©
National Instruments Corporation
11-1
11
Bus Interface
The b
u
s interface circ
u
itry of USB-621
x
devices efficiently moves data
between host memory and the meas
u
rement and acq
u
isition circ
u
its.
All USB-621
x
devices are j
u
mperless for complete pl
u
g-and-play
operation. The operating system a
u
tomatically assigns the base address,
interr
u
pt levels, and other reso
u
rces.
USB-621
x
devices incorporate USB-STC2 technology to implement a
Hi-Speed USB interface.
USB Signal Stream
USB-621
x
devices have fo
u
r f
u
lly-independent USB Signal Streams for
high-performance transfers of data blocks. One USB Signal Stream is
available for each meas
u
rement and acq
u
isition block:
•
Analog inp
u
t
•
Analog o
u
tp
u
t
•
Co
u
nter 0
•
Co
u
nter 1
Data Transfer Methods
The two primary ways to transfer data across the USB b
u
s are as follows:
•
USB Signal Stream
—A method to transfer data between the device
and comp
u
ter memory
u
sing USB b
u
lk transfers witho
u
t intervention
of the microcontroller on the NI device. NI
u
ses USB Signal Stream
hardware and software technology to achieve high thro
u
ghp
u
t rates
and increase system
u
tilization in USB devices.
•
Programmed I/O
—A data transfer mechanism where the
u
ser’s
program is responsible for transferring data. Each read or write call in
the program initiates the transfer of data. Programmed I/O is typically
u
sed in software-timed (on-demand) operations. Refer to the
Output Data Generation Methods
, for more information.
