How SCSI Works
Before the general shift to serial connections, there were a number of different parallel options for connecting hard drives and other devices. One of these options was the Small Computer Systems Interface.
What is Ultra SCSI?
One of the few interfaces equally suited for internal and external use, SCSI developed as a high-speed option for existing computer systems that needed something to handle the bandwidth needs of hard drives and scanners. As most computers did not include a controller, the most common way to add support was with an adapter such as a PCI SCSI card. There are three main aspects to consider when installing an adapter:
- Controller: The controller determines which signaling protocol the device uses as well as the bus width and speed.
- Cable: Usually either 50-pin or 68-pin, the cable connects all the devices in a daisy-chain fashion.
- Terminator: In order to prevent reflections from interfering with the signals, every chain must be terminated to ensure a clean signal.
What Should You Look Out for When Installing SCSI?
Installing a SCSI controller card is relatively easy. All you have to do is open up the computer case and plug the card into an open PCI slot. From there you simply install the drivers and then start connecting devices to the card. There are however a few things that you do have to pay attention to:
- Width: Ultra SCSI is 8 bits wide and uses a 50-pin cable at 20 MHz for 20 MB/sec; Ultra Wide SCSI uses a 68-pin cable for 40 MB/sec at 20 MHz. You can combine 8-bit and 16-bit devices on the same chain, but in that case, you have to be careful about termination as different lines may need to terminate at different places along the cable.
- Device Limits: In most cases you can use up to seven devices on a chain if you have a cable of 1.5 meters or less. Increasing the length of the cable limits you to four devices. Also, even though some devices do include a termination function, it's a good idea to use a standalone terminator with every SCSI Card.
Using SCSI
While very few systems support the protocol today, it's easy enough to plug in a card if you really need access to a particular device that relies on the connector. All you need is an open PCI slot for the adapter and a set of drivers for your system and device. It's particularly handy for devices like older scanners that may not be available for any other interface. You can use it with external drives as well, but most controllers are very much slower than more recent options like USB 3.0.