Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 102)

Computer to PC Communication
Chapter 9
9Ć3
Figure 9.1
The Application and Network Layers
CMD STS FNC ADDR DATA
DST SRC CMD STS TNS
Data
(From Application Layer)
From
Application
Layer
1006–I
x
0 x
Network
Layer
Packet
Legend: x = low hex digit of CMD byte supplied by application layer
The network layer uses these fields of a Data Highway command:
the destination (DST) and source (SRC) byte specify the address of the
station that is receiving the command and the address of the station that
is sending the command.
a command (CMD) byte at the network layer indicates whether the
message is a command or reply.
a status (STS) byte at the network layer indicates whether the message
was successfully executed by the sending station.
a transaction (TNS) field identifies the particular command and reply
cycle the message belongs to.
Again, these bytes and fields are described in much greater detail later in
this manual. Notice (Figure 9.2) that the network layer (DST, CMD, STS,
TNS,and the application layer) is treated as data by the data link layer.
We say that this data is framed by the data link layer.
The data link layer uses these fields of a Data Highway command:
start of text (STX)
end of text (ETX)
enquiry (ENQ)
acknowledge (ACK)
data link escape (DLE)
negative acknowledge (NAK)
start of header (SOH)
end of transmission (EOT)
block check character (BCC)
These control characters are described in greater detail later in this
manual. The type of protocol you choose for the data link layer
determines the meaning of these control characters.