Carmo
12 cameras transferring data could have the following problems:
1)
There is not enough bus power to power all 12 cameras. Most FireWire boards supply a total of 15W (12V DC) power to a camera per FireWire board. Most FireWire cameras require 3-5W of power. That means you should be able to power 3-5 cameras per FireWire board. More than that and the cameras simply won't show up. Please refer to your camera camera documentation to find out exactly how much power each camera consumes.
2)
There is not enough DMA contexts to simultaneously acquire from 12 camera. Most OHCI compliant chipsets only allow a user to stream 4 isochronous video channels simultaneously from a single FireWire board. If you attempt to configure more cameras than that, then you will receive insufficent resource error. In this case buy additional FireWire boards and plug 3-4 cameras per board. Please note that there are some FireWire boards (using VIA or Agere chipset) that a user to stream 8 isochronous video channels simultaneously from a single FireWire board. Read the FireWire board documentation for the exact number of ischronous receive DMA contexts available.
3)
There is not enough FireWire bandwidth to acquire all the images. On a single 1394a board there is a cumulative bandwidth of 4096 bytes available. Since you are using a 15fps camera, each camera only require 640 bytes of bandwidth. If you attempt to more bandwidth than is availble, you will receive insufficient resource error. That means in this situation you are limited by the number of isochronous receive DMA context available on a single FireWire board. So with a standard 4 channel FireWire board, you will only be using 2560 bytes of the total 4096 bytes per FireWire board.
4)
There is not enough PCI bandwidth to transfer data. PCI bandwidth is theoretically limited to 133 MB/s. If you exceed the PCI bandwidth, the you will see distorted images on all or some of the cameras. No errors will be returned. Each 15Hz VGA camera generates about 4.4 MB/s worth of data. So for 12 cameras that is about 53 MB/s. While that is way below maximum bandwidth of PCI, you should consider that network traffic and video traffic will also consume PCI bandwidth. While not absolutely necessary, consider using PCIe FireWire board instead of PCI FireWire boards.
5)
FireWire Hubs and repeaters/extenders can cause problems. Many users attempt to user FireWire hubs to plug as many cameras as possible to a system. Some people use repeaters/extenders to attempt to go beyond the recommend 4.5m cable length of FireWire. In some situtions, these hubs and repeaters cause a lot of trouble on the FireWire bus. Most problems are manifested as cameras that don't show up and/or cameras that display distorted images. My recommendation is to attempt direct cable connection between your cameras and the FireWire boards. If you need longer cable lengths, consider using industrial 10m cables or fibre connectors.
Hope this helps,
JohannS
