Endress+Hauser WirelessHART Fieldgate SWG70 - Mapping Formats; Dynamic Process Variables; Status Mapping

WirelessHART Fieldgate SWG70 Modbus Interface
Endress + Hauser 121
16.3 Mapping formats
16.3.1 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 spe-
cific 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 "AMS via Ethernet" on page 54.
For example, the number 123456.00 as defined in the IEEE-754 standard appears as
follows:
This number will be transmitted from the Fieldgate 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 10.3.1 "Modbus Settings" on
page 70.
16.3.2 Status mapping
For each HART device, two 16-bit unsigned input registers are used to map the relevant sta-
tus 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
– Bit 1: Identifications
– Bits 2 – 15: Not used
• 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. See
Chapter 16.2 "Rules for mapping" on page 119.
1-bit Sign of Fraction 8-bit Exponent 23-bit Fraction
Byte 0 Byte 1 Byte 2 Byte 3
0x00 0x20 0xF1 0x47