Varec Network Adapter 4000, Installation And Operation Manual, Page: 51 / 80

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4000 Theory of Operation
7.6.2 Damping
The ATT implements a damping scheme to stabilize the level and temperature in
turbulent tanks. The amount of damping is specified by a damping value in the
'DAMPING' parameter under the Config Menu. This parameter can be entered via the
Model 1200 using the Alter menu.
A damping value of 0 to 16 can be specified. A damping value of 0 implies no damping.
A damping value of 16 implies that level and temperature is averaged over 16 readings.
7.7 Encoder Battery Backup
The ATT includes battery backup for the encoder. This allows level to be maintained
across a power failure. Several consideration are made to protect the battery from being
fully discharged.
1. The battery must be physically enabled by the microprocessor. This allows the bat-
tery to be shipped connected without being discharged.
2. Service personnel can turn the battery off. This will allow the battery to be disabled
when a tank or a transmitter is to be taken out of service for an extended period of
time.
3. On-board circuitry limits the battery backup time to 24 hours. This allows battery
life to be extended even if service personnel forget to disable the battery during
extended power outage conditions.
The battery is automatically turned on when the ATT is calibrated with a given level.
7.8 Temperature Measurement
The ATT measures temperature directly using a high accuracy 16 bit analog to digital
converter. Temperature inputs can be either a 3-wire Copper or Platinum RTD.
Optionally the temperature can be manually entered. The 4120 Multi-Element
Temperature transmitter can be used as a replacement to a spot temperature bulb
(RTD).
7.9 Host Communication
The Modbus host interface permits the ATT to directly communicate with any distributed
control system utilizing the Gould Modbus protocol. The Modbus protocol defines two
data formats, ASCII and RTU. The RTU format specifies that all data is in binary. The
ASCII format specifies that all data is in ASCII (producing messages twice as long as RTU
format messages). The ATT only supports the RTU message format.
The Modbus protocol permits a host computer to view field devices as having analog
input registers, analog output registers, digital input registers, and digital output
registers. Measured parameters, configuration parameters, and status conditions are
mapped to Modbus analog and digital registers.
A non-standard Modbus floating point register extension has been defined for use by
the ATT. This format extension permits floating point data to be directly read from and
written to any Modbus device. The IEEE 754 standard floating point format is used. Each
floating point register consists of four bytes.
To provide compatibility with future enhancements, data written to undefined registers
are accepted and ignored. Data read from undefined registers returns a value of zero (0).