2. Introducing NECTA Line Scan Cameras 26
2.5 USB 3.2 GEN 1X1 SUPERSPEED INTERFACE
NECTA cameras exploit the USB 3.2 Gen 1x1 standard interface to full the bandwidth requirements of
fast image sensors and communicate with PC. In order to maximize usable bandwidth of installed USB
3.2 Gen 1x1 controller, user can choose between isochronous endpoint and bulk endpoint for transmit-
ting video stream. The command stream (controlling camera features) is supported by a bidirectional
interrupt endpoint providing quick response and low latency.
2.5.1 Isochronous endpoint
The super-speed isochronous endpoint used by NECTA allows up to 3 burst transactions for each 125 µs
service interval (micro-frame); each burst transaction supports up to 16 packets made by 1024 B, reach-
ing up to 3x16x1024 B as payload for a single micro-frame. Total allowed bandwidth is 375 MiB/s for
NECTA USB 3.2 Gen 1x1 connection. The isochronous endpoint allows user to reserve a given amount of
bandwidth for each device, regardless of the connected device number to the host.
USB 3.1 Gen1 MicroFrame tick
Up to 3 bursts (mult value)
Up to 16 packets (burst length)
Figure 2.4: USB 3.2 Gen 1x1 Isochronous transactions structure
In order to exploit the maximum throughput, the computer must be able to support that load, therefore
particular attention must be paid in choosing cables and USB 3.2 Gen 1x1 host controllers (see Section
5.1 for details).
2.5.2 Bulk endpoint
The super-speed bulk endpoint used by NECTA transfers data using bursts of 1 to 16 packets, 1024 B
long. The theoretical maximum bandwidth is 500 MiB/s for a single host, even if the camera will only
use up to 375 MiB/s of the available bandwidth. Unlike an isochronous endpoint, a bulk endpoint can not
reserve the bandwidth for itself: all the available bandwidth is shared with all the other devices present
on the same host, even if connected to other ports. For this reason, when a NECTA is connected to one
USB3 port, it is not recommended to connect other USB devices, such as USB drives or Webcams, to
the same host. When a NECTA camera is connected as bulk endpoint, frame loss phenomena will occur
when other devices will be operated.
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