Chapter 2
Using Your NI 653X
2-28
ni.com
The preloading process causes a small delay between the start command in
software and the actual start of data transfer. If this delay is a concern, you
may disable the preloading by calling the following function/VI before the
software start command:
•
NI-DAQ C interface—In the
Set_DAQ_Device_Info
function, set
the ND_FIFO_Transfer_COUNT to ND_NONE.
•
LabVIEW—In the DIO Parameter VI, set the Scarabs Preload Enable
attribute to None.
Note
Because output data is preloaded to the NI 6534 buffer, you cannot use DAQ events
(called progress events in the CWDO object of Measurement Studio) to monitor the
progress of a handshaking output operation. A DAQEvent is fired when data is preloaded
into the NI 6534 onboard memory from the PC memory, so the event indicates a data
transfer from the PC memory, not the progress of data output from the NI 6534 to an
external device.
Monitoring Line State—Change Detection
You can configure your NI 653
X
to acquire data whenever the state of one
or more data lines change. Once the NI 653
X
detects a change in one of the
selected lines, it captures data within 50–150 ns and outputs a pulse on the
REQ pin. This mode increases CPU and bus efficiency because you can
monitor activity on input lines without continuously polling or transferring
unnecessary data during periods of inactivity.
Tip
When you use the NI 653
X
alone, it detects whether a change occurred, but when you
use the NI 653
X
and an NI 660
X
counter/timer device (using a RTSI line), the relative time
between changes can be acquired by the NI 660
X
.
Deciding the Width of Data to Acquire
You can choose between a width of 8, 16, or 32 bits. Use the following
table to find the valid combinations of ports and timing controllers you can
use based on the width of data you want to acquire.
Table 2-4.
Port and Timing Controller Combinations
Transfer
Width
Possible Port
Combinations
Timing Controllers
That Can Be Used
8 bits
Port 0 (DIOA<0..7>)
Group 1
Port 2 (DIOC<0..7>)
Group 2
