Varec TankGate Interface 8315 Series - Manual (Page 33)

8315 TGI Software Description
Varec, Inc. 23
PntCheckSum Parameter
The PntCheckSum is a CRC-16 checksum of the Configuration Parameters of a point. Most
points have the PntCheckSum parameter; a few like the CLK (Clock) Point, DIAG (Diagnostics)
Point, and the register points (BREG, IREG, and DREG) do not. Any time a Configuration
Parameter is changed, the Tank Gate Interface re-calculates the checksum. The PntCheckSum is
used at initialization to validate each point in the database.
PntStatus Parameter
The PntStatus parameter gives information about point execution. Refer to the information
provided for each Software Block about the particular use of the PntStatus parameter.
Elapse Parameter
The Elapse parameter usually indicates the time of the last point execution. It is used as the
basis for automatic Change of State notifications.
Value Parameter
The Value parameter indicates the real-time value of the point. In the case of DI points, the Value
represents a Coded value (i.e., On, Off, Opened, Closed, etc.) AI points represent the Value in an
IEEE floating point format. In most cases, the Value parameter is one of the most important
parameters for a point.
Level & Temp Parameters
The Level & Temp parameters indicate the real-time tank level and temperature of the tank
gauge point. There are many Software Blocks available in the Tank Gate Interface associated with
tank gauge communications, including Software Blocks for every major tank gauge
manufacturer.
4.3 Gathering Point Data
For Host to Tank Gate Interface communications, there are two primary methods for gathering
point data. These methods are listed below:
4.3.1 Standard Request-Response Format
The Host issues scan requests, and the Tank Gate Interface responds with the data.
The Scan method has the advantage of being deterministic, the response time to a particular
request will always be the same.
This method has the limitation that if there are many points defined in the Tank Gate Interface,
it often takes several requests to obtain all of the data. Most of the time data has not changed
since the last request, so this results in inefficiencies in update times. This situation is often the
case in systems where there are many digital points that do not change frequently.
4.3.2 Change of State Format
The Host issues scan requests based on a Change of State condition. Some systems refer to this
as Event Mode. The Tank Gate Interface keeps track of the items that have changed since the
last Host request. The Host can issue a single request of ‘tell me what has changed since my last
request.’ The Tank Gate Interface will respond with the results, which include addressing
information about the data. In a typical system, this results in a significant improvement in the
throughput rate for data.