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4000 Theory of Operation
7.11 Modbus Support over Mark/Space Host Interface
The ATT permits Modbus communications over the Mark/Space data highway. The Modbus request message is placed
into a Mark/Space message frame. The ATT processes the request message and places the Modbus reply in a Mark/
Space reply frame.
This option is only available with systems using the Model 6840 Tank Polling Unit.
7.12 Compatibility with Varec Tank Polling Unit
The Mark/Space communications provided by the ATT are compatible with the Varec Tank Polling Unit (TPU). This
permits the ATT to be multi-dropped on the same Mark/Space data highway as the Varec 1800, Varec 1900, and MFT/
HIU devices.
If the EIA485 communications interface is installed in the ATT, it can be multi-dropped on an EIA485 data highway,
RTU 8130 along with any Modbus compatible device, including the Model 6850 Field Interface Converter.