Trace Memory Circular Buffer; Map Reading Trace Memory - GE 750 Manual

CHAPTER 4: MODBUS MEMORY MAP
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stamp, and the sampling frequency. There is also the 'Start Index' and 'Trigger Index' which
determine how to 'unravel' the data samples; see the description of the circular data
structure below.
The 'Trace Memory Channel Selector' determines which data channel samples can be read
from the 'Trace Memory Samples' registers at addresses 2120h to 215Fh; refer to format
F26 for a complete listing of the available data channels. For example, to read Vc voltage
samples a value of 6 is written to this register. Note that this register also determines the
data format of the samples.
The 'Trace Memory Sample Selector' (TMSS) determines which block of 64 samples can be
read from the 'Trace Memory Samples' registers. The number of samples stored for one
channel is variable and depends on the 'Buffer Organization' setpoint. The 'Number of
Trace Memory Samples Stored' register indicates the number of samples that have
already been accumulated for the selected Trace Memory Number; this register may have
a value less than the total number of samples that can be stored if a trigger just occurred
and there are an appreciable number of post-trigger samples to capture.
The samples are stored in a circular buffer; the figure below shows how a waveform might
be stored for one channel. The 'Trace Memory Start Index' determines where the
waveform begins in the buffer. Likewise, the 'Trace memory Trigger Index' determines
where the trigger point is. In this example the Start Index is 96 and the Trigger Index is 224.
5000
2500
0
-2500
-5000
0 16 32 48 64
FIGURE 4–1: Trace Memory Circular Buffer
The following example illustrates how information can be retrieved from Trace Memory:
1. A SCADA system polls the Number of Trace Memory Triggers register once every
minute. It now reads a value of 4 from the register where previously the value was 3
which means that a new data has been captured. The SCADA system proceeds with
the following steps to read all the waveform data. After these steps it resumes polling
the Number of Trace Memory Triggers register.
2. Read the TRACE MEMORY BUFFER ORGANIZATION
3. Write a value of 4 to the Trace Memory Number Selector.
4. Read all the Trace Memory Information registers and store to SCADA memory.
5. Set variables TMCS and TMSS to 0.
6. Write TMCS to Trace Memory Channel Selector.
7. Write TMSS to Trace Memory Sample Selector.
8. Read 64 samples from Trace Memory Samples and store to SCADA memory.
9. Increment TMSS by 64.
10. If TMSS < Total Number of Samples then go to Step 5.
80 96 112 128 144 160
Phase A Current
setpoint
READING TRACE MEMORY
176 192 208 224 240 256
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