Project 06/2005
Danaher
Motion
78 Rev
E
M-SS-005-03l
3.12.4. Cyclic vs. Non-Cyclic Communication
SERCOS communicates across the fiber-optic ring with two types of data:
cyclic and non-cyclic. Cyclic data are sometimes referred to as real-time, and
are updated at a regular rate (SERCOS update rate). The SERVO
STAR
system supports update rates from 1 millisecond. The MC requires that the
cyclic data include position and velocity command (transmitted from the
controller) and position and velocity feedback (transmitted from the drives.)
These data must be updated once each SERCOS update.
Non-cyclic data are updated irregularly and on a demand basis via the
service channel. The service channel is roughly equivalent to a 9600-baud
serial link between the drives and the controller, except that it is transferred
on the fiber optic cable along side the cyclic data. Using the service channel,
you can request data from the drive or set a parameter that resides in the
drive. Each SERVO
STAR
drive has I/O points. You can use the service
channel to access this I/O or you can use a Telegram type 7 to configure the
I/O as cyclic data. The service channel is non-deterministic. Consequently,
non-cyclic data transmits considerably slower than cyclic data.
3.12.5. IDNs
SERCOS commands are organized according to an Identification Number or
IDN. SERCOS defines hundreds of standard IDNs, which support
configuration and operation. Manufacturers, such as Danaher Motion,
provide IDNs specific to their products. IDNs are numbered. Standard IDNs
are from 1 to 32767 (although only several hundred are used to date) and
manufacturer IDNs are numbered from 32768 to 65535.
If you are using a SERVO
STAR
drive, you normally use only a few IDNs for
special functions (e.g., for querying drive I/O or changing the peak current of
a drive).
SERCOS requires that you define every IDN used on each drive. The
SERVO
STAR
system eliminates most of this step because the MC
automatically defines all necessary IDNs for SERVO
STAR
drives. This
process would otherwise be tedious as there are quite a few IDNs that must
be defined.
Most requests from the service channel are responded to immediately. For
example, when you set the digital-to-analog converter (DAC) on the drive,
the value is placed in the DAC by the drive immediately upon receipt. The
drive acknowledges the command almost as soon as it is received, thereby
freeing the service channel for subsequent commands. However, some
functions of the drive that are accessed from the service channel require a
much longer time for execution. For example, incremental encoder-based
systems must be oriented on power up. The wait for verification that the
process has completed successfully can take many SERCOS cycles, far too
long to tie up the service channel. To support these functions, SERCOS
developed procedures.
With procedures, the master starts a procedure and optionally halts and
restarts it. Procedures allow processing in the service channel without
blocking other communication. For example, waiting for a registration mark is
a procedure. In this case, the motor is turning and the drive is searching for a
registration mark. By using a procedure, the service channel remains
available for other communication during the search.
