Echelon LNS - Manual (Page 254)

LNS Programmer's Guide 240
Performance Considerations
The performance of the monitoring application when reading configuration properties
from the database depends on disk performance. When reading configuration property
values from the device, rather than from the LNS database, the performance of
monitoring configuration properties varies depending on the implementation method.
When configuration properties are implemented within configuration files, the device
may provide one of the following three access methods:
• Direct memory read/write.
• The LonTalk file transfer protocol (FTP), with random and sequential access.
• LonTalk FTP with sequential access.
The direct memory read/write is the preferred method for applications running on
Neuron-chips or Smart Transceivers, as long as the configuration file fits within an area
of directly addressable memory space of the Neuron Chip or Smart Transceiver. This is
the most efficient method when reading or writing individual configuration property
values. A device implementing direct memory read/write access will have an output
network variable of type
SNVT_address
, and will not have a network variable of type
SNVT_file_request
or
SNVT_file_status
.
The LonTalk file transfer protocol is an interoperable way for devices to share data files
with one another. The file types 0, 1, and 2 are defined by the LonMark program for
specifying configuration parameters. For more information on LonTalk FTP, see the File
Transfer L
ON
W
ORKS
engineering bulletin, which can be downloaded from:
http://www.echelon.com/support/documentation/bulletin/005-0025-01D.pdf
LonTalk FTP with random and sequential access method requires an implementation of
an FTP server on the device, and can be used with any host processor. It can also be used
for a Neuron Chip or Smart Transceiver hosted device, if serial non-volatile memory is
needed for configuration property storage, the configuration property storage
requirements exceed the capacity of directly addressable memory, or if additional data
not related to configuration properties is also stored in files on that device. The random
access version of the LonTalk FTP protocol allows you to access configuration properties
individually. In addition, random access LonTalk FTP requires the device to be online
when you read the configuration properties, while directory memory read/write does not.
Random access LonTalk FTP also requires a network variable on the device that is
dedicated to controlling the position of the file pointer (SNVT_file_pos). LonTalk FTP is
the most efficient method to use when reading or writing all of the configuration property
values in a device at once, or when reading or writing configuration property values
stored in large contiguous blocks within the file.
LonTalk FTP without support for random access is considerably less efficient than the
other two access methods, as it does not allow individual configuration property access,
and a full transfer of all configuration property values is required for each modified
value.
A device implementing one of the two FTP methods will have an output network
variables of type
SNVT_file_status
, and an input network variable of type
SNVT_file_req
. A network variable of type
SNVT_file_pos
is used to control the
position of the read/write pointer in a file used for random access. Therefore, a device
with a
SNVT_file_status
and a
SNVT_file_req
network variable, but without a
SNVT_file_pos
network variable, only implements sequential access.