A common belief among engineers is that, to get the best
performance, a PC based data acquisition module must plug into the
PCI bus. Properly implemented, however, a data acquisition module
can use the PC's USB 2.0 port to pump data into a PC as fast as
PCI-based cards. The keys to achieving this performance are a
hardware USB interface on the module and optimized driver
software for the host system.
Because the Universal Serial Bus (USB) was originally developed
to replace lowspeed peripheral cabling, many engineers fail to
see its potential as a high performance data acquisition channel.
The original USB specification did offer only a modest bit rate,
but USB 2.0 handles 480 Mbits/sec, fast enough to handle 60
Mbyte/sec data streams. Even with the protocol reserving some
bandwidth for interrupts and control transfers, and the header
overhead on data packets, the bus can easily sustain more than
10 Mbytes/sec of continual data transfer. This is fast enough
to support extremely high performance data acquisition (DAQ)
hardware.
Utilizing the USB as a data portal provides many advantages for
data acquisition (DAQ). For one, the external connector and
"plug-and-play" software installation of USB peripherals means
that users do not have to open their PC, then set-up and
configure the hardware in order to begin acquiring data. The USB-
based DAQ module sets itself up upon installation. Addressing
and other potential resource utilization conflicts resolve
automatically.
Software for handling the data also becomes simpler to implement.
The host system USB drivers separate data streams into logical
channels called "pipes." This means that the host system software
will automatically form a logical connection from a DAQ channel
to a destination within the application software, simplifying
software and hardware set-up. Applications simply need to
identify the logical pipe they wish to connect with and the
system software ensures that data travels to the right
destination. Application software does not need to know the
details of peripheral addressing, interrupt placement, or other
installation dependent parameters as it must for PCI plug-in
cards. The USB also has electronic advantages. The bus can power
the peripheral (within limits) so that the DAQ system does not
need its own power source. This further eases system installation
and use and has the added benefit of removing the sensitive A/D
converters and amplifiers from the electrically noisy environment
inside the PC's housing. Unlike PCI plug-in cards, a USB-based
DAQ module is easily shielded for achieving high bit-level
resolution. The benefits of USB operation are compelling for a
DAQ system, but implementing the system does require care in
order to ensure a high-bandwidth connection. Engineers often
assume that high data-rate systems should use the USB data
transfer mode with the highest raw bandwidth and least overhead
penalty: isochronous transfer. But isochronous transfers are a
"best-effort" channel. Data that suffers from errors cannot be
resent. The proper mode to use for DAQ on USB is the bulk
transfer mode.
1). This mode supports the resending of corrupted packets,
ensuring data accuracy, and allows fairly large 512-byte blocks,
keeping overhead effects down.
The drawback is that bulk transfers do not have guaranteed
timing. The USB host controller assigns bandwidth for bulk
transfers but reserves priority for interrupt and control
transfers. Thus, the bandwidth obtained by using bulk transfers
is an average - not a sustained - data rate while the DAQ module
needs to send data at a constant speed.
There are two approaches to dealing with the uncertainty in the
instantaneous bandwidth available for bulk transfers. One is to
keep the data rate low in order to maximize the likelihood that
bandwidth available will always exceed the DAQ module's data
rate. This is the simpler approach, but results in severe
underutilization of USB's potential.
The alternative approach is for the DAQ module to incorporate
FIFO buffering to hold data while waiting for the bus to become
available. Dual buffers are needed in the module - one to fill
while the other is emptying. The larger the buffers, the more
tolerant the module becomes of shifting bandwidth availability
and the closer its data rate can approach the average bandwidth
available.
In addition to the FIFO buffering on the DAQ module, the software
drivers should allocate buffer space in the host system at the
receiving end. This decouples the host system's data processing
activity from the data acquisition so that neither activity can
delay or impede the other's performance.
While FIFO buffering maximizes the DAQ module's ability to
utilize the available bulk transfer bandwidth, however, it is not
the only way to improve the achievable speed on the USB. The
module's USB interface hardware and latency in the host
controller software can seriously constrain achievable USB data
rates if not implemented with high performance in mind. For the
interface hardware, using a state-machine hardware controller
optimized for 512-byte transfers (the largest bulk transfer
packet) produces a much faster interface than the use of a
software-dependent USB microcontroller.
On the host side, careful driver design can reduce latency by
speeding the host system's response to incoming USB data. The
traditional Windows drivers allocate buffer space in response to
an incoming USB transfer, taking many milliseconds getting ready
to receive data once the DAQ module declares data ready to send.
Drivers that are proactive instead of reactive, pre-allocating
buffer spaces of the right size can decrease host latency by an
order of magnitude. These are not simply theoretical suggestions.
Data Translation has achieved a data rate of 250k samples/sec on
12 simultaneous channels with its DT9836 USB data acquisition
module and 2M samples/sec on 2 simultaneous channels at with the
DT9832A. Experience has shown that the USB can provide reliable,
sustainable data rates as great as 10.9 Mbytes/sec, corresponding
to sample rates of 5.45M samples/sec. The potential exists to
take USB-based data acquisition to even higher levels with
additional buffering and driver optimization.
But a high data rate is not all that is required of a high-
performance DAQ module. Issues such as accuracy, aperture
uncertainty, and noise levels are equally important. Further, the
inclusion of additional capabilities such as digital I/O lines,
counter/timers, analog outputs, and quadrature encoders can
greatly increase the utility of a DAQ module. The DT9832 and
DT9836 series offer all of these features.
Accuracy in a high-performance DAQ module has many facets. The
most obvious is the A/D converter's resolution. The Simultaneous
Series offer 16-bit A/D conversion: 2 channels at 2.0 MHz for
the DT9832A, 4 channels at 1.25 MHz for the DT9832, and 6 or 12
channels at 225 kHz for the DT9836. The modules are fabricated
with 12-layer boards, keeping the signal integrity intact and
typically achieving an ENOB (effective number of bits) rating
of greater then 14-bits.
Accuracy also involves timing, however, especially in a multi-
channel DAQ module. In order to properly compare and correlate
sampled signals it is important to know the relative timing of
samples from one channel to the next. Most DAQ systems use a
single A/D converter with a multiplexer front-end to handle
multiple channels. This results in each channel's signal being
sampled at a different time, forcing the use of interpolation to
bring the signal data into temporal alignment and resulting in
relative timing errors or phase noise. Ideally, the samples for
all channels should be made simultaneously to eliminate phase
noise.
The DT9836 and DT9832 series modules do not use a multiplexer.
Each channel has its own track-and-hold stage with an independent
successive-approximation A/D converter (see Figure 4). The
converters use a common clock and the track-and-hold stages have
a common trigger, so that the modules offer true simultaneous
sampling. The channels have an aperture delay of 35 nsec with
uncertainty (aperture jitter) of 1 nsec, and channels are well
matched so that there is less than 5 nsec difference between
channels. This virtually eliminates phase noise in data.
With sample rate, bit accuracy, and timing accuracy at high
levels, the availability of additional features is simply a
performance bonus. The Simultaneous Series offer these extras, as
shown in Figure 2. Each module includes two 16-bit D/A channels,
16 digital output channels, 16 digital input channels, two 32-bit
counter timers, and 3 quadrature encoders. The additional digital
I/O lines provide considerable flexibility for incorporating
functions such as time stamping, pattern recognition, and
synchronization with external events. The counter/timers offer a
convenient means of triggering test events while the quadrature
decoders simplify the use of the module with X/Y positioning and
rotation.
Supporting these additional signal lines means that the data
channel must be fast enough to handle the additional signal and
control bits without impacting the module's sampling rate. The
USB is fast enough. It has proven that it can provide the needed
capacity, and offers significant ease-of-use benefits. There is
no longer a need to pry open a PC to assemble a high-performance
DAQ system. Simply plug one in to the USB port.
For related images and graphs, see:
http://www.datx.com/white_papers/High-Performance-DAQ-on-USB-white-paper.pdf
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