Siemens SIMATIC S5 Manual page 253

Access commands for the S5-115U, S5-135U and S5-155U programmable controllers
What is important Observe these rules for consistent data access:
with regard to
1. Data consistency is switched on and off on the IM 308-C.
consistency?
2. Data consistency is switched off only by a certain byte (switch-off byte,
3. Data consistency is switched on by any other byte in the consistent area
4. If data consistency is switched on when one or more bytes in a consistent
5. If you want to read or write only one byte in the consistent area and this
6. During access to a consistent area (input or output area), no other I/O
7. The length of the consistent data depends on the baud rate and CPU type.
Example
If, for example, you perform read access on an IP 267, you must complete
the read access before attempting write access to the IP 267. Failure to
observe this precaution means that consistency is not ensured.
Switch-on and Bytes with which you switch on consistency (switch-on bytes) are always
switch-off bytes shown as white fields below. Bytes with which you switch off consistency
(switch-off bytes) are shown as gray-hatched fields.
ET 200 Distributed I/O System
EWA 4NEB 780 6000-02a
indicated by gray hatching in the illustrations below).
(switch-on byte, no hatching in the illustrations below).
area are read or written, the IM 308-C waits until data consistency is
switched off again (switch-off byte).
During this wait, the IM 308-C cycle is stopped, i.e. the IM 308-C does
not poll the DP slaves and the SINEC L2 DP bus must wait. Once the
switch-off byte has been accessed, the IM 308-C releases SINEC L2 DP
access to the dual-port RAM.
byte is not the switch-off byte, you must always read or write the
switch-off byte as well to ensure that data consistency is switched off
again.
address outside this area may be accessed because if it were, the
IM 308-C could no longer process the data consistently.
The maximum lengths are stored in the file COMET.INI.
With this byte consistency is switched on.
With this byte consistency is switched off.
B
B-13