Communication Via Data Areas - Siemens SINUMERIK 880 SW 6 Planning Manual

1 General Overview
1.2.2 PLC program structure
Time-controlled transfer
The time-controlled transfer of interface signals corresponds to cyclic transfer except that the
signals are processed periodically, that is to say after determined time intervals, independent
of the PLC cycle. As a result, input signals transferred with time control can change their con-
dition during a PLC cycle. Similarly, output signals transferred with time control can take effect
more quickly in certain circumstances.
Request-controlled signal transfer
In the case of request-controlled signal transfer, the data and signals are transferred only when
requested by the PLC user program. Examples of request-controlled DB processing are pro-
vided:
• In the command channel (request by special bits)
• In the PLC/NC interface (request by calling FB 63 or FB 64, not with S5-155U) and
• In the NC/PLC data channel (FB 61, FB 62).
Common memory area
In one special instance (PLC/PLC, DB 59) the user is able to implement communication via a
jointly used memory area. The PLC operating system is not activated in this case. The PLC
provides semaphores for the required handshake mechanism. The user can decide on the use
of semaphores.

1.2.2.4 Communication via data areas

The DBs are supplemented by the data areas of the input bytes (IB), output bytes (QB) and
flag bytes (FY).
Process image in I/O area
The IBs and QBs contain the image of the machine signals, that is the process image. The in-
coming machine signals are transferred by the PLC operating system to the IBs (process input
image, PII) at the beginning of the PLC cycle (as with the cyclic processing of DBs described
above). The PLC operating system reads the output bytes (process output image, PIQ) at the
end of the cycle and switches the relevant machine signals.
The PII remains constant during a PLC cycle (provided that the user program does not imple-
ment any changes). Changes in the PIQ take effect at the end of the PLC cycle.
When the power is switched off the PII and the PIQ of the PLC 135 WB remain stored. When
the PLC 135 WB is then restarted it resumes the PLC user program from where it was
interrupted. However, the PIQ is not output to the inputs/outputs at the end of the cycle.
Output is only resumed after the first complete cycle has be executed. On the S5 155U the
PIQ is deleted on restart.
On a cold and warm restart, the PIQ is deleted and the PII updated according to the machine
signals (PLC 135 WB and S5 155U).
The assignment of the machine signals to the I/O signals (bits) in the I/O area is determined by
machine data. Bits in the I/O area not assigned by signals can be used as flags by the PLC
user program. To prevent these flags from being deleted with the process images when the
PLC is restarted, a delete limit for the IBs and QBs can be specified in the machine data.
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01.93
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SINUMERIK 880 (PJ)