Inhaltsverzeichnis
Werbung
5 Operation
58
5.2.4
Cyclic data exchange
®
In the PROFIBUS-PA
the cyclic transmission of the analogue values to the automation system is
done in 5 byte long blocks. The measured value is displayed in the first 4 bytes in a floating decimal
point format to the IEEE 754 standard (see IEEE floating decimal point).The 5th Byte contains status
information to the measured value, this is then implemented to the Profile 3.0 specification
(page 59).
IEEE floating decimal point number
Converting a Hex value into an IEEE floating decimal point number for measured value recording.
The measured values are displayed in an IEEE-754 number format as follows and transmitted to the
master class 1.
Byte n
Bit 7
Bit 6
Bit 0
7
6
5
4
3
2
1
VZ
2
2
2
2
2
2
2
Exponent
VZ
* 2
Formula value = (-1)
Example: 40 F0 00 00 hex = 0100 0000 1111 0000 0000 0000 0000 0000 binary
0
(129-127)
Value = (-1)
* 2
* (1 + 2
2
= 1 * 2
* (1 + 0.5 + 0.25 + 0.125)
= 1 * 4 * 1.875 = 7.5
The process temperature is transmitted in a cyclic data exchange from the TMT184 temperature
head transmitter.
TMT184 ---> Automation system
Process
Input byte
parameter
0, 1, 2, 3
Temperature
4
Status temperature
Output data
Display value
The display value offers the possibility to transmit a value calculated in the automation system
directly to the head transmitter. This measured value is a purely display value that can be displayed,
for example, using an RID 261 PROFIBUS-PA display. The display value contains a 4 byte measured
value an 1 byte status value.
Byte n+1
Bit 7
Bit 6
Bit 0
-1
-2
-3
-4
-5
2
2
2
2
2
2
0
2
7
Mantissa
(Exponent -127)
* (1 + Mantissa)
-1
-2
-3
+ 2
+ 2
)
Access mode
Read
Read
Byte n+2
Bit 7
Bit 0
-6
-
-8
-9
-10
-11
-12
-
2
2
2
2
2
2
2
13
-14
-15
2
2
Mantissa
Comment/Data
format
32 Bit floating deci-
mal point (IEEE-
754)
page 59
TMT184
Byte n+3
Bit 7
Bit 0
-16
-23
2
bis 2
Mantissa
Factory default
units
° C
-
Endress+Hauser
Werbung
Kapitel
Inhaltsverzeichnis
Fehlerbehebung
