The Safety-Related As-Interface Slave - Siemens SIGUARD LS4-4 Anleitung

2.2.2 The safety-related AS-Interface slave

The safety-related information coming from the AS-Interface slave is transmitted via
the non-safety-related transmission channel of the standard AS-Interface. Here the
safety-related data transmission is subject to the same transmission mechanism as on
the standard AS-Interface, that is to say, the 4-bit information that is supplied by the
slave to the AS-Interface integrated circuit will be transmitted. From the point of view of
the technicalities of transmission, information will be transmitted from the master to the
slave and back again, but the safety-technology-related flow of information will be
passed by the slave to the AS-Interface monitor, which will be "listening in" on the
entire exchange of information and keeping a constant check on it. The safety-related
user data have been defined in the following way:
Only one bit of user information is transmitted. The two possible states of this bit
have the meaning clear (=1) and not clear (=0).
Example:
Emergency stop not activated = clear ("hazardous movement enabled")
Emergency stop activated = not clear ("hazardous movement disabled")
In the not clear state, the value (0,0,0,0) will be statically supplied to the four input
bits of the AS-Interface slave integrated circuit.
In the clear state, a different value will be supplied to the four input bits with each
cycle. The values constitute a sequence of eight four-bit values which differ in
pairs, so that every slave in the system has its own unambiguous sequence. After
the eighth sequence has been successfully transmitted, the system switches back
again to the first sequence in an endless loop. The sequence is stored in a code
table of the AS-Interface slave, and has to be generated by means of a definite set
of rules. This is programmed into the system by the manufacturer when the AS-
Interface slave leaves the factory.
Data bit
D0
D1
D2
D3
Table 2.2-1:
30
Light path clear
Code sequence
Code sequence
Code sequence
Code sequence
Assignment of the data bits of the safety-related AS-Interface slave
Light path interrupted
0
0
0
0