-blocking. Whenever a Stream block would otherwise wait, the error code -QERR_WOULD_BLOCK is returned instead. When combined with a state machine, non-blocking mode is suitable for periodic tasks because it does not interfere with the sampling rate. The Compare to Error block may be used to check for specific error codes. The Stream Poll block may be used to wait or poll for the connection to complete or other events.

If the connection is successfully established then the stm output refers to the connected stream. This stream signal is passed as an input to other Stream blocks to refer to the stream. In this case, the err output will be zero.

If the connection cannot be successfully established then the err output will be a negative error code. The stm output is never zero, even when the connection cannot be established, because it is a reference to the stream rather than the stream handle itself. Hence, it cannot be used directly to determine whether the connection has been established.

Streams must be closed using the Stream Close block when they are no longer in use. A stream that is not closed will remain open until the model terminates.

Remote hosts are identified by a Universal Resource Identifier (URI), such as tcpip://remotehost:8000 or serial://remotehost:1?baud=57600. QUARC uses URI's for all its communications because it provides a uniform, extensible and flexible means of identifying the communication protocol to use and the associated communication parameters. Refer to Universal Resource Identifiers for more information.

The URI may be specified in the dialog parameters or via an input port. Which option is used is determined by the Source of URI parameter. When the URI is specified via an input, the input is typically driven by a Model Argument block, which allows a model argument to be used to assign the URI at runtime.

When the URI is specified as a dialog parameter and is not evaluated then the URI is configuration-dependent. A separate URI may be entered for each model configuration as well as for normal simulation. Thus, it is possible to employ a different URI for each target, without making a new diagram. The URI that is entered becomes the URI for the currently active configuration when the Simulation mode is set to External. It becomes the URI for normal simulation when the Simulation mode is set to Normal.

Streams have an associated buffer that is independent of any buffering in the underlying protocol. The stream buffer is designed to maximize use of the available bandwidth. For example, the Stream Send block simply buffers data until the stream buffer is full and only then does it flush the stream buffer to the underlying communication channel. Similarly, the Stream Receive block simply returns data from the stream buffer. It only reads from the underlying communication channel when there is not enough data in the stream buffer, at which point it attempts to read enough bytes to fill the entire stream buffer.

Helpful Hints

Firewall

If the server is remote then any firewall on the server must be configured to allow incoming access on the port being used.

Multiple NICs and UDP

When using the UDP protocol, the network interface card (NIC) through which datagrams are sent may be specified using the nic option. This option is particularly useful when broadcasting datagrams in order to restrict broadcast messages to a particular NIC. Without the nic option, most operating systems send broadcast messages through all the NICs. However, Windows 7 and above do not broadcast to all NICs in this case, but chooses one arbitrarily. Hence, specifying the nic option allows the NIC used for broadcast messages to be controlled. Refer to the UDP Protocol for details.

Input Ports

uri

A string specifying the URI to which to connect. The string must be a null-terminated vector of characters represented as a vector of uint8 quantities. It may be variable-sized. This string is typically provided either directly or indirectly by a Model Argument block or String Constant block.

This input is only available if the Source of URI parameter is set to External input port. Refer to the documentation below on the Source of URI parameter for details.

Output Ports

stm

A reference to the stream. This reference is always valid but will not refer to a connected stream if the connection was not established or is still pending. It has a data type of t_stream_ptr. It cannot be plotted and may only be connected to the stm input of one of the Stream blocks. It may not be connected to a Stream Accept block. Attempting to do so will result in an error being returned by the Stream Accept block.

err

An int32 error code indicating the results of the operation. If the connection is successfully established, then this output will be zero. If the stream is non-blocking and the connection is in progress then -QERR_WOULD_BLOCK is output. Otherwise another negative error code is returned. See Error Codes for the different error codes and their values. Use the Compare to Error block rather than the error code itself to check for specific error codes.

Parameters and Dialog Box

Source of URI

Indicates whether the URI should be determined from the dialog parameters or an input port. If this field is set to Specify via dialog (do not evaluate) then the URI is specified via the URI of host to which to connect parameter. In this case, the URI is not evaluated as a MATLAB expression but is interpreted as a literal string. However, format specifiers are recognized. Refer to qc_perform_substitutions for a list of the format specifiers available.

If format specifiers which change frequently, such as '%{time}' or '%{instance}', are used in the URI then the Stream Connect block cannot be placed in a referenced model, because the URI will change from the time the code is built to when the code is run. As a result, Simulink will insist that the code be rebuilt because block parameters in referenced models are inlined. Instead, specify the source of the URI as External input port and pass the URI from the top-level model. Use a String Constant block in the top-level model to generate the URI.

If this field is set to Specify via dialog (evaluate) then the URI is also specified via the URI of host to which to connect parameter. However, in this case, the URI is evaluated as a MATLAB expression. This option is convenient for using a variable in the MATLAB workspace for the URI.

If this field is set to External input port then the URI of host to which to connect parameter is ignored and an extra input port is provided which determines the URI.

URI of host to which to connect (tunable offline)

The URI identifying the remote host to which to connect. This parameter identifies the communication protocol and associated parameters. For example, tcpip://quanser-dev:18000 connects to the remote host called quanser-dev using TCP/IP on port 18000. This field is only used if the Source of URI parameter is set to Specify via dialog (do not evaluate) or Specify via dialog (evaluate). Refer to Universal Resource Identifiers for more information about URIs and the communications protocols supported by QUARC.

Send buffer size in bytes (tunable offline)

The size of the buffer used by the stream for sending data. This buffer is independent of any buffering in the underlying communication protocol. Increasing the buffer size may increase transmission performance. The buffer size must be at least as large as the maximum vector that will be sent. For example, if a double 3-vector is transmitted using the Stream Send block then the stream buffer size must be at least 24 bytes in length since a double is 8 bytes and a 3-vector is being sent.

Receive buffer size in bytes (tunable offline)

The size of the buffer used by the stream for receiving data. This buffer is independent of any buffering in the underlying communication protocol. Increasing the buffer size may increase reception performance. The buffer size must be at least as large as the maximum vector that will be received. For example, if a double 5-vector is received using the Stream Receive block then the stream buffer size must be at least 40 bytes in length since a double is 8 bytes and a 5-vector is being received.

Byte ordering (tunable offline)

The order in which bytes are transmitted or received. Little endian ordering means that the least significant byte of individual values, such as doubles, is transmitted or received first. Big endian ordering means that the most significant byte of individual values is transmitted or received first. The remote host must use the same byte ordering.

Sample time

The sample time of the block. A sample time of 0 indicates that the block will be treated as a continuous time block. A positive sample time indicates that the block is a discrete time block with the given sample time.

A sample time of -1 indicates that the block inherits its sample time. Since this is a source block, only inherent the sample time when it is placed in a conditionally executed subsystem, like a Triggered Subsystem, Enabled Subsystem, Function Call Subsystem or in a referenced model.

To use the fundamental sampling time of the model, set the sample time to qc_get_step_size which is a QUARC function that returns the fundamental sampling time of the model.

The default sample time is set to qc_get_step_size.

Non-blocking (tunable offline)

Check this option to make the stream non-blocking. In this case, all blocks using the stream become nonblocking; this block will not wait for the connection to be established and Stream blocks using this stream will not wait for data to be sent or received. Instead, whenever a block would otherwise wait, the error code -QERR_WOULD_BLOCK is returned. See Error Codes for the different error codes and their values. Use the Compare to Error block rather than the error code itself to check for specific error codes.

Active during normal simulation (tunable offline)

Indicates whether this block should execute during normal simulation. If this option is not checked then the block will not connect to the remote host during normal simulation. This parameter has no effect on generated code.

Targets

See Also

• Stream Listen block
• Stream Accept block
• Stream Close block
• Advanced Communications
• MATLAB Stream Functions
• Communications Using the C Language

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