Varec 4000 manual Mark/Space Host Interface, Function Number, Number Exception

Theory of Operation Tank Gauge Transmitter

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
Table 7-1:	Modbus Functions

Number		Exception
01		Illegal Function
02		Illegal Address
03		Illegal Data
04		Busy
Table 7-2:	Modbus Exception Responses

7.10Mark/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.

40 Installation and Operations Manual