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CMG-DCM
6 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 4.2, “Disk”)
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 (see Section 4.2, “Disk”.)
Writing to the hard disk is performed robustly, so that no data will be lost if a
write is aborted (see Section 6.1, “File systems”.) 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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