Rules for Failsafe Operation of an S5-95F
S5-95F
18.2.2 Discrepancy Times
As a rule, the Read result for an input signal is identical in both subunits. However, in certain
situations there can sometimes be a discrepancy. Reasons for this may be
•
A hardware failure, such as failure of a sensor or input module
•
A difference in the instant of access of the two PLCs
•
Different switching instants in the case of two-channel sensors or contacts
In order to differentiate between a hardware failure and a fleeting, coincidental signal change, the
S5-95F subjects all failsafe input signals to a discrepancy analysis. All you, as user, have to do is
provide the permissible discrepancy time in DB1; at the end of this time, at the very latest, the
signals must once again coincide. You may define a different discrepancy time for each input byte.
From the instance at which the discrepancy is detected until it has been rectified, or until the
specified discrepancy time has elapsed, the two subunits use the last valid value for the user
program.
If a discrepancy remains between input signals for a longer period than specified in DB1, the S5-95F
assumes a hardware failure and responds with an error message and reacts accordingly.
Safety Note
The discrepancy times must be selected so that the sum of discrepancy time and total
PLC response time is less than the error tolerance time of the relevant process
function.
Table 18-2.
Discrepancy Times for Digital Inputs
Type of Input
Address
Discrepancy Time Programmable in DB1
OB2 interrupt DI
IB 59
Discrepancy time, short
: Approx.1 ms
OB3 interrupt DI
IB 32 to 33,
IB 59
Discrepancy time, short
: Approx.1 ms
Discrepancy time, average
: Approx. 5 ms
Discrepancy time in
: n* OB1 cycles
IB 0,2 to 30
Discrepancy time in
: n* OB1 cycles
Discrepancy time in
: n* OB13 cycles
IB 32 to 33
Discrepancy time, short
: Approx.1 ms
Discrepancy time in
: n* OB1 cycles
Discrepancy time in
: n* OB13 cycles
IB 59
Discrepancy time, short
: Approx.1 ms
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Standard DI
18-10
EWA 4NEB 812 6210-02
