WirelessHART Fieldgate SWG70 Instructions d'exploitation télécharger pdf (Page 98)

A Appendix: Modbus Interface WirelessHART Fieldgate SWG70

96 Endress+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

• 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 0Byte 1Byte 2Byte 3

0x00 0x20 0xF1 0x47

1 2 ... 93 94 95 96 97 98 99 100 101 102 103 ... 123 124

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WirelessHART Fieldgate SWG70 Instructions d'exploitation Page 98