Siemens SIMATIC S5 CP 523 Manual page 52

CP 523
5.2 Access to the Transfer Memory
The CPU can write data to the transfer memory at any time.
After the CPU has written the relevant data into word 0,
• the CP 523 fetches data from the transfer memory
• the CP 523 updates the transfer memory with current data
• the CPU can read the current data from the transfer memory
This has the following consequences for the order in which data is transferred from the CPU to the
transfer memory:
• Words 2, 4 and 6 in the transfer memory must be written first if the job requires it. If you want
to print a message text into which three variables are to be inserted, for example, the actual
variable values must be transferred in words 2, 4 and 6.
• Finally, the job number must be written to the transfer memory in word 0.
CPU transfers
data to words
2, 4 and 6 of
the transfer
memory
(T PW X).
Note:
If you do not observe this order, the new job may be executed with the wrong values.
Special feature of the CPU 944 when accessing the transfer memory
If you are using the CPU 944, you may not write data to word 0 of the transfer memory with T PW
<Start address of the module>. You must write the data to word 0 byte for byte in the following
order:
1. T PY <Start address of the module>
2. T PY <Start address of the module + 1>
EWA 4NEB 811 6044-02a
CPU transfers CP reads the
data to word 0 transfer
of the transfer memory.
memory
(T PW X).
Figure 5-2. Using the Transfer Memory
CP writes
current data to
the transfer
memory.
(byte 0)
(byte 1).
Address Assignment
CPU can read
current data
from the
transfer
memory
(L PW X).
5-3