R400 Experion PKS Series A Fieldbus Interface Module User's Guide 539
July 2010 Honeywell
18. Appendix I
18.1 Link Parameters Reference
Background
The architecture of the Fieldbus communications entity is based on the ISO OSI
Reference model, but it includes only three layers as shown in the following illustration.
SMKP
SMK
Device Aps
Network
Mgmt Agent
DIR OD
NMIB
FMS
FAS
Data Link Upper Level Functions
Data Link Media Access Functions
PH - SAP
Physical Medium Independent
Physical Medium Dependent
MAU
MAU
FMS
MAU
DL-SAPs Only
DL-SAP/CEPs
Via
ALME
Via
DLME
Via
PLME
Physical Medium Physical Medium
Physical Medium
Application
Layer
Data Link
Layer
Physical
Layer
VCRs
Figure 51 Communications entity architecture.
The layer protocols provide Virtual Communication Relationships (VCRs), which are
communication channels between applications. The Fieldbus Message Specification
(FMS) provides VCR application-messaging services, such as reading and writing
variables. Some device Applications Processes (APs) may access the Fieldbus Access
Sublayer (FAS) directly rather than use the FMS. The System Management Kernel
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