Echelon Neuron C - Manual (Page 205)

Neuron C Programmer’s Guide 193
• If your device is installed with unicast connections (that is, one device
sends a network variable or message to one other device), the default
number of network buffers is probably sufficient.
• If your device is installed with multicast acknowledged or multicast
request connections (that is, one device sends a message to a group of
devices and expects a response from each), the number of network input
buffers should be at least equal to the size of the largest group.
• If your application frequently has blocking sequences, that is, when tasks
that do not complete for a relatively long time, the default application
buffer count might be insufficient.
If, for example, a device sends a message with the acknowledged or request
service to 63 different devices, the sender device could receive 63 almost
simultaneous acknowledgments or responses. In general, large acknowledged
connections should not be used because the same message delivery reliability can
be achieved using repeated messaging, with far less network traffic. Network
variable updates using the repeated delivery service do not generate any
acknowledgements, and therefore do not require any input buffers.
The exact number of network input buffers required is a function of both bit rate
and the system clock rate, so some experimentation might be necessary to
determine the minimum number of buffers.
Compiler Directives for Buffer Allocation
The following sections describe the compiler directives used for setting the size
and number of different types of buffers.
The compiler issues warnings when any of the buffer size compiler directives are
used and the resulting settings are too small to accommodate all possible
network management messages from being properly received or responded to.
Each of the buffer directives can include an optional modifier specification after
the number of buffers. The modifier can be either of the following keywords:
• minimum
The minimum keyword specifies that the number of buffers indicated for
the pragma directive is the minimum number of buffers required for the
application. The compiler defines at least as many buffers as specified in
the directive, but could define additional buffers if the application
requires them. This keyword encourages modular development, wherein
different modules could have different buffer count or size requirements.
• final
The final keyword specifies that the number of buffers indicated for the
pragma directive is the absolute number of buffers to be used by the
application. The compiler defines exactly as many buffers as specified in
the directive, and issues an error if additional buffers are required. This
keyword promotes self-documenting source code.
Example: The following two directives specify the incoming network buffers.
The first directive specifies that the compiler should define at least 66-byte