Guralp Systems CMG-DM24 - Manual (Page 29)

Operator’s Guide DM-24 Digitiser
Issue G January 2003 27
SETTING DIGITISER COMMUNICATION PARAMETERS
If the digitiser has a duplex link for handshake communications with a seismic
instrument and a GPS, its COM ports must be configured to match the communications
parameters of the attached devices or computers. If the digitiser is connected directly to
a computer running SCREAM, SCREAM can auto-detect the communication baud
rate under most conditions. If data from the digitiser must pass through a telemetry link
or into a Guralp Storage and Acquisition Module (SAM), it is very important that the
baud rate and other COM port parameters for the digitiser match that of the telemetry
device, whether it be a modem, a radio or the SAM.
Data Port Baud Rate. The Baud Rates window of the digitiser configuration set-up
allows you to program the baud rate and stop bits for the digitisers COM port.
The baud rate you choose must satisfy two conditions. It must be high enough
to allow all the transmission of all data generated by the digitiser at the
sampling rates you have chosen. For three streams of data at 100 Hz, for
example, 9600 baud will usually be sufficient. If you wish to transmit 200 Hz
data, however, the baud rate must be at least 19200. The second condition for
the baud rate, is that it matches that of the telemetry equipment. While modern
modems often offer transfer rates up to 56 kbaud, the telephone or transmission
lines may not support these rates. The same holds true for radio telemetry.
Usually, transmit and receive rates of the data port will be the same. If not, you
may select different data rates by removing the check in the box marked
Identical TX/RX rates. Make sure that the COM port in SCREAM, the SAM
or the communications device is also configured accordingly.
Stop Bits In most circumstances this can be left set at 1. The Stop Bit option gives the
user the choice of setting the serial transmitted data stop bits to 2 if required
over ‘difficult’ transmission lines. Whilst it can be an aid with say, a radio
link, it will add an overhead to the data of
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