Streaming: CoreTimer, STREAM_DATA_CAPTURE_16, and next channel interaction.

Hi All,

With a channel list  defined like:

POS_IN_NAMES = ["CORE_TIMER", "STREAM_DATA_CAPTURE_16", "AIN0", "AIN1", ... "ANIN13", "CORE_TIMER", "STREAM_DATA_CAPTURE_16"]

We're noticing that the values reported in "AIN0" are not what is expected.  The values seem to increase or decrease with each sample (the channel is connected to a low-impedance source, and properly ground referenced).

If I remove "CORE_TIMER", "STREAM_DATA_CAPTURE_16" from the channel list, the reported data remains steady.

I suppose that we're doing something wrong in the setup of reading Core_Timer.  Any good ideas?

Data and code below.

With Core Timer (notice column #3 is decreasing):

18829.0, 61014.0, -10.0296916962, -10.5794315338, -5.2149720192, -9.17303276062,
21133.0, 61075.0, -10.0252714157, -10.5810098648, -5.21307754517, -9.17177009583,
23437.0, 61136.0, -10.018324852, -10.5806941986, -5.21339321136, -9.17208576202,
25741.0, 61197.0, -10.0107460022, -10.5810098648, -5.21623516083, -9.17082214355,
28045.0, 61258.0, -10.0164299011, -10.5810098648, -5.21244621277, -9.17113876343,
30350.0, 61319.0, -10.0107460022, -10.5810098648, -5.2143406868, -9.17082214355,
32653.0, 61380.0, -9.98422241211, -10.5810098648, -5.29454374313, -9.17082214355,
34957.0, 61441.0, -9.99779987335, -10.5810098648, -5.21118307114, -9.17177009583,
37261.0, 61502.0, -9.99180030823, -10.5810098648, -5.21686697006, -9.16924381256,
39565.0, 61563.0, -9.98327541351, -10.581325531, -5.21307754517, -9.17019081116,

WIth CoreTimer removed from channel list (the AIN0 channel is now in column 1):

5.85449361801, -10.5803785324, -5.20549917221, -9.17492771149,
5.84186172485, -10.5797472, -5.2137093544, -9.17208576202,
5.86523103714, -10.5800628662, -5.20613098145, -9.17240142822,
5.84912490845, -10.5797472, -5.20676231384, -9.17303276062,
5.84723043442, -10.5797472, -5.2099199295, -9.17145442963,
5.83838796616, -10.5800628662, -5.20770978928, -9.17208576202,
5.85165166855, -10.5797472, -5.21213054657, -9.17177009583,
5.84975671768, -10.5794315338, -5.2102355957, -9.17145442963,
5.84849357605, -10.5797472, -5.20897293091, -9.17240142822,
5.84280920029, -10.5797472, -5.20865678787, -9.17082214355,

Thanks...

Code below:

from labjack import ljm
import time
import sys

filename = "trickle_data.csv"

OFILE = open(filename,'w')

MAX_REQUESTS = 200 # The number of eStreamRead calls that will be performed.

#Possible Analog Input resolutions
AN_IN_RES_DEFAULT = 0
AN_IN_RES_16BIT = 1
AN_IN_RES_19BIT = 2
#Make your choice here...
AN_IN_RES_CURRENT = AN_IN_RES_16BIT

#How long each CORE_TIMER tick is, in seconds.
CPU_CORE_TIMER_TICK = 1.0/40000000.0

# Open LabJack
handle = ljm.openS("T7", "Ethernet", "192.168.0.50", )

info = ljm.getHandleInfo(handle)
print("Opened a LabJack with Device type: %i, Connection type: %i,\n" \
    "Serial number: %i, IP address: %s, Port: %i,\nMax bytes per MB: %i" % \
    (info[0], info[1], info[2], ljm.numberToIP(info[3]), info[4], info[5]))

# Stream Configuration
POS_IN_NAMES = [
    #"CORE_TIMER",
    #"STREAM_DATA_CAPTURE_16",
    "AIN0",
    "AIN1",
    "AIN2",
    "AIN3",
    "AIN4",
    "AIN5",
    "AIN6",
    "AIN7",
    "AIN8",
    "AIN9",
    "AIN10",
    "AIN11",
    "AIN12",
    "AIN13",
    "CORE_TIMER",
    "STREAM_DATA_CAPTURE_16"
    ]

NUM_IN_CHANNELS = len(POS_IN_NAMES)

#Add positive channels to scan list
aScanList = ljm.namesToAddresses(NUM_IN_CHANNELS, POS_IN_NAMES)[0]

#max scan rate = 100,000 samples / Number of Analog channels.
#max scn rate here = 100,000/14 = 7142 scans/second.
scanRate = 10

#The number of scans returned by each call to the LJM_eStreamRead function.
#Increase this parameter to get more data per call of LJM_eStreamRead. A
#typical value would be equal to ScanRate or 1/2 ScanRate, which results
#in a read once or twice per second.
#This results in a blocking-read of the DAQ device.  IF multiple daq's are
#present, then start a new thread for each device, or consider adjusting
# http://labjack.com/support/ljm/users-guide/constants/ljmstreamscansreturn
scansPerRead = 1

# Configure the analog inputs' negative channel, range, settling time and
# resolution.
# Note when streaming, negative channels and ranges can be configured for
# individual analog inputs, but the stream has only one settling time and
# resolution.

#The value passed for Resolution_Index is from 0-8, where 0 corresponds
#to default, 1 is roughly 16-bit resolution (RMS or effective), and 8
#is roughly 19-bit resolution.
#For stream readings the default of 0 corresponds to Resolution_Index=1.

aNames = ["AIN_ALL_NEGATIVE_CH", "AIN_ALL_RANGE", "STREAM_SETTLING_US",
          "STREAM_RESOLUTION_INDEX"]
aValues = [ljm.constants.GND, 10.0, 0, AN_IN_RES_CURRENT] #single-ended, +/-10V, 0 (default),
                                          #0 (default)
ljm.eWriteNames(handle, len(aNames), aNames, aValues)

try:
    # Configure and start stream
    print(aScanList[0:NUM_IN_CHANNELS])
    scanRate = ljm.eStreamStart(handle, scansPerRead, NUM_IN_CHANNELS, aScanList, scanRate)
    print("\nStream started with a scan rate of %0.0f Hz." % scanRate)

    print("\nPerforming %i stream reads." % MAX_REQUESTS)
    totScans = 0
    i = 1
    while i <= MAX_REQUESTS:
        print("Getting read #%s."%str(i))
        ret = ljm.eStreamRead(handle)   #this will block until the data is ready.
      
        # Note that the Python eStreamData will return a data list of size
        # scansPerRead*TOTAL_NUM_CHANNELS, but only the first
        # scansPerRead*NUM_IN_CHANNELS samples in the list are valid. Output
        # channels are not included in the eStreamRead's returned data.
        data = ret[0][0:scansPerRead*NUM_IN_CHANNELS]

        ostr = ""
        for idx in range(len(data)-1):
            ostr = ostr + str(data[idx]) + ', '
        #last point
        ostr = ostr + str(data[idx +1])+'\n'
        OFILE.write(ostr)
        print(ostr)
        scans = len(data)/NUM_IN_CHANNELS
        totScans += scans
             
        i += 1

    OFILE.close()
    print("\nTotal scans = %i" % (totScans))
    print("LJM Scan Rate = %f scans/second" % (scanRate))
    print("Skipped scans = %0.0f" % (totSkip/NUM_IN_CHANNELS))
except ljm.LJMError:
    ljme = sys.exc_info()[1]
    print(ljme)
except Exception:
    e = sys.exc_info()[1]
    print(e)

print("\nStop Stream")
ljm.eStreamStop(handle)

# Close handle
ljm.close(handle)