ProSoft Technology MVI69-101S - Manual (Page 94)

MVI69-101S ♦ CompactLogix or MicroLogix Platform Reference
IEC 60870-5-101 Slave Communication Module
Page 94 of 149 ProSoft Technology, Inc.
March 16, 2009
This example shows that the module sends all 40 measured scaled points in one
single message, if the message is not greater than 252 bytes. However, some
master devices may not support messages containing this number of bytes. If the
master for a given application supports only 100 bytes, the following
communication procedure would occur:
As shown in the previous diagram, the module sends out 15 messages (15
points, 15 points and 10 points) instead of sending the whole 40 points in one
single message.
Note: This example shows the case where IOA Length = 3 bytes, COT Length = 2 bytes and
CASDU Length = 2 bytes.
Important: If the database parser gets a point index that is not valid, the whole database is set as
invalid and no points are reported. Because the index 0 is not valid (the protocol does not support
this index value), the driver considers it invalid. For example, if you set the size of the ASDU to 1
and you set a value of 1000 for a point index, this is also invalid as the indexes can only go from 1
to 255.
Balanced and Unbalanced Modes
The module supports balanced and unbalanced modes. In balanced mode, each
station may initiate message transfers. If the links from the master unit to several
slaves share a common physical channel, then these links must be operated in
an unbalanced mode to avoid the possibility of more than one device attempting
to transmit on the channel at the same time.