VEGA WHA-GW-WIRELESSHART GATEWAY - Manual (Page 86)

22
1981
2011
-0
7
86
WHA-GW-*
Operation
Dynamic variable mapping format
For each HART device, the 5 possible CMD 3 floating-point dynamic variables are
sequentially mapped (the CMD 3 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 CMD 3 dynamic variables follow the IEEE-754 (IEC 559) single-precision floating-point
format (see following table).
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. Note that the Modbus floating value will be
transmitted in the "big-endian" style. 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
where "00" – the less significant byte – is the first one to be transmitted.
0x20 Cold start A power failure or device reset has
occurred.
0x10 More status
available More status information is available via
Command 48 "Read additional status
information".
0x08 Loop current fixed The loop current is being held at a fixed
value and is not responding to process
variations.
0x04 Loop current
saturated The loop current has reached its upper (or
lower) endpoint limit and cannot increase
(or decrease) any further.
0x02 Non-primary
variable out of limits A device variable not mapped to the PV is
beyond its operating limits.
0x01 Primary variable out
of limits The PV is beyond its operating limits.
Sign of the Fraction 8-bit Exponent 23-bit Fraction
Byte 0 Byte 1 Byte 2 Byte 3
0x00 0x20 0xF1 0x47
Bit Parameter Description