WirelessHART Fieldgate SWG70, Operating Instructions Manual

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A Appendix: Modbus Interface WirelessHART Fieldgate SWG70
96 Hauser
A.3 Mapping formats
A.3.1 Dynamic process variables
For each HART device, the 5 possible Command 3 floating-point dynamic variables are sequentially
mapped (units code values are not mapped). If a device does not support a specific dynamic value,
a "NaN" (Not a Number) floating-point value is returned (namely, 0x7F, 0xA0, 0x00, 0x00).
The CMD3 dynamic variables follow the IEEE-754 (IEC559) single-precision floating-point format
The same format is also used for the Modbus 32-bit floating point values.
The Modbus protocol does not explicitly specify any 32-bit data element. However, the usage of 2
consecutive 16-bit registers is the de-facto standard way to map a single-precision IEEE-754
floating-point value. The Modbus floating value will be transmitted in the "big-endian" or "little
endian" style according to the swap selection in the Modbus dialog, see Chapter 8.4.4.
For example, the number 123456.00 as defined in the IEEE-754 standard appears as follows:
This number will be transmitted from the Gateway to the Modbus master in the following sequence:
47 F1 20 00
in "big-endian" style where "00" – the less significant byte – is the first one to be transmitted. The
sequence can be changed to "little-endian", see Chapter 10.2.1.
A.3.2 Status mapping
For each HART device, two 16-bit unsigned input registers are used to map the relevant status
information. The status information is delivered as a series of independent flags. Within an input
register, the unused bits are always returned as "0".
• ModStat: Modbus Specific Status
– Bit 0: Cache validity (= 1 when cache empty). If the bit is set for a long period, this
usually indicates that CMD 3 publishing is disabled for the device concerned- 13: Not used
– Bit 1: Identification ongoing (=1 when Fieldgate is identifying a device)
– Bit 3: Device off-line(= 1 when device is off-line).
• DevStat: The "Device Status" information according to the HART 7 specification
– Bit 0: Primary variable out of limits
– Bit 1: Non-primary variable out of limits
– Bit 2: Loop current saturated
– Bit 3: Loop current fixed
– Bit 4: More status available
– Bit 5: Cold start
– Bit 6: Configuration changed
– Bit 7: Device malfunction
For more information see the Modbus Specific Status and HART Device Status tables in
Chapter A.2.1.
1-bit Sign of Fraction 8-bit Exponent 23-bit Fraction
Byte 0 Byte 1 Byte 2 Byte 3
0x00 0x20 0xF1 0x47