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AS-Interface Analyser
6.2
More Critical Networks
A net of similar quality which is used in an application which stops even if a slave
fails for a short time only, should be examined more precisely. Every "yellow" or
"red" slave basically indicates the risk of a reduced availability.
Something similar applies to applications used by operators who cannot handle
AS-Interface. The examination with the analyser as well as the detailed clarificati-
on of the repetitions` occurrence decreases the danger of taking expensive repair
services.
It should be inspected also if the effort for avoiding repetitions` can be justified eco-
nomically. The evaluation of the consecutive error helps to decide rationally: If
repetitions` only occur in the lower classes – even though this net has been obser-
ved for a longer period – a good forecast for a further trouble-free run is likely to
be made.
In some cases eventual risks can be evaded by determining in the software or on
the master's level whether the failure of a particular slave should effect an imme-
diate stop of the facility or whether it is sufficient to stop the facility after the end of
a facility cycle. The second option can be handled more easily.
6.3
Facility and Safety Engineering
Facility and safety engineering represents potentially two exceptions:
• In the field of facility engineering there are applications, whose breakdown must
be absolutely avoided for cost reasons. Here it can be suggested to arrive close
to the theoretical ideal of "no telegram repetitions". Eventually you even try to
avoid repetitions of "only green" slaves.
• A second special case is safety related facilities according to "Safety at Work".
Here repetitions are acceptable also because they will be intercepted by the
system and do not limit the security. In order to ensure a turn off after max.
40 ms, it is fixed here that the safety monitor already responds after the fourth
telegram repetition. Therefore, with safety slaves a consecutive error of the
class 4 leads to a turn off and so to a reduced availability of the facility. Therefore
repetitions are judged here more critically.
6.4
Example
Fig. 16 exemplarily shows how the representation of the consecutive error can af-
fect the validation. The slave 1 communicates badly in a net with excess length.
The overview with hold time indicates that the slave is red. According to the „ad-
vanced statistics" about 17 % repetitions arise with this slave. However, the control
of the consecutive errors shows that all repetitions refer to the class 1. After a run-
time of more than 2 minutes the slave exhibits many errors which belong to class
1 only.
Validation: The net should be stabilized indeed because is it not working "clean".
Depending upon the cause of the disturbance this stabilisation can be achieved by
using an AS-i bus termination or an AS-i tuner. If EMC disturbances cause the pro-
blems they must be eliminated. But due to the fact that only one repetition of each
call is necessary the error is tolerable first. That applies also to the relevant times
Subject to reasonable modifications due to technical advances.
SIEMENS AG · Low-Voltage Controls · Phone +49 911 895 59 00 · Fax +49 911 895 59 07 · Internet: http://www.siemens.com/lowvoltage/technical-assistance
Valuation of Errors
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