Guralp Systems CMG-DM24 - Manual (Page 57)

Operator’s Guide DM-24 Digitiser
Issue G January 2003 55
GPS serial port or the data port, the terminal port is not optically coupled to the outside
world. Care should be taken not to run a terminal over very long RS232 cables.
The DSP software consists of 4 cascaded programmable filter/decimation stages
allowing multiple data output rates to be simultaneously selected. The first stage is set
to decimate the data by 10 resulting in a data output rate of 200 samples/sec. The
following 3 stages can be set individually for decimation factors of 2, 4, 5, 8, and 10
allowing data to be output at lower rates requiring less storage and transmission
bandwidth. For example, a system can be configured to provide data at 200, 50, and 10
samples/sec covering the whole of the seismological broad band range.
The configuration of the DSP is programmable (in the field) via the host H8
microprocessor. The H8 communicates with the DSP via its high speed 8-bit ‘host port’,
which allows the operating mode/configuration to be altered and the resulting
processed/filtered data to be acquired.
The primary digital interface for the systems is the multiple serial port card. Each card
can contain 1 or 2 dual UARTs (Universal Asynchronous Receiver Transmitters) and
upto 2 cards can be fitted to a SAM/DM unit. This allows a system with upto 8 serial
ports to be configured.
On a DM unit with analogue inputs a serial port is usually configured to send the data
packets to a (local) SAM unit for storage/acquisition or via a modem or radio link to the
central recording station. The second serial port is available for use with a local GPS
receiver for time synchronisation, or alternatively the first (data) port is used for time
synchronisation from the central station.
The multiple serial port card is usually configured as several data inputs for a SAM unit
allowing it to collect data from upto 8 other SAM or DM units located locally, using
RS232 or RS422 links, or more remotely using radio links or telephone modems.
Each of the serial ports on a module can be configured for a wide range of standard
baud rates (with different settings available for transmit and receive channels), allowing
a wide range of data links to be used depending on the required data rates.
The first dual UART supports full modem interface on one port and hardware
handshake on the second. The second dual UART is configured for data line interface
only, supporting software handshake. Each dual UART is optically isolated to avoid
ground loops that could degrade the performance of the ADC’s.
The serial port module includes 32k RAM for data buffering and formatting by the
transmission/reception process.