Echelon LonTal Stack - Manual (Page 53)

LonTalk Stack Developer’s Guide 41
The packet sum is an 8-bit value that, when added to the first three bytes, results
in an 8-bit zero result.
The code packet hast the following features:
• Asynchronous method of presenting itself at any time (by using an escape
sequence).
• Checksum verification.
• Data transfer initiation.
• Duplication detection using a sequence number. Duplicates may be sent
when an expected response is lost or broken and does not occur within a
time-out period.
Data outside of the code packet is restricted to either a message or a multi-byte
local network interface command or response and is always enclosed with a length
byte in the front and a checksum at the end. The length byte does not account for
the checksum at the end—the inclusion of the checksum is implied. The
checksum covers the data that preceded it; therefore, it does include the length
byte.
Note: All data outside a code packet must include an escape prefix before any
escape data values. For example, if a 0x7E value appears in the sequence it must
be followed by another 0x7E (the first 0x7E value is the escape prefix).
Checksum bytes and length bytes must be preceded with escape prefixes.
The following table summarizes the layout of the data message used in Layer 5
mode and for the niNETMGMT local network interface command (Layer 2 mode
is required for the LonTalk Stack). When sending local network interface
commands (NI Commands other than the niCOMM or niNETMGMT) that have
additional data, the commands are contained in a data message and they are
preceded by a CpMsg code packet.
BYTE 0 BYTE 1 BYTE 2..N BYTE N+1
Length
(bytes to
follow
except
checksum)
NI
Command SICB starting w/ message header Message
Checksum
of bytes 0-N
The following table summarizes the layout of the data message used in Layer 2
mode.
BYTE 0 BYTE 1 BYTE 2..N BYTE N+1
Length
(bytes to
follow
except
checksum)
NI
Command Priority/AltPath/DeltaBacklog byte,
NPDU,
CRC (2 Bytes)
Message
Checksum
of bytes 0-N