Four memory slots supporting up to 32GB of main memory using 1GB, 2GB, 4GB, or 8GB DIMMs. 2.8GHz and 3.2GHz systems: 1066MHz DDR3 ECC SDRAM. Eight memory slots (four per processor) supporting up to 64GB of main memory using 1GB, 2GB, 4GB, or 8GB DIMMs. 2.66GHz and 2.93GHz systems: 1333MHz DDR3 ECC SDRAM. VGA output using Mini DisplayPort to VGA Adapter or DVI to VGA Display Adapter (optional). Additional dual-link DVI output using Mini DisplayPort to Dual-Link DVI Adapter (optional). Additional DVI output using Mini DisplayPort to DVI Adapter (optional). Support for analog resolutions up to 2048 by 1536 pixels. Support for digital resolutions up to 2560 by 1600 pixels. Option to install two ATI Radeon HD 5770 cards. ATI Radeon HD 5870 with 1GB of GDDR5 memory, PCI Express 2.0, two Mini DisplayPort outputs, and one dual-link DVI port. ATI Radeon HD 5770 with 1GB of GDDR5 memory, PCI Express 2.0, two Mini DisplayPort outputs, and one dual-link DVI port. Double-wide, 16-lane PCI Express 2.0 graphics slot with one of the following graphics cards installed:. Multichannel audio through Mini DisplayPort. Analog stereo line-level input and output minijacks. Optical digital audio input and output TOSLINK ports. Front-panel headphone minijack and internal speaker. Five USB 2.0 ports (two on front panel, three on back panel). Four FireWire 800 ports (two on front panel, two on back panel). Hyper-Threading technology for up to 8 virtual cores. Turbo Boost dynamic performance up to 3.06GHz. 8MB of fully shared 元 cache per processor. One 2.8GHz Quad-Core Intel Xeon W3530 “Nehalem” processor. Hyper-Threading technology for up to 16 virtual cores. Turbo Boost dynamic performance up to 2.66GHz. 12MB of fully shared 元 cache per processor. Two 2.4GHz Quad-Core Intel Xeon E5620 “Westmere” processors. Mac notebook computers with USB-C or Thunderbolt 3 can charge over that port using a compatible USB-C power adapter and cable. Learn how to resolve Wi-Fi and Bluetooth issues caused by wireless interference. USB 3 devices can create wireless interference that affects Wi-Fi and Bluetooth devices. 
Others need more power than your Mac can provide. Some devices can be powered by the USB port on your Mac.
If your device came with an AC power adapter, use it. Check with the maker of your device, and install all available Apple software updates as well. Some devices need their own software, such as drivers or firmware. Plug the device directly into your Mac instead of a USB hub or other device, and if necessary test with a different USB port on your Mac or device. Test with another cable or adapter, if available. 
Check all connections: Unplug the device from your Mac, then plug it back in, and make sure that all cables and adapters are securely connected at both ends. If your Mac doesn't recognize a USB device after you plug it into your Mac:
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