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4.2 Source Accesses
The Modbus Plus driver supports four possible source accesses.
The first source defines a local address which allows any device on the
Modbus Plus network to write the data to the X-Link. This type of source is
ideal for data that changes infrequently, but must be forwarded immediately,
such as errors or exceptions. Programming must be provided on the remote
device to update the data when necessary.
The second type of source supported by the Modbus Plus driver is an X-Link
initiated read. This source defines an external address which the X-Link uses
to read the data from a remote node on the network. This access also requires
you to specify when to read the data. This type of data source does not
require any programming in the remote node, but generates extra network
bandwidth to poll the data.
The next source is Modbus Plus global data. As each station passes data on
the network, it has the opportunity to transmit up to 32 integers to all the
other stations on the network. This global data can be accessed as a source
for an X-Link mapping. To access global data, you provide a station address
(1 to 64) and an offset into that station's global data table.
The last source is Modbus Plus network statistic counters. To access the
counters stored on the SA85 card, provide an offset specifying the number of
words to skip from the beginning of the diagnostic counters.