Echelon OpenLDV - Manual (Page 70)

62 Sending and Receiving Messages with the OpenLDV API
• Domain length (2 bits)
• Data (variable length, depending on the packet type and the data)
CRC
The cyclic redundancy check (CRC) is computed over the NPDU and the Layer 2
Header. The CRC is generated using the ITU-T (CCITT) CRC-16 standard
polynomial. When the link layer receives a packet with a CRC error, or a packet
that is less than 8 bytes in length, a transmission error statistic is incremented; if
a packet is received that is too long for the input buffer, or if there are no input
buffers, the missed packet statistic is incremented.
Layer 5 Buffer Structure
The following sections describe the Layer 5 buffer structure shown in Figure 6.
Bit transmission order within a byte is “most significant first”, meaning that the
most significant bit is transmitted first. Byte transmission order is also “most
significant first”, meaning that the most significant byte of a field is transmitted
first.
For a more complete description of the Layer 5 buffer structure, see the ISO/IEC
14908-1 Interconnection of information technology equipment – Control Network
Protocol - Part 1: Protocol Stack.
Application Layer Header
The application layer header contains the network interface command (and
queue) and a byte that indicates the length of the rest of the message. The most
significant nibble of the network interface command contains the command code
(niCOMM for network messages or niNETMGMT for local network interfaces
messages), and the least significant nibble contains the queue code. These
nibbles combine to form the command/queue byte, which is the network interface
command.
An OpenLDV application sends these commands using the ldv_write() function,
and receives them using the ldv_read() function. The OpenLDV Developer
Example contains an example implementation of a network interface API. You
can use the NiSendMsgWait() and NiSendResponse() functions, included as
part of this example API, to send enqueued commands more conveniently. You
can also use the application-specific message dispatcher, also implemented as
part of the OpenLDV Developer Example, to receive these messages.
See Network Interface Commands on page 76 for a description of the network
interface commands.
Message Header
The message header describes the various attributes of the LonTalk message
contained in the data field. The message header field is defined by the union of
two structures:
• ExpMsgHdr — for sending and receiving explicit messages that are not
processed by the network interface