Basler slA1000-30fm, User Manual, Page: 143 / 166

Using Multiple Cameras on a Single Bus and Managing Bandwidth
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10.2 Using Multiple Cameras Where 1394a
and 1394b Devices are Mixed
The descriptions in the previous section assume that all of the devices on the bus are IEEE 1394b
devices. If the bus has mixed IEEE 1394a devices and IEEE 1394b devices, determining how to
share bandwidth between devices is a bit more difficult. As a first step toward understanding the
situation, consider the difference between 1394a devices and 1394b devices:
„
A 1394a device can transmit at what is known as S400 speed (400 Mbit/s).
„
During a single bus cycle, a device operating at S400 speed can transmit a single packet of up
to 4096 bytes. Alternatively, several devices operating at S400 speed can transmit packets
during a single bus cycle as long as the sum of the bytes in the packets is 4096 bytes or less.
„
A 1394b device can transmit at what is known as S800 speed (800 Mbit/s).
„
During a single bus cycle, a device operating at S800 speed can transmit a single packet of up
to 8192 bytes. Alternatively, several devices operating at S800 speed can transmit packets
during a single bus cycle as long as the sum of the bytes in the packets is 8192 bytes or less.
The next thing that we must consider in a bus that has mixed 1394a and 1394b devices is the speed
at which each device will transmit:
„
A 1394a device will always be capable of transmitting at S400 speed on a mixed 1394a /
1394b bus. (The 1394a device can transmit at slower speeds, but we are assuming that you
always want to transmit at the fastest speed.)
„
A 1394b device will transmit at S800 speed if all of the devices in its path to the host PC,
including the adapter card in the PC, are 1394b devices. If the path to the PC passes through
any 1394a device, then the 1394b device will transmit at S400 speed.
Figure 36 illustrates some situations where 1394a devices and 1394b devices are mixed on a single
bus. If you look at the figure, you will notice:
„
Camera 1 will transmit image data at S400 speed. This is simply because the camera itself is a
1394a device and S400 is the maximum speed for 1394a devices.
„
Camera 2 will transmit image data at S400 speed. The camera is a 1394b device, which
means that it is capable of S800 speed. But the camera’s path to the host PC passes through
a 1394a adapter, so this limits the camera’s actual maximum speed to S400.
„
Camera 3 will transmit image data at S800 speed. This is because the camera is a 1394b
device and its path to the host PC passes through only 1394b devices.
„
Camera 4 will transmit image data at S400 speed. This is simply because the camera itself is a
1394a device.
„
Camera 5 will transmit image data at S400 speed. The camera is a 1394b device, but its path
to the host PC passes through a 1394a hub, so this limits the camera’s maximum speed to
S400.
„
Camera 6 will transmit image data at S400 speed. This is simply because the camera itself is a
1394a device.
„
Camera 7 will transmit image data at S800 speed. This is because the camera is a 1394b
device and its path to the host PC passes through only 1394b devices.