Siemens SIMATIC S5 Manual page 65

IP 265
6.2 Transfer Times for Data Exchange Between CPU and IP 265
Input and output data is transferred continuously between the IP 265 and the I/O bus. If the IP 265
is in the RUN state,
• the output data of the I/O bus (8 bytes) is read into the IP 265,
• the IP 265 checkback signals are written into the input data area (8 bytes) over the I/O bus.
Data exchange via the I/O bus is carried out cyclically by the CPU. The input and output data is
transferred between the IP 265 and the CPU in 8-byte data blocks and in one data cycle (OB1:
S5-100 Manual) .
Remember:
A CPU scan cycle consists of two separate processes:
• Program cycle (PCyc)
The STEP 5 statements of the CPU user program are executed.
The I/O bus is inactive.
• Data cycle (DCyc)
The input and output data is transferred via the I/O bus between the CPU and the IP 265.
Program cycle and data cycle constantly alternate.
CPU transfers
output data to the
IP 265
1st PCyc of the CPU
Note
The scan cycle diagram shows that a response of the IP 265 to changes in the output
data (control word, input parameter) are evaluated by the CPU user program in the next-
but-one program cycle at the earliest. This can have consequences for programming the
user program.
EWA 4NEB 812 6130-02a
Transfer of the
input and output data
IP 265 receives
output data from
1st DCyc
acknowledgement
2nd PCyc of the CPU
Figure 6-2. Transfer of Input and Output Data
Control of the IP 265 by the CPU User Program
Transfer of the
input and output data
the CPU and
initializes the
2nd DCyc
CPU can
evaluate
input data
3rd PCyc of the CPU
Time t
6-3