Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 18)

Publication 1757-UM006A-EN-P - May 2002
1-4 The Fieldbus Communication Model
Communication Layer Description
The following table provides a summarized description of the
communication layers that make up the F
OUNDATION Fieldbus. The
Fieldbus Foundation maintains a complete library of detailed
reference specifications including a Technical Overview, and Wiring
and Installation Guides.
Table 1.A Communication Layer Descriptions
Layer Functional Description Associated Terms
Physical Defines the transmission medium for fieldbus signals and the message conversion tasks
to/from the Communication Stack.
Based on the Manchester Biphase-L Encoding technique, so a F
OUNDATION Fieldbus (FF)
device interprets a positive transition in the middle of a bit time as logical “0” and a
negative transition as logical “1”.
Complies with existing International Electrotechnical Commission (IEC 1158-2) and the
Instrumentation, Systems, and Automation Society (ISA S50.02) physical layer
standards. And, it can be used with existing 4 to 20mA wiring.
H1, 31.25 kbit/s signal rate
H1 Link
H1 Segment
HSE, High Speed Ethernet
Data Link (DLL) Defines how messages are transmitted on a multi-drop network. It uses a deterministic
centralized bus scheduler called a Link Active Scheduler (LAS) to manage access to the
fieldbus. It controls scheduled and unscheduled communications on the fieldbus in a
publish/subscribe environment.
Identifies device types as Basic Device, Link Master, or Bridge. A Link Master device
type can become a Link Active Scheduler (LAS) for the network. Compel Data (CD) message
Pass Token (PT) message
Time Distribution (TD)
message
Live List
Link Active Scheduler (LAS)
Fieldbus Access
Sublayer (FAS) Defines the types of services used to pass information to the Fieldbus Message
Specification layer. The types of services are defined as Virtual Communication
Relationships (VCR).
The VCR types are Client/Server, Report Distribution, and Publisher/Subscriber. The
Client/Server type handles all operator messages. The Report Distribution type handles
event notification and trend reports. The Publisher/Subscriber type handles the
publishing of User Application function block data on the network. Virtual Communication
Relationship (VCR)
Fieldbus
Message
Specification
(FMS) Defines how fieldbus devices exchange User Application messages across the fieldbus
using a set of standard message formats. It uses object descriptions that are stored in
an object dictionary (OD) to facilitate data communication. The OD also includes
descriptions for standard data types such as floating point, integer, Boolean, and
bitstring.
A Virtual Field Device (VFD) mirrors local device data described in the OD. A physical
device may have more than one VFD.
Provides these communication services to standardize the way the User Applications
such as function blocks communicate over the fieldbus - Context Management, Object
Dictionary, Variable Access, Event, Upload/Download, and Program Invocation.
Uses a formal syntax description language called Abstract Syntax Notation 1 (ASN-1) to
format FMS messages and applies special behavioral rules for certain types of objects. Object Dictionary (OD)
Virtual Field Device (VFD)
Network Management
Information Base (NMIB)
System Management
Information Base (SMIB)