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Theory of Operation Tank Gauge Transmitter
40 Installation and Operations Manual
Table 7-1: Modbus Functions
Table 7-2: Modbus Exception Responses
7.10 Mark/Space Host Interface
The Mark/Space interface provides compatibility with existing Mark/Space data highways. The interface operates in
one of two modes.
• Transmitter Emulation Mode
• Varec 1800 Transmitter Emulation
• Varec 1900 Transmitter Emulation
• Mark/Space Modbus Communication Mode
When operating in the 1800 or 1900 emulation mode, the ATT is compatible with all existing Varec field interfaces
such as the Tank Polling Unit (TSU) or RTU 8130.
The ATT automatically determines if it is being polled as an 1800/1900 transmitter. When polled, the ATT will respond
based on the configured emulation mode. The ATT can be configured to support the following encoding formats:
• Imperial Fractional
• Imperial Decimal
• Metric, 0-20 meter
• Metric, 0-30 meter
When emulating an 1800 or 1900, battery low status can be reported as one of the dry-contact inputs or as an invalid
level.
Note! The ATT will accept but ignore the Raise/Lower Displacer commands.
Function Number Function
01 Read Digital Outputs
02 Read Digital Inputs
03 Read Analog Output Registers
04 Read Analog Input Registers
05 Write Single Digital Output
06 Write Single Analog Output Register
08 Loopback Test
15 Write Multiple Digital Outputs
16 Write Multiple Analog Output Registers
65 Read Floating Point Registers
66 Write Floating Point Registers
Number Exception
01 Illegal Function
02 Illegal Address
03 Illegal Data
04 Busy