St 700 Ff Pressure Transmitter Operation; Operational Considerations; Las Capability; Special Non-Volatile Parameters And Nvm Wear-Out - Honeywell ST700 User Manual

4. ST 700 FF Pressure Transmitter Operation

4.1

Operational considerations

There are a number of considerations that must be noted when configuring a transmitter to
operate in a fieldbus network.

LAS Capability

The transmitter is capable of operating as the Link Active Scheduler (LAS). The LAS is a
fieldbus feature which controls traffic on the network, such as controlling token-rotation
and coordinating data publishing. This fieldbus function is active in only one device at any
given time on a network. Devices which can be designated as the LAS may be an operator
station or a field device. The transmitter can be designated as LAS, in the event of a failure
of the primary LAS, control in the field could continue.
ATTENTION
Note that the transmitter can be used only as "backup" LAS.

Special Non-volatile parameters and NVM Wear-out

All function block parameters designated as Non-Volatile (N) in the FF specifications are
updated to non-volatile memory (NVM) on a periodic basis. NV_CYCLE_T parameter in
the resource block specifies this update interval.
To provide predictable restart behavior in the transmitter, the following Non-Volatile
parameters are updated to NVM each time they are written over the fieldbus.
•
MODE.TARGET for all blocks
•
SP.VALUE for the PID block
Since these are user-written parameters, these additional updates to NVM contribute
negligibly to NVM wear out. However, users are cautioned to not construct control
configurations where the above parameters are written continuously (via a computer
application for example) or at rates greater than the NV_CYCLE_T interval. This
consideration helps to minimize the possibility of NVM wear-out.
In the case of MODE this must not be a problem. When users wish to provide set-points to
the PID block via a computer application, users should use RCAS mode with its
corresponding set point value RCAS_IN. RCAS_IN is updated only at the
NV_CYCLE_T update rate and this mode supports full shedding functionality and PID
initialization necessary for a robust application.

Mode Restricted Writes to Parameters

Some block parameters have restrictions on having write access to them. These are
specified in the FF specifications. Writing to certain function block parameters are
restricted based on the block's Target and/ or Actual mode.
Revision 2.0 FOUNDATION Fieldbus ST 700 Pressure Transmitter User's Guide Page 87