2
02
0-
11
146
Appendix
Figure 8.2
Bit cell jitter
Segment or Field Device Jitter
This parameter monitors the current maximum jitter of all active devices connected to the seg
-
ment. The H1 jitter level is a value derived from the device jitter values. Due to noise levels
caused by additional effects, the diagnostic device indicates a first warning at 75 % (2.4
s) of
the maximum allowable jitter level. This is an empiric value that can be modified according to
your requirements. If the jitter level exceeds 3.2
s, a final warning is issued.
The segment jitter is the maximum value of all device jitter values. A high jitter level causes
communication problems and a lack of operational reliability. The transmitted bit cell jitter can
-
not exceed 10 % of 1-bit time. E. g., at 31.25 kbits/s the duration of 1-bit time is 32
s. Thus, the
maximum bit cell jitter cannot exceed 3.2
s. Your system can run at a higher jitter level, but at
the expense of a reduced level of immunity against influences on electromagnetic compatibility.
Jitter can have the following causes:
•
Crosstalk
•
Electromagnetic interferences (EMI)
•
Simultaneous switching outputs
•
Device dependency
•
Bad wiring practice
A
Reference event; first zero crossing point
B
Actual zero crossing point
C
Bit cell jitter, deviation from the ideal timing
D
Ideal zero crossing point
Type
Values
Precision
0.1
s
Measuring range
0
s ... 8
s
C
L
t
V
1 bit time
0.5 bit time
0.5 bit time
A
B
D
C
