Echelon OpenLDV User Manual download pdf (Page 66)

58 Sending and Receiving Messages with the OpenLDV API

Constructing Messages

You can construct outgoing messages for OpenLDV application using application

buffer structures, and send that data to the network interface using the

ldv_write() function. Use the ldv_read() function to retrieve data from the

network interface, using the same application buffer structures. The following

section describes the application buffer structure.

The OpenLDVdefinitions.h header file contains example code that defines the

formats of these application buffer structures for Layer 5 devices. This header

file is included with the OpenLDV Developer Example; see Chapter 4, The

OpenLDV Developer Example, on page 89.

The ldv_read() and ldv_write() functions take a msg_p parameter, which is a

pointer to a buffer for the data that is to be received or sent. These functions also

take a len parameter, which specifies the size (in bytes) of the buffer or data to

write. See The OpenLDV API on page 15 for a description of these functions.

Application Buffer Structure

Figure 5 on page 59 shows the application buffer structures used by OpenLDV

ONWORKS interface devices. All OpenLDV LONWORKS interface devices support

the serial interface control block (SICB) buffer format. Some L

ONWORKS

interface devices also support an extended (LdvEx) buffer format. You can use

the ldv_open_cap() function to specify which format to use; see ldv_open_cap()

on page 25.

The SICB buffer format begins with a simple header, the Network Interface

Header. The structure of this header depends on the type of command being

processed:

• For commands that use a message queue, the header includes 4 bits for

the queue type, four bits for the command, and a byte for the length of

the payload.

• For commands that do not use a message queue (such as the immediate

commands), the header includes a byte for the command and a byte for

the length of the payload.

For some commands, the value of the length byte can be zero. An optional,

variable-length data field (as indicated by the header’s length byte) follows the

header.

The LdvEx buffer format encapsulates the SICB format; it adds extra timestamp

information and extended data (where applicable; this extended data is not

interpreted by the OpenLDV software). Unless specifically stated, an

“application buffer” refers to the SICB format buffer.

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Echelon OpenLDV User Manual Page 66