What Is an SCSI External Hard Drive?

The SCSI bus is a general-purpose I / O bus. Using a SCSI bus on a microcomputer can maximize the capabilities of the I / O devices and enable multiple I / O devices to be connected to the host computer to work together. The SCSI bus has become an ANSI standard in 86, and has now developed to SCSI-2, and is developing SCSI-3. This article introduces the SCSI bus and its development in more detail.

Like microchannel, VME, and NuBu, SCSI is a true bus that defines standard physical and electrical connections for devices. The principle difference between SCSI and other buses is that SCSI makes the application of a variety of peripheral devices more convenient. Its communication protocol treats peripheral devices as logical devices that use a defined set of commands, thus eliminating hardware incompatibilities.

The SCSI specification lists individual commands for many types of devices. These devices are: hard disk (random access), magnetic tape (sequential access), printers, and so on. The specification includes compulsory orders, operating orders, and orders set by each vendor. The SCSI bus treats the hard disk as a random access device with a specified capacity and can respond to standard formatting, read, and write commands. The actual hardware may be a hard drive using MFM, RLL, ESDI, or some proprietary interface. A built-in SCSI controller translates commands into control signals for a dedicated interface.

In addition, SCSI extends the physical bus beyond the case of a typical microcomputer. The single-ended driver can be used to extend the SCSI bus to 6 meters by cable, and the differential driver can be extended to 25 meters. This solves the problem of requiring each type of peripheral device to be placed in the computer box, and also enables large peripheral devices such as image scanners to be connected. Such devices often cannot be placed anywhere.

The connectivity and economics of the hardware provide users with a lot of ability to use the required I / 0 requirements. The SCSI standard connector can easily connect multiple peripheral devices, and its standard command set simplifies the system designer's choice of purchasing SCSI peripheral devices while ensuring compatibility. The versatility of SCSI has made it a standard for most companies' system-level and external storage device products. This is a meaningful thing.

In order to explain the role of Taiwan CSI as a formed peripheral interface I / O bus standard, we must first understand the various components of the SCSIANSI specification, including hardware specifications, timing signal specifications, and device-independent command set descriptions.

SCSI hardware specifications describe its physical characteristics in detail: cables, signal output pins, connector types, and more. There are two types of SCSI connectors commonly used: standard 50-pin rubber cable connectors for internal peripherals and center-type 50-pin cables for external devices. Apple has selected an alternative external 25-pin high-density connector for its Macs with single-ended drives (Recently, Macs use a standard 50-pin rubber SCSI connector on their built-in hard drives, while externally connecting devices Still using 25-pin connector). Both Amiga and Nex computers use Apple's external connectors and signals, which allows them to access devices and cables made for Macs. Considering that the interface signals will be more complicated in the future, IBM announced that it has selected 60-pin connectors for its SCSI host adapters for its microchannels.

Although this seems to be a failure of the SCSI hardware specification, peripheral devices generally use standard SCSI 50-pin connectors. So in most cases, you can first connect the first SCSI peripheral device to the host computer with a suitable cable, and then you can use standard cables to connect other SCSI peripheral devices.

Just like any other standard bus, the wires in a SCSI cable are like transmission lines, which can produce undesired effects such as signal reflections and uneven load on the wires connecting the drives. To mitigate these effects, termination matching resistors must be connected to the two physical ends of the SCSI cable. The termination resistor is usually connected to the beginning of the SCSI bus (typically a plug-in resistor box is used). If the SCSI bus terminal is improperly connected, it will cause the device to work unstable or the entire bus to fail.

The SCSI signal specification specifies the signal delay, minimum pulse width, and hold time of the SCSI bus. But users don't need to worry about these, because the SCSI protocol chip will manage most timing restrictions. The SCSI protocol chips are Adaptic's AIC-6250 (installed on the SCSI host adapter produced by Adapt 7ee and IBM) and NCR's 5380 and 5390 (used on Mae machines and other models) ^[1] .

SCSI uses an interlocked symbol-exchange method to 'send data to the entire bus. This communication protocol is based on indirect answering symbol exchange. The bus state of SCSI is called phases. There are eight possible bus phases. Except for one, the rest use asynchronous transmission. In the data phase, information can be exchanged in one of two ways, synchronous or asynchronous. Synchronous transfers are used only if both the host computer and the target device allow it. Data is transmitted on the bus one byte at a time in parallel to the entire bus, using eight data lines. The typical asynchronous transfer rate is about 2 megabytes per second, and the synchronous transfer rate can reach up to 5 megabytes per second.