4 - 4 750/760 FEEDER MANAGEMENT RELAY – COMMUNICATIONS GUIDE
READING THE DATA LOGGER CHAPTER 4: MODBUS MEMORY MAP
11. Increment CS by 1.
12. If TMCS < total number of channels available then go to Step 4.
13. Done.
All the Trace Memory data for the most recent trigger is now stored to SCADA memory. The
SCADA software could now ‘unravel’ the circular buffers using the Start Index and Trigger
Index registers that were read in Step 3 and store them permanently to file or perhaps
generate a graphical picture.
Reading the Data Logger
All Data Logger information can be read from Modbus registers 2200h to 225fh. Familiarity
with the Data Logger settings is required to understand this section; refer to the Data
Logger section in Chapter 5 of the instruction manual for details. Reading the Data Logger
is very similar to reading Trace Memory and the description for the latter should be used
with note of the exceptions described below.
The data channels in the Data Logger are programmable and are determined by the
Channel 1 to 8 Source setpoints. These setpoints determine the format of data samples for
a given channel.
In Continuous mode:
• The Number of Data Log Triggers Since Last Clear is always equal to one.
• The Data Logger only maintains one buffer of 4096 samples for each channel. Since
the buffer is being continuously updated the ‘Data Log Start Index’ will change once
the buffer is full and new data is added (overwriting old data and thus moving the
location of the starting index). At slow sampling rates this is less important but when
sampling is performed at a high rate it is very likely that the start index will move
between subsequent reads of the memory map.
• The “Data Log Trigger Index” will always index the last sample added to the buffer.
Thus, it too, will also be quickly and continuously changing if the sampling rate is high.
• The “Data Log Trigger Cause” will be set to zero and the time and date are when the
last sample data was written to the buffer.
Accessing Data via the User Map
The 750/760 has a powerful feature, called the User Map, which allows a computer to read
up to 120 non-consecutive data registers (setpoints or actual values) by using one Modbus
packet. It is often necessary for a master computer to continuously poll various values in
each of the connected slave relays. If these values are scattered throughout the memory
map, reading them would require numerous transmissions and would burden the
communication link. The User Map can be programmed to join any memory map address
to one in the block of consecutive User Map locations, so that they can be accessed by
reading these consecutive locations.
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