Parameter 60.3 defines whether the drive is the master or a follower on the
communication link. Typically, the speed-controlled process master drive is also
configured as the master in the communication.
The communication on the master/follower link is based on the DDCS protocol, which
employs data sets (specifically, data set 41). One data set contains three 16-bit words.
The contents of the data set are freely configurable using parameters 61.1 M/F data 1
selection…61.3. The data set broadcast by the master typically contains the control
word, speed reference and torque reference, while the followers return a status word
with two actual values.
The default setting of parameter 61.1 M/F data 1 selection is Follower CW. With this
setting in the master, a word consisting of bits 0…11 of 6.1 and four bits selected by
parameters 6.45…6.48 is broadcast to the followers. However, bit 3 of the follower
control word is modified so that it remains on as long as the master is modulating, and
its switching to 0 causes the follower to coast to a stop. This is to synchronize the
stopping of both master and follower.
Note: When the master is ramping down to a stop, the follower observes the decreasing
reference but receives no stop command until the master stops modulating and clears
bit 3 of the follower control word. Because of this, the maximum and minimum speed
limits on the follower drive should not have the same sign – otherwise the follower would
be pushing against the limit until the master finally stops.
Three words of additional data can optionally be read from each follower. The followers
from which data is read are selected by parameter 60.14 in the master. In each follower
drive, the data to be sent is selected by parameters 61.1 M/F data 1 selection…61.3.
The data is transferred in integer format over the link, and displayed by parameters
62.28…62.36 in the master. The data can then be forwarded to other parameters using
62.4…62.12.
To indicate faults in the followers, each follower must be configured to transmit its status
word as one of the above-mentioned data words. In the master, the corresponding target
parameter must be set to Follower SW. The action to be taken when a follower is faulted
is selected by parameter 60.17. External events (see parameter group 31 Fault functions)
can be used to indicate the status of other bits of the status word.
For block diagrams of the master/follower communication are presented on pages 642
and 643.
Program features 37
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