I/O System S900
Communication Interface for PROFIBUS DP, CI920
16 3BDD010422R0101
Total amount of I/O data [bytes] = 60 + 30 = 90
Number of available bytes = 216 –90 = 126
Number of possible HART variables = 126 / 4 = 31
Cycle time
The PROFIBUS master defines the transmission rate to be used within the
system. The internal cycle time is 5 ms for processing 128 binary signals and
20 ms for processing 64 analog signals. The response time of the entire
system depends upon the total amount of I/O data of all PROFIBUS stations
and of the processing capabilities of the higher-level control system.
The S900 output data are synchronized with the higher-level bus. Therefore,
the internal cycle time has to be considered only once.
The following approximation formula is valid: T
R
= T
i
+ 2 x T
B
+ T
OCS,
with
T
R
= response time
T
i
= internal cycle time
T
B
= fieldbus cycle time
T
OCS
= OCS cycle time
With a bus cycle time of 5 ms and an OCS cycle time of 5ms, the response
time for 128 binary signals is T
R
= 5 + 2 x 5 + 5 = 20ms
The following illustration gives you an overview of the expected PROFIBUS
cycle time, depending on the transmission rate and number of slaves. The
cycle time also depends on the total amount of I/O data. An output data
amount of 20 bytes and an input data amount of 80 bytes were assumed in the
following example, corresponding to an S900 station with the following
configuration:
• 32 digital outputs
• 48 digital inputs with state
• 16 analog inputs
• 8 analog outputs
• 8 secondary HART variables
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