SCSI - Standards

What is SCSI ?

SCSI (Small Computer System Interface) is a general-purpose parallel bus system.
It was originated from Shugart's 1979 SASI (Shugart Associates System Interface) and Shugart and NCR presented it to the ANSI in 1981.
It became official in 1986, when the ANSI-committee X3T9.2 defined the SCSI-1 spec in document X3.131-1986.

SCSI is now an acknowledged and very well standardized multipurpose interface. and supports a wide variety of devices which includes hard disks, removable disks, magneto-optical devices, tape drives, printers, CD-ROMs, scanners, communication devices .... and much more.

See > SCSI Bus Termination / Terms - Definitions


SCSI-1 defines a universal 8-bit (narrow) I/O-Bus that allows connection of up to 8 devices, incl. the SCSI (host) adapter.
Each SCSI device, connected to an SCSI-bus must have a unique ID (address) in the range of 0 to 7.
SCSI - ID #7 (by default) is occupied (not avail. for devices) by the SCSI (host) adapter.
SCSI-1 was a high-speed bus system, compared to this time's peripheral devices.


SCSI-2 development and approval started while SCSI-1 still was not officially approved.
In 1986, when SCSI-1 was officially approved, SCSI-2 already was in the process of being developed.
The official SCSI-2 designation is X3.131-1994.
SCSI-2 provided a better formal definition, removed some oddities and obsolete items, added some extensions, and most importantly, added the ability to double and even quadruple data transfer speed on the SCSI bus with it's Fast SCSI and Wide SCSI options.


With SCSI-2, Fast SCSI was defined as an option.
Despite the tendency of the market to define Fast-SCSI as different, but faster SCSI,
Fast SCSI is only an additional synchronous data transfer mode with tighter timing to achieve the 10MByte/sec maximum data rate.
As with any other synchronous transfer mode, only data transfers are synchronous, the commands are transferred asynchronous.


Another SCSI-2 option is Wide SCSI.
Two types are defined, 16-bit and 32-bit.
Wide SCSI can be combined with Fast SCSI and thus it can provide up to a 40 MByte/sec data rate.
The X3T9.2 committee defined a 68-pin SCSI cable for 16-bit and a combination of a standard 8-bit and a 68-pin cable for 16-bit and 32-bit Wide SCSI.
Wide SCSI host adapters can address standard SCSI devices without problems - assuming that a correct adapter cable is used.

P&P - Plug and Play - SCSI

Plug and Play - SCSI is an extension to the generic Plug and Play specification.
P&P is an approach for defining an auto-configuring environment for ISA boards.
One of the key features of P&P-SCSI is SCAM.

SCAM - SCSI-Configured AutoMagically

SCAM is a protocol for automatic SCSI ID assignment.
It is included in the SCSI-3 parallel interface drafts.
The SCAM master (typically the host adapter) scans the bus for attached SCSI devices.
For compatibility, it also needs to find and identify legacy, i.e. standard, SCSI devices.
Thus the SCAM master gets a map of the attached devices and assigns a valid soft ID to each SCAM-compliant SCSI device.
After this process, the SCAM master keeps this device table in a non-volatile memory to provide - if possible - an identical ID setup for further boot processes.


Every SCSI device must have a unique ID on the bus.
For this reason, most devices have three ID jumpers to set the SCSI ID from 0 to 7.
ID 7 (in most cases) is reserved for the SCSI (host) adapter.
ID 0 is for a boot device and is usually a hard drive.
ID 1 is normally used for a second hard drive, although this is not a requirement.
ID 2 and up are normally used for other devices with ID 2 being popular with CD ROM's.
With the exceptions of ID 0 and 7, there is no particular ID that has to be assigned to any device type.
Additionally, every ID can have up to seven sub-units identified by a LUN (Logical Unit Number).
Thus, you can address multiple devices through one ID.

Termination and TRMPWR

The SCSI bus needs to be terminated.
This means, both ends of the bus must have a circuit of some sort to eliminate signal reflections that otherwise would occur from the physical ends of the bus.
The termination circuit needs some power.
In order to supply this power there is a line called TRMPWR or Termination Power on the bus.
Usually, the TRMPWR source is the host adapter. However, multiple devices can supply TRMPWR without causing damage.

The 'classic' passive termination ('Alternative 1' in the SCSI-2 spec) came with SCSI-1 and consisted of a 220 Ohm pull-up and a 330 Ohm pull-down resistor on each signal.
The active termination ('Alternative 2' in the SCSI-2 spec) consists of a 110 Ohm resistor on each signal pulled up to a 2.85 Volt reference supply.

SCSI Connectors

There are many different connectors for SCSI.
Some of them were defined with SCSI-1 and are now obsolete, like the DB-50 connector.
The most common ones today are the 50-pin Centronics-type SCSI-1 connector, the 50-pin High Density SCSI-2 connector.
Most new host adapters and external devices use the SCSI-2 HD.

SCSI - Cables / Cable Lengths

Cable lengths were defined up to 6 meters maximum in SCSI-1 and SCSI-2, for a single-ended SCSI bus and up to 5 MHz data rate.
Keep this in mind, if you use long cables, and don't forget the cable lengths for the internal device cables.
With Fast SCSI-2 the highest possible data rate doubled to 10 MHz in synchronous mode and the maximum cable length was halved to 3 meters.
SCSI-2 allows up to a 10 cm cable stub length from the device to the main bus cable.
Sometimes this length is exceeded, causing higher capacitive loading.

A differential SCSI bus can use the full cable length up to 25 meters (approx. 82 feet) and keep the max. data rate of 10 MHz.
This is especially important with external devices, as most external single-ended cables can't cope with 10 MHz.

SCSI - Signal levels

Single-Ended SCSI signal levels vary from 0V to +5.25V.
All signals are active low.
True is a voltage level between 0 and +0.8 V and false is a voltage between +2 and +5.25 V.
Differential SCSI signals conform to the EIA RS-485 interface.

Single-Ended and Differential SCSI ?


There are two different electrical SCSI interfaces, Single-Ended and Differential SCSI.
Single-Ended SCSI has single ended TTL transceivers that allow up to 5 MHz data transfer rates with up to 6 meters cable length or up to 10 MHz data transfer (Fast SCSI) with up to 3 meters cable length.
Differential SCSI use (RS-485-style) transceivers that allow up to 10 MHz data transfer rate, but with cable lengths up to 25 meters.

Single-Ended and Differential devices are not electrically compatible with each other.
If you try to mix them, maybe you will destroy the Single-Ended devices on the bus as also the differential devices.

However, a new transceiver technology, Low-Voltage Differential (LVD), will combine the best features of single-ended and high-powered differential transceivers.
LVD will also enable higher speeds.
To ease the migration to LVD, most new SCSI devices will support Universal transceivers which include both single-ended and LVD transceivers.

Synchronous / Asynchronous Transfer

SCSI can use two handshaking modes, Asynchronous and Synchronous.
The main difference is: Asynchronous is basically a classic REQ/ACK handshaking system for each data packet, while with Synchronous, multiple REQ's can be issued before receiving ACK's, thus increasing the overall transfer rate.

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