Although modern computers have USB or FireWire ports, most ‘classic’ Apple Mac OS machines have a Small Computer System Interface (SCSI) port, which accommodates older high-speed devices, including disk drives, printers and scanners.

SCSI is a parallel port, so it needs separate wires for every data bit. The original 8-bit form of SCSI uses eight wires, as well as control circuits. So, although fast, the cost of cables and other problems make SCSI unsuitable for long distances.

The SCSI ports on many early machines run at less than 1 megabyte per second (MB/s), which is actually slower than the maximum speed of a contemporary hard disk drive. This older form of SCSI only operates in asynchronous mode, although later versions also work in the faster synchronous mode. Most modern SCSI ports operate at between 10 and 80 MB/s.

Although SCSI comes in several versions you can usually connect a device to any type of bus by using a suitable adaptor cable. However, if one device uses an older version of SCSI the entire bus usually slows down to accommodate it.

Variations of SCSI

The following versions of SCSI are in common use:-


This, the original form of SCSI, uses an 8-bit connection, as defined by the American National Standards Institute (ANSI) standard X3T9.2. It appears on the early Mac OS machines, conveying data at up to 5 MB/s. Standard single-ended (unbalanced) wiring is used, permitting cables up to six metres in length, although some specialist hardware has differential (balanced) wiring, extending this limit to 25 metres. Connections are made via one of the following types of connector:-

Most Mac OS computers have a DB25 or HDI30 socket, whilst SCSI devices have a matching plug or two C50 sockets. Up to eight devices, including the computer, can be connected at once.

Fast Narrow SCSI-2

This improved protocol increases the maximum speed to 10 MB/s but retains compatibility with SCSI-1, although you may need to replace some SCSI cables by higher quality products. A total cable length of up to 12 metres is permitted.

To add to the general confusion, some devices with this interface use a 50-way high-density D connector (HD50) instead of the original C50 connector.

Fast Wide SCSI-2

This version conveys 16-bit data at up to 20 MB/s via a 68-way high-density D connector (HD68). All devices incorporate active termination, eliminating the need for an external terminator (see below). A total cable length of up to 12 metres is permitted.

Up to 15 devices can be connected, whilst 32-bit data can be accommodated at 40 MB/s by using paired HD68 connectors or a special 104-way connector.

Ultra Narrow SCSI-3

This basic form of Ultra SCSI uses a special technique to double the original speed of narrow SCSI to 20 MB/s whilst retaining full compatibility with SCSI-1 and the narrow form of SCSI-2.

Ultra Wide SCSI-3

This 16-bit form of Ultra SCSI works at up to 40 MB/s, conveying data via a 68-way high-density D connector (HD68). All devices incorporate active termination, eliminating the need for an external terminator (see below). However, an external active terminator, known as an Ultra Wide SCSI-3 terminator, can also be used if required.

Up to 15 devices can be connected, whilst 32-bit data is accommodated using paired HD68 connectors or a special 104-way connector. A typical PCI card, providing two SCSI ports, will often be found to have a pair of micro-C 68-way connectors.

Ultra 2 Narrow SCSI

This interface uses low-voltage differential (LVD) technology, but is compatible with older forms of SCSI. It uses lower voltage signals, enabling it to run at up to 40 MB/s. The Ultra 2 SCSI terminator, although similar in appearance to an Ultra-Wide SCSI-3 terminator, is designed specifically for Ultra 2 SCSI devices. HD68 connectors are preferred for this kind of interface.

Ultra 2 Wide SCSI

This also uses LVD technology (see above) and is compatible with Ultra Wide SCSI-2, but runs at speeds of up to 80 MB/s. The Ultra 2 Wide SCSI terminator, although similar in appearance to an Ultra Wide SCSI-3 terminator, is designed specifically for Ultra 2 Wide SCSI devices. HD68 connectors are preferred for this kind of interface.

Ultra 3 Narrow SCSI

This development of SCSI, also known as Ultra 80 or Fast-80, works at up to 80 MB/s, with HD68 connectors preferred.

Ultra 3 Wide SCSI

This later version of SCSI, also known as Ultra 160, works at up to 160 MB/s, with VHD68 or HD68 connectors preferred.

Ultra 320 SCSI

An emerging SCSI format, providing speeds of up to 320 MB/s.

  SCSI in Detail

SCSI ID Numbers

Each device on a SCSI bus must be assigned an exclusive ID number, which is usually determined in the hardware of each item. Typically, this number is selected by an ID switch on the back of a SCSI device, or by internal arrangements, such as a series of links or a tiny switch located on a drive card.

IDs and Buses

The assignment of device IDs is dependent on the number of SCSI buses provided by your computer and whether these buses are shared between internal and external devices.

Later versions of the Classic Mac OS allow multiple SCSI buses, accommodating the external bus and internal bus in a later ‘classic’ machine or a computer with built-in SCSI ports as well as a SCSI card. With more than one bus you can, for example, connect a drive with an ID of 1 to one bus and connect another device, also with an ID of 1, to the other port.

Unfortunately, in some machines, a single SCSI bus is shared between all the internal and external devices, which seriously limits its usefulness. When using an older version of SCSI, with 0 for the startup drive, 1 for the internal CD-ROM and 2 for an internal backup drive, you only have the IDs from 3 to 6 available for additional devices.


Most SCSI systems need termination at one or more points along the bus. A terminator, usually consisting of a pack of resistors, absorbs any data reflections that might upset the operation of the chain. Termination can be provided by:-

Pass-through terminator

Often built into a C50 plug and socket coupler, usually providing termination in the middle of a chain of SCSI devices.

Block terminator

Usually in the form of a C50 plug that can be plugged into a spare socket at the end of a chain.

Internal termination

This is inside a device and often activated by a switch. Any device with permanent (unswitched) internal termination must go at the end of the chain. Or you can dismantle the device and remove its terminator packs (see below).

Active termination

Similar to internal termination, but with automatic switching. Although sometimes used with the original form of SCSI, where it can fail to work, it’s more common in Wide SCSI, where a standard terminators can’t be used.

Internal Termination

As already mentioned, some devices have internal termination, and, if fitted with a single connector, can be placed at the end of a chain. However, if the device hasn’t got a switch and you want to disable the termination, you may be able to remove the terminator packs (usually three eight-pin packages in a line) from within the device. Trouble is, you have to look inside to see if they’re there. Once removed, the device must go in the middle of a chain or be externally terminated.

Always follow these rules:-

Termination Rules

Generally speaking, you should observe the following rules for SCSI termination:-

  1. Termination is usually required at the first device in the chain, or at the point where the system cable from the computer is extended. However, if your machine has built-in termination, you don’t need a terminator at this point: see your computer manual for details.
  2. Termination is always required at the last device in the chain.
  3. Only two terminators should be fitted in a chain: two's company, three's a crowd!

Having said all this, you should break these rules if your system doesn’t work. For example, you could try a terminator in the middle of a chain, with or without terminators at the beginning or end. If you have serious problems you should also try changing the sequence of devices in the chain.

SCSI Troubleshooting

Here’s a quick review of what can cause problems on a SCSI chain:-

SCSI ID Conflict

Always make sure that every device has a unique ID number.


Try to follow the rules, but if this doesn’t work, don’t worry about breaking them. Sometimes you won’t need to fit any terminators at all and in other situations you’ll need to fit three!

Cable Length

The cable length of an entire SCSI chain, including the wiring inside each device, shouldn’t exceed the maximum allowed by SCSI. For SCSI-1 this is usually six metres, although if pushed to the limit, as in real-time digital audio processing, the length may have to be limited to one metre or less.

A SCSI regenerator can be used to convey your data over a longer distances, and also lets you disconnect part of a SCSI chain without disrupting the entire system. For distances of up to 25 metres, or possibly more, you can use a SCSI converter to change the signal into differential form. For runs up to 300 metres you can use a pair of fibre-optic converter boxes.

Cable Quality

SCSI cables often get damaged, causing errors or slow operation, even though they look normal. The metallic shield under the cable’s outer layer, also known as the screen, is particularly important.

Powering-up Sequence

In most instances you can switch on your computer and SCSI devices at the same time. However, if you do have problems, such as drives that fail to mount, slow computer operation and system freezing, you should try switching on external drives, in particular CD-ROM drives, before powering the computer itself.


On rare occasions a SCSI bus can be upset by the proximity of a video monitor, power supply or fluorescent lighting. Just try moving things around, including the order of items on the chain.

Other Hardware Problems

Mac OS Software Problems

Before panicking about a SCSI problem, restart your machine and try again. If that and everything else has failed, try clearing your parameter RAM (PRAM). And in the Classic Mac OS you should also check for a conflict of SCSI system extensions in your Extensions folder. If at all possible, choose just one such extension and use it for all of your drives.

SCSI Pin Connections

The wiring for unbalanced SCSI on a C50 connector is shown below. The wire numbers are for a ribbon cable or an insulation displacement connector (IDC) inside a computer or drive box.

C50WireFunction C50WireFunction
11Ground 262TPWR
23DB0 274Ground
35Ground 286Ground
47DB1 298Ground
59Ground 3010Ground
611DB2 3112Ground
713Ground 3214NATN
815DB3 3316Ground
917Ground 3418Ground
1019DB4 3520Ground
1121Ground 3622NBSY
1223DB5 3724Ground
1325Ground 3826NACK
1427DB6 3928Ground
1529Ground 4030NRST
1631DB7 4132Ground
1733Ground 4234NMSG
1835DB Parity 4336Ground
1937Ground 4438NSEL
2039Ground 4540Ground
2141Ground 4642NC/D
2243Ground 4744Ground
2345Ground 4846NREQ
2447Ground 4948Ground
2549- 5050NI/O

The meanings of the SCSI circuit codes are as follows:-

DB0 to DB7Data circuit
TPWR+5 volt supply for external terminator
NATNNot Attention
NBSYNot Busy
NACKNot Acknowledgement of received data
NRSTNot Reset
NMSGNot Message
NSELNot Select device
NC/DNot Control Information, otherwise Data
NREQNot Request to send
NI/ONot Input, otherwise Output (showing data direction)

The following table lists the wiring for a DB25 connector, as used on Mac OS computers:-

DB25WireFunction DB25WireFunction
74Ground 2513TPWR
817DB0 179NATN
95Ground 616NBSY
2111DB1 53NACK
1420Ground 415NRST
2224DB2 214NMSG
1621Ground 1910NSEL
1018DB3 158NC/D
1822Ground 11NREQ
2312DB4 32NI/O
2023DB Parity    

whilst the following shows the relationship between ribbon cable wire numbers and DB25 pins:-

PinWire PinWire
11 1420
214 158
32 1621
415 179
53 1822
616 1910
74 2023
817 2111
95 2224
1018 2312
116 2425
1219 2513

©Ray White 2004.