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The purpose of link-layer packets is to provide a mechanism for getting command packets into a par-
ticular SKB device. Standard command packets are placed in the data payload section of the link-
layer packet. Graphically, the RS-485 link packet is as follows:
SOH
The START-OF-HEADER byte, always defined as 0x81. If a
LEN, PAYLOAD, or CRC byte is has a value of 0x81, the byte
should be actually transmitted twice. This indicates that the
bytes should be interpreted as a valid packet byte rather than
a SOH.
DEST
The destination device address where the packet is being
sent. All other devices will ignore the rest of the packet bytes.
SRC
The device address of the sender. This byte is remembered
and used to send ACK or response packets.
TYPE
Defines the purpose of the packet: a value of 0 means this is a
packet containing data, and a value of 1 means this packet is
an ACKnowledgement to a previous data packet.
LEN
The total number of payload data bytes (8-bits) . This number
does NOT include the CRC bytes, which are concatenated to
the end of the packet prior to actually transmitting.
PAYLOAD
These bytes represent the non-RS-485-specific data portion of
the link-layer packet. Typically, a complete command (or
response) packet is included in this field. The RS-485 sub-
system will pass this payload up to the session/application
layer of the software.
CRC
The cyclical redundancy check (CRC) is calculated on the
assembled link-layer packet (including any/all byte swapping).
The RS-485 protocol is based on a command-acknowledge architecture: For each link-layer packet
sent, an acknowledgement (in the form of and ACK packet) is provided.
RS485 Packet Example
Assume that a device with network address of 1 is present on the RS-485 network. The master will
connect input channel 1 to output channel 2 on the first logical switch by sending the following com-
mand:
Application Notes
1 byte
1 byte
1 byte
1 byte
2 bytes
< 256 bytes
2 bytes
83
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