Design and Operation
13
3.1.1 Host Interface
The CMC 156 communicates with the computer via the Host Interface (parallel
printer interface) at its rear side.
If the computer is equipped with a parallel printer port, too, use the 25-pole
cable (VEHK0002) that was supplied with the test set to connect the CMC 156
to the computer.
If the computer is equipped with a USB port, use an OMICRON CMUSB-P
converter (ordering number VEHZ2007) instead of the VEHK0002 to connect
the CMC 156 to the computer.
3.1.2 Processor
A signal processor is the core of the test set hardware. It has the following
tasks:
• Communications with the host computer
• Signal generation
• Control of the test set hardware
• Input signal measurement and filtering
Its Harvard architecture and the built-in arithmetic unit make the signal
processor ideally suited for digital signal processing tasks.
For example, the signal processor needs for multiplication + adding + parallel
data transfer only two machine cycles (30ns). The CMC 156 uses this
advantage of the signal processor for signal generation and digital filtering.
3.1.3 D/A Converter + Reconstruction Filter
For D/A converters 16-bit PCM converters are used. Their high resolution
allows coverage of the entire amplitude range without range switching.
This is important in particular for fault simulation for protective devices,
because range switching in the test set via relays may cause erroneous
tripping when switching from a load to a fault condition.
The reconstruction low pass filter forms the desired output signal from the
stair-step output signals of the D/A converters and suppresses the high
frequency component, which is periodically repeated with the sampling
frequency. The 3dB bandwidth is at 3.1kHz.
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