KROHNE BM 100A PROFIBUS PA EN - Manual (Page 5)

Supplementary documentation BM 100 A with PROFIBUS-PA
KROHNE Messtechnik GmbH & Co. KG · Ludwig-Krohne-Str. 5 R D-47058 Duisburg 5/7
Tel.: 0203-301 309 R Fax: 0203-301389 · e-mail: [email protected]
Units
Independent from the units within the local operating display Level and Distance values will be transmitted in „m“
always. Depending from the volume table Volume will be transmitted as cubic metres (m3) or tons (t).
1.3 Meaning of measurement and status information
Each configured measurement value is a 4 Byte Float Format according IEEE Standard 754 Short Real Number. A
status byte follows each measurement value. That means each measurement value consists of 4 byte value plus 1
byte status. Other measurement values will directly follow as 5 Byte package if configured.
First an example of the float format:
Float Format
Byte n Byte n+1 Byte n+2 Byte n+3
Bit7 Bit6 Bit7 Bit6 Bit7 Bit7
VZ 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0 2 -1 2 -2 2 -3 2 -4 2 -5 2 -6 2 -7 2 -8 2 -9 2 -10 2 -11 2 -12 2 -13 2 -14 2 -15 2 -16 2 -17 2 -18 2 -19 2 -20 2 -21 2 -22 2 -23
Exponent Mantissa Mantissa Mantissa
Example: 40 F0 00 00 (hex) = 0100 0000 1111 0000 0000 0000 0000 0000 (binary)
Calculation: Value = (-1)
VZ
* 2
(Exponent - 127)
* (1 + Mantissa)
Value = (-1)
0
* 2
(129 - 127)
* (1 + 2
-1
+ 2
-2
+ 2
-3
)
Value = 1 * 4 * (1 + 0,5 + 0,25 + 0,125) = 7,5
Status (1 Byte)
Quality Quality-Substatus Limits
Gr Gr QS QS QS QS Qu Qu
2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0
0 0 = bad
0 1 = uncertain
1 0 = good (Non Cascade)
Status = bad
0
0 0 0 0 1 = configuration error
0 0 0 0 1 1 = device failure
0 0 0 1 0 0 = sensor failure
0 0 0 1 1 1 = out of service
Status = uncertain
0 1 0 0 0 1 = last usable value
0 1 0 0 1 1 = initial value (value before first measurement)
0 1 0 1 1 0 = sub-normal (appears if level or interface level has been frozen)
Status = good
1 0 0 0 0 0 = ok
1 0 1 0 0 1 = maintenance required
Status = Limits
0 0 = ok
0 1 = low limited
1 0 = high limited
1 1 = constant
Check the first two quality bits in order to get the quality information of the measurement value:
Good (non Cascade)
measurement value is ok and can be used without restrictions
Uncertain measurement value can be used but the accuracy can not be guaranteed (e.g. measurement
value has been frozen or A/D converter is saturated)
Bad measurement value is bad don´t use it
The „Quality-Substatus“- and „Limit“-Bits can be used for further diagnostics or limit checking.
Attention: The status should be watched always because a valid number will be transmitted even if the status of
the measurement is bad or uncertain. This is the only chance to check the meaning of the
transmitted measurement values.