F_Ch_Ai - Siemens SIMATIC S7 F Manual

Fail-Safe Blocks
Non-Linearized Value Checking
Depending on the measurement type and measurement range, there is a rated
range of the analog input module, in which the analog signal is converted to a
digitized non-linearized value. To this end, there is an overrange and an
underrange in which the analog signal can still be converted. Overflow and
underflow apply beyond these limits. The F channel driver indicates whether the
non-linearized value lies within the rated range of the module. If the value lies
under the rated range, the output parameter QCHF_LL = 1 is set. If the value lies
above the rated range, the output parameter QCHF_HL = 1 is set. In the case of
overflow or underflow, the output QBAD = 1 is also set, and, depending on the
parameter assignment, a substitute value or the last valid value is output.
In the event of channel faults (e.g. wire break), the module outputs 16#7FFF
(overflow) as a non-linearized value. Accordingly, the F channel driver F_CH_AI
detects an overflow and sets the output QCHF_HL = 1 and QBAD = 1.
NAMUR Limit Value Checking
In the NAMUR guidelines for analog signal processing, limit values are defined for
life zero (4 to 20 mA) analog signals where there is a channel fault:
3.6 mA < analog signal < 21 mA.
By default, the above NAMUR limits are set for limit value checking. If other limit
values are to be set, the input parameter CH_F_ON = 1 must be set and the input
parameters CH_F_HL and CH_F_LL must be set in mA with corresponding new
limit values. In the event of overflow or underflow of the active limit values, the
output QBAD = 1 is set, and, in the case of a life zero analog signal, a substitute
value or the last valid value is output, depending on the parameter assignment
(input SUBS_ON).
Note
The selectable limit values must be under the upper limit of the overrange and
above the lower limit of the underrange of the module. Values outside the NAMUR
range are thus also possible, unless the module automatically limits the measured
values.
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Fail-Safe Systems
A5E00085588-03