Instruction manual

Theory of Operation Tank Gauge Transmitter
40 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.