System overview
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Multiple AS-i safety monitors can be used within an AS-i system. In this way, a safety-
related AS-i slave can be monitored by multiple AS-i safety monitors.
2.2.2
The safety-related AS-i slave
The safety-related information of the AS-i slave is transmitted by way of the non-safety-
related transmission channel used by Standard AS-i. The same transmission mechanism
applies to a safety-related transmission as it does to the Standard AS-Interface, that is to
say, the 4-bit information that is delivered to the AS-i slave-IC will be transmitted. From a
transmission perspective, information is transmitted from master to slave and back again,
but the safe data information is sent from the slave to the AS-i safety monitor only, which
“listens in” to the entire exchange of information and monitors what is transmitted. Here the
safety-related user data are defined as follows:
• Only 1 bit of user information is transmitted. The two possible states are interpreted as
meaning
free
(=1) and
not free
(=0).
Example:
Emergency stop not activated =
free
("hazardous movement approved")
Emergency stop activated =
not free
("hazardous movement not approved")
• In the
not free
state the values 0,0,0,0 are statically registered in the 4 input bits of the
AS-i slave-IC.
• In the
free
state, with every cycle a different value is registered in the 4 input bits. The
values amount to a sequence of 8 4-bit values which vary in pairs, in such a way that
each slave in the system has its own unique sequence. After the eighth sequence has
been successfully transmitted, the system switches to the first sequence again (endless
loop). The sequence is registered in a code table of the AS-i slave and can be generated
in keeping with defined rules. It is assigned by the manufacturer of the AS-i slave as part
of the manufacturing process.
Data bit
Light path clear
Light path interrupted
D0
Code sequence
0
D1
Code sequence
0
D2
Code sequence
0
D3
Code sequence
0
Table 2.2-1:
Allocation of the data bits of the safety-related AS-i slave
