Varec 4000 - Manual (Page 48)

Theory of Operation Tank Gauge Transmitter
38 Installation and Operations Manual
7.3 Status Inputs and Outputs
Status inputs and outputs are accomplished via a separate I/O Module. The ATT will communicate with the Module via
the ATTI port. The state of the inputs will be mapped into Modbus registers to allow a host computer to read them.
Control of status outputs will also be available to a host via Modbus registers. This unit is currently under development.
7.4 ATTI Communications Port
The ATTI Communication Port is a HART device compatible port. However, it does not power HART devices and is not
designed to be intrinsically safe. It is used to communicate with the 1200 Hand Held Terminal for configuration. In the
future it will support digital I/O extensions to the ATT including, 4120 Multi-Element Temperature Transmitter and
4040 Tankside display
7.5 Host Communications Interfaces
The ATT supports a plug-in host communication interface module. The following host communication modules are
supported in the initial release of the ATT.
•
EIA485 Modbus
•
Mark/Space
•
GPE Loop Communications
•
TIWAY Communications
•
L&J Tankway Communications
•
Varec Mark Space Matrix
•
GSI Modbus
7.6 Level Measurement
The HART Level Encoder utilizes a proprietary incremental count technique for determining liquid level. Two code disks
are provided in the encoder with each disk containing a number of slots. Optical sensors for each disk detect disk
rotation by sensing the passing of these slots. The primary disk contains 128 slots and is used to calculate the actual
liquid level. The secondary or check disk contains only four slots and is used to verify or check the level from the
primary disk. The count obtained from these sensors is stored in encoder memory.
7.6.1 Back-up Battery
The encoder's back-up battery provides continuous operation during power outages. The battery is non-rechargeable
with a continuous operating lifetime of 10 days or more and a shelf life of ten years. To preserve the battery life,
automatic battery shut down occurs after 24 hours of continuous power outage. The encoder monitors the voltage of
the back-up battery and provides a battery low warning to the host in the event the battery needs replacement or has
been turned shutdown due to an extended power outage.
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.