Interface: USB 3.0, IEEE 1394, USB 2.0,
SCSI, USB 1.1, Parallel Port
My own preference is in that order. USB 3.0 and IEEE 1394 are faster than any scanner, so variations are due to the speed of individual scanners, not the interface. However USB 1.1 and Parallel interfaces are not as fast.
A fast interface is required to scan 20 or 60 MB of data in less than one minute. Otherwise if the interface cannot keep up with the data, we see the scanner carriage back-scanning (reversing stop and start motion) when scanning very large images.
SCSI and IEEE-1394 typically require a controller board to be added, although that's simple today with Plug & Play in Win98 and later.
SCSI is the historic fast interface, normally much faster (the better models) than USB 1.1 or parallel, especially true for large data sizes. All operating systems support SCSI, but you will need to add ASPI in WinXP and Win2000 (see Appendix C). SCSI won't be so popular on new scanners, it's apparent that IEEE 1394 is taking over at top-end.
IEEE 1394 (also called Firewire by Apple - same as IEEE 1394) is the latest and greatest. It requires at least Win98SE or later, or Mac OS 9.0. Advantages over SCSI include longer cables, which can also be connected with power on, and 1394 is simpler, no device ID or termination. 1394 compares to SCSI for speed, and compares to USB for easy installation. The 1394 bus is rated "up to" 400 Mbits/second (up to 50 MB/second, in the same way the SCSI bus is rated up to 10 or 20 MB/second). In every case, these numbers are bus speeds, not device speed. Scanners cannot go nearly that fast. The idea is that each of several devices can all run full speed simultaneously on the fast bus, even if no one device can ever approach full bus speed. 1 or 2 MB/second is very fast for a color scanner, due to the CCD exposure time.
USB (Universal Serial Bus) scanners are very popular, perhaps not the fastest scanners, but easy to install, and a huge improvement to bypass the conflicts of trying to share a parallel port. USB 1.1 requires Win98 and later, and Macintosh since the iMac/G3/G4 OS 8.6. USB 1.1 dominates the low-end scanner market today, and IEEE 1394 seems to have won the high-end.
And today, USB 3.0 is a class apart, very fast. USB 2.0, not so much.
The USB 1.1 bus speed is 12 Mb/sec, which is 1.5 MB/second (Mb = bits, MB = Bytes), but no one device ever exceeds half of that, so the USB 1.1 advantage is not speed (and implementations do vary).
Some compact USB scanners are powered from the USB cable and port (no separate power cord) which can be ideal for a portable computer (these scanners may not work on a non-powered USB hub however, they need powered hubs).
Here is one USB/SCSI speed comparison:
Scanner: HP 6200 (has both USB and SCSI)
Computer: 400 MHz P2, 256 MB, Win98
This comparison uses the same scanner on the same computer, and scans the same area.
8.0 x 10.0 inches
Asus P2B motherboard
|150 dpi RGB
|300 dpi RGB
|600 dpi Line art
There is a small difference in speed with a small image, but a large difference on large data. Firewire and SCSI scanners are typically 2x or 3x faster than USB 1.1 for large data. Film scanners typically create images from 20 MB to 60 MB, or even more, and need a faster interface. But 6x4 inch photo prints are only perhaps 6 MB in size, and the smaller size doesn't see such a big difference in speed.
The USB 2.0 Hi-Speed bus is 480 Mb/second (60 MB/sec), and is starting to appear in scanners and motherboards. You can add a USB 2.0 port if your computer doesn't have one. A 2.0 device on a 1.1 port, or a 1.1 device on a 2.0 port, only goes 1.1 speed. Note USB 2.0 specifications include three ratings: Low Speed 1.5 Mb/sec (for mouse or keyboard), Full Speed 12 Mb/sec (like USB 1.1), and Hi-Speed 480 Mb/sec. It needs to actually say "Hi-Speed". If the spec just says USB 2.0 or USB 2.0 Full-Speed, it is probably not faster than USB 1.1. If buying now, look for USB 2.0 Hi-Speed in both motherboards and scanners.
EPP (Enhanced Parallel Port) is part of the IEEE 1284 parallel specification. Parallel port scanners are still available (for Windows, the Mac doesn't use parallel ports), but USB is replacing them, and I would discourage parallel scanners. The appeal of parallel scanners was that they are inexpensive, and easy to install. Or rather, easy unless you also plan to use a printer on the same port. Parallel ports are simply not designed to be shared, or to support multiple devices on one port, and there is often trouble when you try. Reason says all those different drivers watching the same one parallel port can only cause trouble. Sometimes it works, but often the printer won't work at all. It mostly depends on the printer, and the bidirectional printers are the most problem.
If the scanner is on a parallel port by itself, it can work fine, and I withdraw my concerns. Sometimes it might work anyway, but I wouldn't bet on it. If two devices must fight each other to access the same port, problems are not a surprise.
The only good plan for using a parallel port is to put only one device on it. You can add another parallel port for a second device. Many printers have both USB and Parallel interfaces, so adding a USB cable allows reinstalling the printer at the USB port, to fix the problem of printer and scanner not working on the same port.
EPP parallel port scanners theoretically can be noticeably faster than USB 1.1, but they rarely are. Implementations do vary, the low-end units of either type typically are not very fast.