Bus Parameters - Omron CJ Series Operation Manual

Appendices
A-2

Bus Parameters

The PROFIBUS Bus Parameters define both the baud rate and the bus timing settings, necessary to
perform the exchange of messages over PROFIBUS. The Bus Parameters settings must be deter-
mined for each and every Master device on the bus, and usually depend on
• the number of I/O data bytes per slave device,
• the number of slave devices connected to the master,
• the number of other masters on the bus.
The Bus Parameter settings consist for one part of a number of settings, which are directly entered into
the PROFIBUS interface hardware registers. These settings implement the necessary timing to enable
the transfer of a single message between the PROFIBUS Master Unit and a slave device. The other
part of the Bus Parameters must be calculated. These parameters implement the overall cyclic timing
as well as the watchdog time to monitor the communication.
The calculation is performed by the PROFIBUS Master Unit DTM. The software implements a formula
to calculate the settings which will be downloaded to the PROFIBUS Master Unit. Most of the parame-
ters have baud rate dependent default values. Some of the parameters can be tuned by the user. How-
ever, this is not recommended, since the communication may not function correctly, due to incorrect
settings or too small timing margins.
This appendix defines the parameters which are shown by the PROFIBUS Master Unit DTM, and pro-
vides the formula used for the timing calculations.
Bus Parameter Definitions
The figure below shows the PROFIBUS Master Unit DTM, Bus Parameter tab, in which the bus param-
eter settings can be made.
The table below defines the bus parameter settings options, shown in the figure. The Unit t
defined as the transmission time for one bit at the selected baud rate.
A-8 CJ-series PROFIBUS Master Unit Operation Manual for NJ-series CPU Unit (W509)
is
BIT