Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 17)

Publication 1757-UM006A-EN-P - May 2002
The Fieldbus Communication Model 1-3
Open Communications Architecture
F OUNDATION Fieldbus is an enabling technology for dynamically
integrating dedicated field devices with digitally based control
systems. It defines how all “smart” final control devices are to
communicate with other devices in the control network. The
technology is based upon the International Standards Organization's
Open System Interconnection (OSI) model for layered
communications.
As shown in Figure 1.1, OSI layer 1 is the Physical Layer, OSI layer 2
is the Data Link Layer, and OSI layer 7 is the application layer or the
Fieldbus Message Specification. A Fieldbus Access Sublayer maps the
Fieldbus Message Specification onto the Data Link Layer. Fieldbus
does not use OSI layers 3 to 6, and layers 2 and 7 form the
Communication Stack. Also, the OSI model does not define a User
Application, but the Fieldbus Foundation does.
Figure 1.1 OSI versus Fieldbus communication model
Physical Layer
Data Link Layer Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
Fieldbus Message
Specification
Fieldbus Access
Sublayer
6
5
4
3
2
1
7
Physical Layer Physical Layer
Communication
Stack
OSI Model
Fieldbus Model
User Application User Application