Siemens S7-300 - Page 273

Analog modules
9.3 SM 336; F-AI 6 x 0/4 ... 20 mA HART
Fail-safe signal modules
Installation and Operating Manual, 01/2010, A5E00085586-10
273
Configuring the discrepancy analysis parameters
The following four parameters for the discrepancy analysis are assigned for each channel
pair in
HW Config :
● Discrepancy time
● Standard value
● Tolerance window %, absolute
● Tolerance window %, relative
"Discrepancy time" parameter
If the standard value of both input channels of a channel pair is outside the assigned
tolerance window and exists longer than the specified discrepancy time but no more than the
duration of the maximum response time, the module detects a discrepancy error. In the case
of a discrepancy error, the module triggers a diagnostic interrupt and sets the process value
to 7FFF
H
. The discrepancy time is reset if the standard value lies within the tolerance window
again.
In
S7 Distributed Safety , the fail-safe value 0 is provided in the PII for the safety program in
place of 7FFF
H
.
In
S7 F Systems , the fail-safe value is provided according to the F-channel driver parameter
assignment.
You calculate the maximum discrepancy time that is permitted in the respective application
using the following formula:
Discrepancy time = Maximum response time (in case of discrepancy) – 2 × Conversion cycle
time × Smoothing – 2 × Conversion cycle time
Note
Calculate the discrepancy time by inserting the values from Chapter "Technical Data of the
SM 336; F-AI 6 x 0/4 ... 20 mA HART (Page 263)" in the formulas shown above.
You can assign the discrepancy time for each channel pair. The entered value is rounded to
an integer multiple of 10 ms. The value "0" is permitted. Small values other than 0 are
rounded to a permissible minimum value.
"Standard value" parameter
You can select which of the two values is to be signaled to the F-CPU for each input channel
pair. During a discrepancy between the two input channels, the last valid standard value
prior to occurrence of the discrepancy is signaled to the F-CPU.
● "MIN": The lower of the two values is signaled to the F-CPU as the standard value.
● "MAX": The higher of the two values is signaled to the F-CPU as the standard value.