Echelon LonTal Stack - Manual (Page 126)

114 Developing a LonTalk Stack Device Application
operating system event which is signaled by the LonEventReady() callback
handler function. From within your application’s main thread, the application
should implement an infinite loop that waits on this operating system event.
Whenever the event is signaled, the application should call the
LonEventPump() API function to process LonTalk Stack events.
You can signal this same operating system event to schedule your main
application thread to perform other functions as well. For example, you could
signal the operating system event from within an interrupt handler to signal the
main application task to process application I/O. Calling the LonEventPump()
function when there are no LonTalk Stack events is acceptable.
The host application should be prepared to process the maximum rate of
L ON W ORKS traffic delivered to the device. Although events are enqueued within
the LonTalk host stack, your application should call the LonEventPump()
function frequently to process events. Use the following formula to determine the
minimum call rate for the LonEventPump() function:
1 −
=
rCount InputBuffe
ate MaxPacketR
rate
where MaxPacketRate is the maximum number of packets per second arriving for
this device, and InputBufferCount is the number of input buffers defined for your
application (that is, buffers that hold incoming data until your application is
ready to process it). The formula subtracts one from the number of available
buffers to allow new data to arrive while other data is being processed.
However, the formula also assumes that your application has more than one
input buffer; having only one input buffer is generally not recommended.
If the application expects periods of inactivity, it can simply wait for the LonTalk
host stack to post an event. If the application expects periods where it is busy for
several milliseconds at a time, it should call the LonEventPump() function
during the busy time to ensure that events are processed. Use the formula above
to determine a baseline for how often to call the LonEventPump() function.
If you do not have measured data for your typical network and you are
developing a device for the TP/FT-10 channel, assume 90 packets per second
arriving for the device. This packet rate meets the TP/FT-10 channel’s
throughput figures, assuming that most traffic uses acknowledged or
request/response service. Use of other service types will increase the required
packet rate, but not every packet on the network is necessarily addressed to this
device.
Using the formula, devices that implement two input buffers and are attached to
a TP/FT-10 network that expect high throughput should call the
LonEventPump() function approximately once every 10 ms.
When an event occurs, the LonEventPump() function calls the appropriate
event function for your host application to handle the event. Your event handler
functions must be designed for this minimum call rate, and should defer
time-consuming operations (such as lengthy flash writes) whenever possible, or
manage them in separate contexts (tasks or threads).
See Appendix D, LonTalk API, for a list of the available event handler and
callback handler functions.