Cycle Synchronization On The Profibus Dp - Siemens Simatic ET 200S Operating Instructions Manual

7.2

Cycle synchronization on the PROFIBUS DP

7.2
7.2.1 Basics
Properties
Reproducible response times (i. e., of equal length) are achieved in SIMATIC with an
constant DP bus cycle, synchronization of the user program on the DP bus cycle, and the
isochronous transfer of I/O data to the I/O modules. The isochronous portions of the user
program are processed synchronously with the DP bus cycle by means of synchronous cycle
interrupts (OB 61 to OB 64). The I/O data are transferred at defined and constant
(isochronous) intervals via the backplane bus of the DP slave to the I/O modules and
switched through isochronously up to the "terminal."
In other words, isochronous operation results in the synchronization of all of the hitherto free-
running single cycles. These include the user program in the CPU, the DP cycle on the
PROFIBUS subnet, the cycle in the DP slave, and, finally, the cycle in the I/O modules of the
DP slaves.
The maximum jitter for the IM151-1 amounts to 10 µs. The jitter of the peripheral devices of
the ET 200S cannot be accounted for due to the existing diversity.
Requirements
• Cycle synchronization is possible with the IM 151-1 HIGH FEATURE with modules, that
• The transmission rate of the PROFIBUS DP is at least 1.5 Mbps (shorter constant bus
• The maximum constant bus cycle time is 32 ms.
• The constant bus cycle time master (Class 1) must be a class 1 DP master. In other
• In constant bus cycle time operation, only one DP master (Class 1) can be active on the
• The cycle synchronization can only then be activated on the ET 200S if the constant bus
• Isochronous operation (constant bus cycle time) of the ET 200S is not guaranteed during
• In constant bus cycle time operation, the ET 200S requires a startup time of
• The bus length must amount to less than 1 m.
ET 200S
Operating Instructions, 12/2005, A5E00515771-03
support cycle synchronization. You can see whether a module supports cycle
synchronization in the device description or HW Config. Other modules in the ET
200S setup are possible. However, these do not support cycle synchronization.
cycle times can be achieved with higher transmission rates).
words, a programming device or PC cannot be an constant bus cycle time master.
PROFIBUS DP. PDs or PCs (Class 2) can also be connected.
cycle on the DP master system was activated.
removal or insertion of electronic modules.
If asynchronous results like "Switch on power module" or "Read/Write data set" are to
cause no cycle injury, a sufficiently large gap between To and Ti must be ensured, i.e.
Tdp is to be increased.
approximately 150 DP cycles to ensure isochronous operation right through to the
terminals.
7.2 Cycle synchronization on the PROFIBUS DP
Functions
7-3