DLG LMP-100, Руководство пользователя

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LMP-100
PROFIBUS/MODBUS GATEWAY
Page 20 of 40 LMP-100 User Manual MAN-EN-DE-LMP100-01.00_14
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Choosing between the three available behaviors requires
understanding of the specific application.
The behavior “On value change” is optimal from the Modbus
network standpoint, since data writing occurs only when the
Profibus master sends a different value.
However, this behavior is only totally safe when the variable to be
written is retentive, i.e., it holds the last value even if the Modbus
slave is turned off and on again. This caution is justified by the
following example: an output rule is configured with the behavior
“On value change”. At one point in time, the Profibus master
changes the value of a digital output from 0 to 1. The LMP-100
transfers this change to the appropriate Modbus slave. Sub
sequentially, the Modbus slave is turned off and on again, and the
non-retentive digital output goes back to value 0.
In this situation, the Profibus master keeps sending value 1, but
as the value has not changed, the LMP-100 does not detect
change and does not forward the current value to the Modbus
slave. This situation was caused by the digital output changing
from 1 to 0 when the device was turned off.
The “User triggered” behavior allows the user to control exactly at
which time writing is made. The control register allows the user to
trigger each output rule individually, writing the value available in
the Profibus output buffer. Therefore, output rules with the “User
triggered” behavior need the value to be written copied to the
Profibus output buffer and the trigger fired in the control register.
The writes triggered by the LMP-100 always carry all the variables
addressed by the rule. An output rule addressing, for instance,
five Modbus variables configured with behavior “On value change”
and that at a given time detects the change of only one of the five
variables will result in a Modbus write command containing the
new value of the variable that has changed and the last value of
the other variables.
Enabled
Sets whether the rule is enabled or disabled. A disabled rule does
not produce activity in the Modbus network.
•
Yes : the rule is enabled.
•
No : the rule is disabled.
In some cases it may be interesting to disable a rule. For
example, a defective Modbus slave can cause considerable