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Installing and Connecting the Basic System
Discrepancy Time
Usually, 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 subunits
•
Different switching instants in the case of two-channel sensors/contacts
In order to differentiate between a hardware failure and a fleeting, coincidental signal change, the
safety-related input signals are subjected to a discrepancy analysis.The analysis is executed by the
operating system; you simply specify a discrepancy time in DB1, within which the signals match
again.
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.
Type of Input
OB2 interrupt DI
OB3 interrupt DI
Standard DI
Note
•
The average discrepancy time (approx. 5 ms) fluctuates with the synchronization
interval. For systems with no load it is
interrupt load it is more than 10 ms.
•
If you select the discrepancy time as a number of OB13 cycles, you must initialize
the OB13 interval in DB1. However, you need not write OB13 in the user program.
4-8
Table 4-3. Discrepancy Times for Digital Inputs
Address
IB 59
Discrepancy time, short
Discrepancy time, short
IB 32 to 33,
Discrepancy time, average
IB 59
Discrepancy time in
Discrepancy time in
IB 0,2 to 30
Discrepancy time in
Discrepancy time, short
IB 32 to 33
Discrepancy time in
Discrepancy time in
IB 59
Discrepancy time, short
Discrepancy Time Programmable in DB1
: approx.1 ms
: approx.1 ms
: approx. 5 ms
: n * OB1 cycles
: n * OB1 cycles
: n * OB13 cycles
: approx. 1 ms
: n * OB1 cycles
: n * OB13 cycles
: approx. 1 ms
5 ms and for systems with maximum
S5-95F
EWA 4NEB 812 6210-02
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