Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 101)

Computer to PC Communiation
Chapter 9
9Ć2
data link – checks the path between stations for errors to ensure that
data is transmitted in a proper sequence, frames messages sent by a
station and checks the integrity of messages received by a station. This
layer is not visible to the person placing Data Highway commands in a
program.
physical link – sets up, maintains, and disconnects a physical link
between two stations. This layer consists largely of hardware (Data
Highway modules and cable). Like the data link layer, this layer is not
visible to the person placing Data Highway commands in a program.
A Data Highway command consists of many fields, each of which
originates from one of the above layers at the sending station. When a
station receives a Data Highway command, it separates these fields so that
a single layer uses only those fields it needs to perform its specific
function.
The application layer uses these fields of a Data Highway command:
a command (CMD) field and function (FNC) field identify the type of
command that is sent.
a status field (STS) contains a code that indicates if the command was
successfully sent from one station to another.
an address (ADDR) field specifies the address in the remote station’s
memory
a data (DATA) field contains the data that is sent from one station to the
other.
These bytes and fields are discussed in greater detail later. For now, notice
(Figure 9.1) that the FNC, ADDR, and DATA fields from the application
layer are treated as data by the network layer. We say that the data is
framed by the fields of the network layer.