Service Channel
The service channel allows high level, non-time-critical
data transfers without disturbing the synchronous
transfer of real-time data. Non real-time messages
such as diagnostics and loop gains are broken into
two byte chunks, sent over the service channel in
the Amplifier Telegram, and re-assembled by the receiving
device. Via the service channel, operators can retrieve
diagnostic and status information, reprogram or display
setup parameters and change operating data on the
fly.
Some manufacturer's drives include a software oscilloscope
function that captures data on drive performance,
sends the data over the service channel two bytes
at a time, and formats the information into an oscilloscope-type
display on the operating terminal.
Cyclic Operation Example
The best way to imagine how a SERCOS I or II system
works is to take a real world example, such as a system
with 14 drives and a 500 microsecond cycle rate. Under
these conditions, the following information can be
transferred every cycle:
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From control to each drive:
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- 32-bit command value (e.g.,
velocity or position)
- 16-bit limit value (e.g.,
torque)
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| From each drive to control:
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- 32-bit feedback value (e.g.,
velocity or position)
- 16-bit feedback value (e.g.,
torque)
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Thus, every 500 microseconds one SERCOS interface
ring can transport a very high resolution velocity
command signal to 14 drives, plus a torque limit command
to each of those drives that can be enabled at will.
In each cycle two feedback values are returned from
each drive. This reduces the customary feedback wiring
to the control to nothing.
In addition to all this real-time data being exchanged,
the service channel provides the equivalent of a 128-kilobaud
serial port to each drive on the ring . This can be
used to set and archive performance variables, to
read diagnostics, etc.
Note that SERCOS interface performance is not limited
to 500 microseconds or 14 drives. Flexibility exists
in the update rate (down to 62.5 milliseconds – 31.25
for SERCOS-III), the number of drives per ring (up
to 254), the amount of data exchanged, and even the
number of rings.
Error Correction/Diagnostics
If a SERCOS interface loop detects an error, it continues
to operate for one cycle using previously valid data.
However, if a second consecutive error is detected,
the drives are shut down in a predefined, orderly
fashion, and a diagnostic message is issued.
This allows vendors to incorporate detailed context-sensitive
troubleshooting procedures in their motion controls.
An operator can go to his control panel and acquire
instant diagnosis of every axis motor and drive at
every section of the machine. Failures can be immediately
pinpointed by the control system and quickly and easily
be repaired as opposed to hit and miss troubleshooting.
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