To obtain the highest data capture rates, a large amount of information must flow from the sensors, through the SGI workstation and into MotionSampler. The cables from the sensors are high-speed serial lines.
The Polhemus system bundles sensor cables into groups of four per serial line. The Ascension unit operates best with each sensor on a separate serial line.
While SCSI-based terminal ports that can provide 32 or more serial lines on an SGI are available, IRIX cannot handle interrupts from a dozen or more serial lines simultaneously.
The Polhemus system communicates with the SGI machine through an ethernet connection, which eliminates most of the IRIX problems if there is not excessive traffic on the network.
The Ascension system may require the use of a terminal server to combine a number of serial devices into an ethernet-based communication device. The XYLogics unit is a particularly good example of the terminal server solution because it can be read directly across the ethernet without any intervening protocols.
One thing to consider about the XYLogics unit is that it places data in packets, so that motion samples do not arrive when the serial unit generates them, but rather in blocks. Therefore, MotionSampler should be configured so that the sampling frequency matches the data generation frequency.
Another consideration is that the ethernet traffic generated by running eleven or more serial devices at a high sampling rate is sufficient to interfere with most networks. Therefore, any other traffic on the network degrades the real-time performance of the motion sampling. The best solution to this problem is to isolate the motion capture gear and computers from the network.
On Indigo2 systems, an EISA-based serial card can provide a high speed link to the Ascension Flock of Birds. These cards can support high baud rates on banks of terminals and are expandable to more than 16 communication lines. With this device, the Flock of Birds can provide data to MotionSampler at 100 samples per second.