Echelon Neuron C - Manual (Page 146)

134 How Devices Communicate Using Application Messages
msg_send();
}
when (msg_fails(TAG1))
{
failures[0]++;
}
when (msg_fails(TAG2))
{
failures[1]++;
}
when (msg_succeeds)
// any message qualifies
{
success++;
}
Preemption Mode and Messages
The Neuron firmware enters preemption mode when there is no application
buffer available for an outgoing message. If the system needs a free application
buffer, it causes the application program to wait and processes only completion
events, responses, and incoming network variables and messages to facilitate
application buffers’ becoming free.
No other predefined or user-defined events are processed unless the
preempt_safe keyword is used in conjunction with a when clause containing an
event expression. The syntax for the when clause is explained in Chapter 2,
Focusing on a Single Device , on page 15.
The watchdog timer is automatically updated during this wait. If the program
waits for more than a configurable number of seconds, the device is reset. This
configurable timer is controlled by the preemption_timeout field of the
configuration structure (config_data_struct), which you can modify using the
update_config_data() function; see appendix B of the ISO/IEC 14908-1 Control
Network Protocol specification for information about this structure. A buffer
wait timeout (also known as preemption mode timeout ) should only occur if a
device is totally blocked from transmitting. This situation could occur under
extreme network congestion or with certain network failures.
A buffer wait timeout could also occur if a program is not properly freeing
completion events. The most common error is to check for completion events in
bypass mode (for example, if (nv_update_completes) ) and not to have a
corresponding completion event check in a when clause.
With network variables, the system can only enter preemption mode if:
• synchronous output network variables are updated, or
• flush_wait( ) is called.
When the system is in preemption mode, further attempts to send a message
from a task associated with a message completion event when clause cause a
device reset if no buffer is available for the new message.
The following sequence is therefore
not recommended:
when (TOGGLE_ON)