ABB AC 800M, User Manual

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Section 2 Transmission Technology RS485 cycle time calculation
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power supply to ensure error-free operation. The bus terminator is usually switched
in the devices or in the connectors.
RS485 cycle time calculation
The bus cycle time should be shorter than the AC 800M program cycle time.
However, a faster data throughput alone is not enough for successful
implementation of a bus network. PROFIBUS DP only requires approx. 1 ms at 12
Mbit/s for the transmission of 512 bits of output data distributed over 32 stations.
The following figure shows different bus cycle times of a DP mono-master system
with 2 bytes in and output data per slave:
Figure 4. Bus cycle times of a DP mono-master system.
Boundary conditions: each slave has 2 bytes of input and output data
Each PROFIBUS master configuration tool (for example Control Builder M)
automatically calculates the bus cycle time. The detailed calculation rules are
described in the PROFIBUS specification.