System information

Section 3. The CR200 Datalogger
3-11
3.10.4 Getting Values from Specific Records
Use either the Collect Data Command or the Get Values transaction when
attempting to collect a specific value or swath of values from an individual
record. These values are referenced by the table and field name, which can be
found in the table definitions collected from a station if the user does not
already know them. The Collect Data Command allows multiple fields to be
specified while the Get Values transaction focus on a single field at a time.
Once the table and field names are known the Collect Data or Get Values
Command packet can be sent to the datalogger specifying the exact values that
are wanted. The Collect Data or Get Values Response packet returns the
values requested or if an error occurs, the error is specified but no values are
returned.
These transactions are designed to minimize complexity, by allowing the
application to dictate the data type of the values that are returned. On a Collect
Data or Get Values transaction, the datalogger must convert the data value to
the data type requested. If the datalogger does not support a requested data
type or the conversion is not possible, the response will indicate that the data
type conversion is not supported. In general, the safest data type to request is
IEEE4.
3.11 Controlling Packet Size
Because of the compact size of the CR200, it has a limited buffer space for
receiving packets. Therefore, an application should not send messages to the
datalogger that exceed 100 bytes including the header. This maximum packet
size information can be obtained from the datalogger if necessary via the Get
Settings transaction. The setting containing this information is named
“MaxPktSize”.
Additionally, the application must not request packets from the datalogger that
exceed the 1000 byte PakBus message limit. The requested number of bytes
from the CR200 datalogger should always be less than this maximum size to
stay within the limits of the CR200 datalogger and the PakBus message limit.