Varec 4000 Status Inputs and Outputs, Atti Communications Port, Host Communications Interfaces

Theory of Operation Tank Gauge Transmitter

7.3Status 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.

7.4ATTI 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

7.5Host 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

7.6Level 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.

7.6.1Back-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.

7.6.2Damping

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.

38 Installation and Operations Manual