Echelon Neuron C - Manual (Page 140)

128 How Devices Communicate Using Application Messages
Important: Assigning values to the msg_out object can invalidate fields in the
msg_in object. After receiving a message, you must examine or save any
necessary fields in the msg_in object
before starting to send a message.
code A numeric message code. See Message Codes on page 123.
len The length of the message data.
data The application data. This field is valid only if len is greater than 0.
MAXDATA is a function of the #pragma app_buf_in_size directive
(see Chapter 8,
Memory Management , on page 173):
MAXDATA = app_buf_in_size – 6
or ,
MAXDATA = app_buf_in_size – 17
(if explicit addressing is used by any message or network variable in
the program)
authenticated This field has the value TRUE if the message was
authenticated, and FALSE if the message was not authenticated.
service The message service type; one of the following values:
ACKD - acknowledged service with retries
UNACKD
- unacknowledged service
UNACKD_RPT- repeated service (message sent multiple times)
REQUEST- request/response service. When a message is sent using
this service, the receiver device returns a response to the
sender device, and the sender processes the response.
The request/response mechanism is described later in
this chapter.
addr An optional field in the incoming message that an application
program can look at to determine the source and destination of the
message. You can find the definition of the msg_in_addr type in the
include file.
To use this field, you must include the file.
duplicate When this boolean flag is TRUE, it indicates the message is a
duplicate request message passed to the application. Duplicate
request messages are passed to the application if the application
response contains data beyond the one-byte message code.
rcvtx The receive-transaction ID that the message used in the device’s
transaction database.
Importance of a Default When Clause
Listing 3 (in The msg_arrives Event on page 126) illustrates an important
technique to be used with messages: Any program that receives application
messages must be prepared to receive unwanted messages and discard them.
Discards can take the form shown in Listing 3, or can be a default case in a
switch statement.