Sampsperchantoacquire =1 gives error -200077

Question

using either DAQmxCfgSampClkTiming or DAQmxCfgImplicitTiming and setting SampsPerChantoAcquire to 1 gives error code -200077
I want to sample 6 channels exactly once as fast as possible - how should I do this?
Thanks!

Answer 1

Hello Britt,
You can use the basic structure of OneSample as a guideline and simply add on however many more channels you require (in this case 5 more). You would do this using the DAQmxCreateAIVoltageChan() function. The NI-DAQmx C Reference Help describes this function as follows:
DAQmxCreateAIVoltageChan
int32 DAQmxCreateAIVoltageChan (TaskHandle taskHandle, const
char physicalChannel[], const char nameToAssignToChannel[], int32
terminalConfig, float64 minVal, float64 maxVal, int32 units, const char
customScaleName[]);
Purpose
Creates channel(s) to measure voltage and adds the channel(s) to the task
you specify with taskHandle. If your measurement requires the
use of internal excitation or you need the voltage to be scaled by excitation,
call DAQmxCreateAIVoltageChanWithExcit.
You would simply call this function with the same taskHandle you are already using and list whatever channels you want as the physicalChannel[] input parameter. Also, as noted in the NI-DAQmx Help, the physical channel syntax can be used for a single channel or a range of channels. As described in this manual:
Physical Channel Syntax
<script type="text/javascript">
</script>
Use this syntax to refer to physical channels and groups of
physical channels in NI-DAQmx.
Physical Channel Names
Physical channel names consist of a device identifier and a slash
(/) followed by a channel identifier. For example, if the physical channel is
Dev0/ai1, the device identifier is Dev0, and the channel identifier is ai1. MAX assigns device identifiers to devices in the
order they are installed in the system, such as Dev0 and Dev1. You also can
assign arbitrary device identifiers with MAX.
For analog I/O and counter I/O, channel identifiers combine the
type of the channel, such as analog input (ai), analog output (ao), and counter
(ctr), with a channel number such as the following:
ai1
ctr0
For digital I/O, channel identifiers specify a port, which
includes all lines within a port:
port0
Or, the channel identifier can specify a line within a port:
port0/line1
All lines have a unique identifier. Therefore, you can use lines
without specifying which port they belong to. For example, line31—is equivalent to port3/line7 on a device with four 8-bit ports.
Physical Channel Ranges
To specify a range of physical channels, use a colon between two
channel numbers or two physical channel names:
Dev0/ai0:4
For digital I/O, you can specify a range of ports with a colon
between two port numbers:
Dev0/port0:1
You also can specify a range of lines:
Dev0/port0/line0:4
Dev0/line0:31
You can specify channel ranges in reverse order:
Dev0/ai4:0
Dev0/port1/line3:0
Physical Channel Lists
Use commas to separate physical channel names and ranges in a list
as follows:
Dev0/ai0, Dev0/ai3:6
Dev0/port0, Dev0/port1/line0:2
The implementation in which you specify 2 samples per channel in DAQmxCfgSampleClkTiming is probably not the best implementation. If you continue to specify 2 samples to read and then only pull 1 sample from the buffer in DAQmxReadAnalogF64 you will eventually overflow the buffer on the device. Also, you will not "lose" data, but I don't think you are getting the data you expect. The DAQmxReadAnalogF64 function will read the oldest available sample from the device; this isn't a problem when you are reading all the data that is in the buffer every time. However, in your implementation, you will be getting old data when you expect more recent. The figure below should demonstrate the problem with this scheme. The arrows represent the data that you are reading with the DAQmxReadAnalogF64.
This behavior is the default setting for the NI-DAQmx driver, but you can change it using the RelativeTo property. The description of this property is as follows:
RelativeTo
Data Type:
int32
Description:
Specifies the point in the buffer at which to begin a read operation. If you
also specify an offset with Offset, the read
operation begins at that offset relative to the point you select with this
property. The default value is DAQmx_Val_CurrReadPos unless you configure a Reference Trigger for the task. If you configure a Reference Trigger, the default
value is DAQmx_Val_FirstPretrigSamp.
Valid values
DAQmx_Val_FirstSample
10424
Start reading samples relative to the first sample
acquired.
DAQmx_Val_CurrReadPos
10425
Start reading samples relative to the last sample returned by
the previous read. For the first read operation, this position is the first
sample acquired or the first pretrigger sample if you configured a reference
trigger for the task.
DAQmx_Val_RefTrig
10426
Start reading samples relative to the first sample after the
reference trigger occurred.
DAQmx_Val_FirstPretrigSamp
10427
Start reading samples relative to the first pretrigger
sample. You specify the number of pretrigger samples to acquire when you
configure a reference trigger.
DAQmx_Val_MostRecentSamp
10428
Start reading samples relative to the next sample acquired.
For example, use this value and set Offset to -1 to
read the last sample acquired.
You can get/set/reset this property using:
  DAQmxGetReadRelativeTo
  DAQmxSetReadRelativeTo
  DAQmxResetReadRelativeTo
However, in your implementation, you would still face problems with overflowing the buffer. Instead, I would recommend that you read all available samples and either average the data or just discard the samples that are not applicable.
Finally, to address your question regarding the sample rate in DAQmxCfgSampClkTiming, the sample rate is accurate down to the specifications of the device you are using. The USB-6008/6009 User Guide here provides the following specification:
Timing accuracy..................................... 100 ppm of actual sample rate
The NI USB-621x Specifications here indicates:
Timing accuracy..................................... 100 ppm of actual sample rate
Message Edited by Matt A on 07-11-2007 10:39 AM
Matt Anderson
Hardware Services Marketing Manager
National Instruments
Attachments:
Dev Range Reverse.JPG ‏2 KB
DAQmx Read.JPG ‏13 KB
Dev Range.JPG ‏3 KB