Azbil F4Q, User Manual

Page: 18 / 76

4-2
„
Overview of a CPL Frame
A message consists of the following two layers. Both the instruction message from
the master station and the response message from the slave station take this form.
• Data link layer:
This layer contains the basic information required for communication. For
example, it tells the destination of the message and the checksum information.
• Application Layer:
Data is read and written in this layer. The content of the layer varies according to
the purpose of the message.
Many slave stations
(up to 31 units)
Master station
Slave station
Instruction message from
master station
Response message from
slave station
Application layer
Data link layer
Application layer
Data link layer
The driver in the data link layer has the following
information:
• The destination (the device address).
• The check sheet for the shipment (the checksum).
The application layer cargo (data)
changes every time according to
the purpose of the operation.
Details on each layer are given in the next section.
4 - 2 Message Structure