I/O Write Accesses - Siemens SIMATIC CM35 Manual

Data Communication with the CM35
4.3.3

I/O Write Accesses

4-14
When I/O write accesses (T PQW) are used, it is essential to remember that
write accesses to the same I/O addresses may overwrite each other before
being transferred to the CM35.
Example, pulse counter:
Start channel 1, then start channel 4 shortly afterwards
L W#16#0181
T PQW 258
: :
L W#16#0881
T PQW 258
If the time between the start of channel 1 and channel 4 is too short, the start
command for channel 1 may be overwritten by the start command for chan-
nel 4.
This is particularly apt to happen if you are using distributed connection of
the CM35 with low bus transmission rates. In this case, the 1st start com-
mand (channel 1) is overwritten by the 2nd start command (channel 4) before
the 1st start command can be sent to the module via PROFIBUS.
The following measures should be taken to ensure that I/O write accesses do
not compete at the same address.
The same I/O address may only be write- - accessed once during one OB1
S
cycle.
When a distributed layout is used, the DP cycle should be at least twice as
S
fast as the CPU cycle (DP cycle and CPU cycle can be determined with
STEP 7).
// Start channel 1
// Module start address +2
:
// Start channel 4
// Module start address +2
CM35 Counter Module
(4) J31069-D0416-U001-A5-7618