Siemens SIMATIC ET 200C, Manual

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5-11
ET 200C Distributed I/O Station
EWA 4NEB 812 6119-02b
The process image is created one word at a time by the CPUs 941, 942, 943
and 944 (AG S5-115U/H series). In the case of ET 200C, this means that two
slave stations can be addressed within one word:
8DI (1)
8DI (2)
...
...
...
Address 0:
1:
2:
3:
4:
8DO (1)
8DO (2)
...
...
...
Address 0:
1:
2:
3:
4:
The following can occur if an input/output value is read in via the process
image:
Assumption: failure occurs in 8DI (1) or 8DO (1).
Since the CPUs 941 ... 944 read/write the process image one word at a time,
they recognize that a failure has occurred in 8DI (1) or 8DO (1). As a result,
no read/write operations are performed by the CPUs on the process image of
8DI (2) or 8DO (2) either, although these are present.
The CPU is set to STOP with NAK (if ”NAK = yes” in COM ET 200). The
CPU is reset to RUN after a STOP-RUN transition although 8DI (1) and 8DI
(2) (or 8DO (1) and 8DO (2)) are not entered in the process image.
8DO (2) behaves in the same way as in case 1.
Assumption: failure occurs in 8DI (2).
Since the above-mentioned CPUs read the process image one word at a time,
they recognize that a failure has occurred in 8DI (2).
The CPU is set to STOP with NAK (if ”NAK = yes” in COM ET 200). The
CPU remains set to STOP even after a STOP-RUN transition.
Note
The following rules must be noted when addressing the CPUs 941 ... 944:
1. Only use load and transfer commands. These commands recognize, one
byte at a time, whether or not a byte is present or missing or
2. Assign each ET 200C module to an even address (e.g. 2, 4, ...) and leave
the odd addresses free. You can then also access the process image.
Special Points of
Interest with
Regard to
Addressing
Case 1
Case 2
Case 3
Address Assignment and Parameterization with COM ET 200