Guralp Systems CMG-DCM - Manual (Page 114)

CMG-DCM Data Communications Module
8 Inside the DCM
The DCM is a fully-functional, Linux-based computer system
especially designed for handling seismic data. It can collect and store
data from several sources and, if required, output it in your preferred
format to other locations on your network or on the Internet. This is
done in the following manner.
Firstly, the DCM receives some data from an instrument connected to
it. This can be any of
•
a digitizer connected through a serial link,
•
a computer running a Scream! server,
•
a CD1.0 or CD1.1 transmitter (optionally), or
•
another DCM or AM.
All the received data is stored in files in the on-board Flash memory.
There are two banks of Flash memory available, which are accessible
as /nand0 and /nand1 in the Linux file tree. Data is normally stored
as GCF (Güralp Compressed Format) files.
As an option, you may be able to configure the DCM to use the
miniSEED or sac formats instead (see Section 6.3, page 86.)
In automatic mode, when the Flash memory becomes more than 75%
full, the oldest data files are moved to the DCM's primary hard disk
until it is less than 50% full. If you prefer, you can configure the DCM
to write to the hard disk at set intervals.
Writing to the hard disk is performed robustly, so that no data will be
lost if a write is aborted (see Section 8.1, page 116.) This means that
you can safely swap hardware in and out at any time. Stand-alone
DCM modules use off-the-shelf Lacie hard disks, which can be easily
removed and installed in most conditions. You can specify other
models of IDE / USB or IEEE 1394 2.5” disk at manufacture. If an
internal disk is not present, and the module has a USB
host interface,
it will look for hard disks connected to its external USB port.
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