Echelon Neuron C - Manual (Page 141)

Neuron C Programmer’s Guide 129
If a message were to arrive and the application fail to process it, that message
would remain at the head of the queue forever, blocking the arrival of any other
messages or network variable events and locking up the device until it is reset.
One example of a message that would be sent to all devices, most of which are not
interested in the message, is the service pin message. Probably only a network
tool would want to process the service pin message; all other devices need to
discard the message.
If a program does not process messages (either implicitly through the use of
when(msg_arrives) or explicitly through the use of msg_receive( )), the scheduler
automatically discards all incoming messages.
A device that uses only network variables need not be concerned with this
phenomenon, because the scheduler then handles all incoming messages.
Application Message Examples
The following example shows how lamp and switch devices could be written using
application messages instead of network variables.
Lamp Program
First, here is the program for the lamp devices:
// lamp.nc - Generic program for a lamp
// The lamp’s state is governed by an incoming
// application message
#define LAMP_ON 1
#define LAMP_OFF 2
#define OFF 0
#define ON 1
// I/O declaration
IO_0 output bit io_lamp_control;
when (msg_arrives) {
switch (msg_in.code) {
case LAMP_ON:
io_out(io_lamp_control, ON);
break;
case LAMP_OFF:
io_out(io_lamp_control, OFF);
break;
} //end switch
} //end when
Switch Program
Here is the program for the switch devices:
// switch.nc - Generic program for a switch
// Send a message when the switch changes state
#define LAMP_ON 1
#define LAMP_OFF 2