A parallel port uses a separate wire for each bit of data — hence an 8-bit link requires at least eight wires. This makes it very fast but also unsuitable for long distances.
Small Systems Computer Interface
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The basic form of SCSI is defined in the American National Standards Institute (ANSI) standard X3T9.2. This 8-bit parallel port can be used to connect computers to disk drives or other high-speed peripherals. It can use single-ended (unbalanced) or differential (balanced) circuits with a maximum cable length of 6 and 25 metres respectively.
Connections and Cables
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Most SCSI devices use 50 way Amphenol connectors (C50), with plugs on cables and sockets on equipment. Some Macs and other devices use a 25 way D connector (DB25) or 30 way miniature connector (HDI-30) instead. This may necessitate an adaptor cable — two if an odd device is in the middle of the SCSI chain!
The following alternative connectors are in use:-
Equipment Connector Type
PowerBook 30 way miniature socket (HDI-30)
Compact Mac 25 way D socket (DB25) •
SCSI device 25 way D socket or flying 25 way D plug ◊ •
SCSI-2 device 50 way micro D socket (HC50)
Wide SCSI-2 device 68 way micro D socket (HC68) ∆
◊ Suitable for direct connection to the DB25 on a compact Mac
• DO NOT plug any other type of serial interface into this connector
∆ For maximum speed you must use a complete chain of Wide SCSI-2 devices
? Some PowerMacs have dual ports.
SCSI Speed
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SCSI accommodates any speed up to 4 megabytes per second (MB/s), although in practice many Macs can only run at 1 MB/s or less. In some cases the disk drives are faster than the SCSI port! Here are some examples of speed ratings for various Macs:-
Model Speed (MB/s)
Mac Classic: 1
Mac IIci: 1.5
Quadra: 3 to 5
PowerMac (External): 5
PowerMac (Internal): 10
SCSI ID Numbers
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SCSI acts as a data bus, accommodating up to 8 devices. Only one of these devices can be a controller, usually the Mac’s processor. Each device has to be assigned a unique SCSI ID (identification) number.
The Mac uses the following numbers:-
Item SCSI ID
Processor 7
Internal Drive 0
Other Devices 1 to 6
? A second internal drive is usually assigned an ID = 1.
? The highest ID gives the highest priority — an important drive should have an ID = 6.
? A device with an ID = 0 is assumed to be the internal drive and is given the highest
priority at startup — followed by ID = 6 and so on.
? Some applications give each device a software ID that’s kept in a Preferences or
Settings file. A drive won’t auto-mount if this ID isn’t the same as on the device itself.
? Older drives, originally used as internal drives, may only work with an ID = 0.
? PowerBooks that include an internal slot for a CD-ROM or alternative drive can
only use six external ID numbers.
SCSI Termination
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Most SCSI systems need to have a terminator fitted at some point (or points) in the chain. A pass-through terminator has a plug and socket, allowing it to be fitted in the middle of the chain. A block terminator has just a plug and can only be fitted at the end of the chain. Some devices have an internal terminator built in (see below).
The first device (or the point where the system cable from the Mac is extended) and last device in a SCSI chain must be terminated, either internally or externally.
? There must only be two terminators in a chain — two's company, three's a crowd!
? Some devices (and plugs) include active termination to automatically adjust
termination as necessary — but they don’t always work!
? The Mac IIfx has a fast SCSI port that uses a special black terminator that can’t be used
on other models. Standard internal terminators shouldn’t be used with a IIfx.
? Sometimes you can break the rules to make SCSI work! For example, try a terminator
in the middle of a chain, with or without a terminator on the last device. Changing
the sequence of devices in a chain can sometimes solve problems.
√Ö Incorrect termination can prevent the internal drive from mounting
— except on models that include extra buffers for the external SCSI port(s).
Internal Terminators
Some SCSI equipment may incorporate an internal terminator. If such a device has a single socket it’s best placed at the end of the SCSI chain.
In some cases an external switch lets you disable the termination. If not, you may be able to remove the terminators from inside the device — they usually consist of three 8-pin packages in a line. Trouble is, you have to look inside to see if they’re really there! Once removed, the device must go in the middle of the chain or be externally terminated.
Å If you’re in doubt about what you’re doing … don’t do it!
√Ö If you really want to replace internal terminators note which way round they go!
? Don’t put back removed internal terminators — it’s easier to use external ones!
SCSI Problem Solving
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Slow data transfer or data errors can be caused by damaged cables — although such damage is rarely obvious. The metallic shield under the cable’s outer layer, known as the screen, is particularly important.
Other causes include:-
z A need to restart the Mac
z Problem with PRAM
z Electrical noise from monitors, power supplies or fluorescent lights
z Inadequate power supply inside Mac or box containing the drive
z Insufficient Mac memory
? If your Mac shows a flashing ? or a Sad Mac try restarting with the SCSI cable
unplugged from the Mac. There could be any number of causes!
? Make sure there aren’t any SCSI system extensions that conflict with each other
— use only one extension, if possible.
Powering-up Devices
It’s best to power external drives before starting a Mac. In practice, switching everything on together rarely causes problems — but a CD-ROM drive should always be switched on first.
If you power-up in the wrong order some drives won’t auto-mount, or you may get a slow display or a freeze. You can use SCSIProbe or similar to mount any awkward drives.
? An unpowered device can prevent startup — it depends on the location in the chain,
whether it’s terminated and whether the Mac has buffers for its external SCSI port.
Cable Lengths
The SCSI cable length limitations can be very inconvenient — in working out the total length of a SCSI chain you must also include the length of cable inside each box! For standard single-ended SCSI the maximum distance is 6 metres.
For longer distances you can use a SCSI regenerator — this also lets you disconnect a portion of a SCSI chain without disrupting the entire system. For distances of up to 25 metres, or possibly more, you can use a converter to change the signal into differential form. For runs up to 300 metres you can use a pair of fibre-optic converter boxes.
? High-speed digital audio processing systems may need cables as short as one metre.
SCSI-2
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SCSI-2 employs an improved protocol that operates at up to 5 MB/s and is fully compatible with standard SCSI (SCSI-1). SCSI-2’s data rate can be increased still further by using either or both of the following:-
Fast SCSI
This is used in recent PowerMacs and works at up to 10 MB/s — but you may have to replace standard SCSI cables by a higher quality product. Fast SCSI devices may be fitted with 50 way micro D sockets (HC50).
Wide SCSI
In Wide SCSI the bus width is expanded from 8 bits to 16 or 32 bits, providing speeds of 20 and 40 MB/s respectively. Such devices come with 68-pin sockets to accommodate the extra wires — if you use an adaptor cable to connect to a 50 way Amphenol connector you won’t get the maximum speed! Wide SCSI also uses active termination to make things easier. As with Fast SCSI, you should use a high quality cable.
Fast and Wide SCSI can be added to a Quadra by fitting a suitable accelerator card into the Processor Direct Slot.
SCSI-3
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Yet another protocol! 16 bit Wide SCSI-3 works at up to 20 MB/s whilst Ultra SCSI-3 runs at up to 40 MB/s. The latter can be fitted to a PowerMac by means of a suitable PCI card.
Pin Connections
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The following list shows the pin connections for a single-ended interface via C50 socket (or ribbon connector inside a Mac) and for a DB25 socket:-
50 Way 25 Way Function 50 Way 25 Way Function
1 7 Ground 26 25 TERMPWR (+ 5V)
2 8 DB0 27 - Ground
3 9 Ground 28 - Ground
4 21 DB1 29 - Ground
5 14 Ground 30 - Ground
6 22 DB2 31 - Ground
7 16 Ground 32 17 NATN
8 10 DB3 33 - Ground
9 18 Ground 34 - Ground
10 23 DB4 35 - Ground
11 24 Ground 36 6 NBSY
12 11 DB5 37 - Ground
13 - Ground 38 5 NACK
14 12 DB6 39 - Ground
15 - Ground 40 4 NRST
16 13 DB7 41 - Ground
17 - Ground 42 2 NMSG
18 20 DB Parity 43 - Ground
19 - Ground 44 19 NSEL
20 - Ground 45 - Ground
21 - Ground 46 15 NC/D
22 - Ground 47 - Ground
23 - Ground 48 1 NREQ
24 - Ground 49 - Ground
25 - N/C 50 3 NI/O
The purpose of the control lines are as follows:-
TERMPWR +5 volt supply (for external terminator)
NATN (Not) Attention
NBSY (Not) Busy
NACK (Not) Acknowledgement of received data
NRST (Not) Reset
NMSG (Not) Message
NSEL (Not) Select device
NC/D (Not) Control Information, otherwise Data
NREQ (Not) Request to send
NI/O (Not) Input, otherwise Output (indicating data direction)
Centronics
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This interface isn’t supported by the Mac but appears on printers used with PCs. It uses eight data wires as well as control lines. Most printers are fitted with a 36 way Amphenol socket that connects via a printer cable to a 25 way D connector (DB25) on the PC.
The Busy and Strobe control lines ensure smooth communication between PC and printer. Many computers contain a Versatile Interface Adaptor (VIA) or Programmable Interface Adaptor (PIA) chip for this interface.
The pin connections for a 36 way Amphenol connector are:
Pin Circuit Function
1 (Not) Strobe Logic 0 pulse to printer to indicate valid data
2 Data 0
3 Data 1
4 Data 2
5 Data 3
6 Data 4
7 Data 5
8 Data 6
9 Data 7
10 (Not) Acknowledge Logic 0 pulse from printer indicating readiness
11 Busy Logic 1 to indicate printer is busy
12 PE Logic 1 to indicate printer out of paper *
13 Select Logic 1 from printer to indicate on line *
14 0 volts
15 Spare
16 0 volts
17 Chassis
18 + 5 volts
19 Pin 1 Return
20 Pin 2 Return
21 Pin 3 Return
22 Pin 4 Return
23 Pin 5 Return
24 Pin 6 Return
25 Pin 7 Return
26 Pin 8 Return
27 Pin 9 Return
28 Pin 10 Return
29 Pin 11 Return
30 Pin 31 Return
31 (Not) Prime Logic 0 pulse to reset printer *
32 (Not) Fault Logic 0 to indicate printer fault *
* Pins not always used or provided
General Purpose Instrument Bus
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The GPIB a a special high-speed interface designed for instrumentation purposes as defined in the IEEE 488 standard. Although unsupported by the Mac you may be able to operate the bus with a GPIB controller connected via an RS-232 interface.
The interface uses eight multi-purpose bidirectional data lines and a number of control lines wired onto a 24 way connector as follows:-
Pin Code Function
1 DIO1 Data Line 1
2 DIO2 Data Line 2
3 DIO3 Data Line 3
4 DIO4 Data Line 4
5 EOI End Or Identify (end of transfer)
6 DAV Data Valid
7 NRFD (Not) Ready For Data
8 NDAC (Not) Data Accepted
9 IFC Interface Clear (initialises)
10 SRQ Service Request
11 ATN Attention
12 SHIELD Screen
13 DIO5 Data Line 5
14 DIO6 Data Line 6
15 DIO7 Data Line 7
16 DIO8 Data Line 8
17 REN Remote Enable
18-24 GN D Ground
Expansion Slots
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An expansion slot provides a fast link to a Mac’s processor. It can accommodate an expansion card for enhanced video, modems, sound processors or Ethernet.
Processor Direct Slot (PDS)
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This is the oldest form of expansion slot. It uses a special 96 pin DIN connector to give direct access to the data and address lines of a 680x0 processor. Different versions are used for each type of processor — you’re stuck if you’ve the wrong card for your machine! The following table summarises some of the varieties of PDS used:-
Computer PDS Name
Mac SE SE-Bus
SE/30 030 Direct Slot
Portable PDS
LC 020 Direct Slot
The 680x0-based Mac IIs and Quadras have their own varieties of PDS as well as NuBus slots (see below). Some machines accept a NuBus adaptor to convert from PDS to NuBus — but these only take small NuBus cards. If you need Ethernet you could fit an Ethernet NuBus card with such an adaptor. Otherwise your only option is connect an external Ethernet box into your SCSI port — hardly elegant, but it works!
Some Macs can accommodate an accelerated graphics card in their PDS slot.
Digital Audio Video (DAV)
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This internal connector is provided on the motherboard of some AV Macs for an MPEG video card, or similar. Some AV models have a card already fitted.
NuBus
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The standard slot for later 680x0 Macs, introduced with the Mac II. The cards talk to the Mac’s processor at 3 MB/s but correspond with each other at over 30 MB/s. With suitable hardware, up to 15 cards can be addressed using numbers from 0 to 14 — address 15 is used by the Mac itself.
Macs can’t accept the larger NuBus cards used in other machines. but an external NuBus expansion rack can be fitted to additional cards that won’t fit inside the machine.
The Quadra 700 and 900 can use cards that partially implement the NuBus 90 standard. These work at double speed compared with older NuBus cards — but only if all the cards in the machine are the newer type. If not, the entire bus reverts to the original speed.
There’s a wide range of networking cards available in NuBus format.
Peripheral Component Interconnect (PCI)
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PCI is the standard for PowerMacs — the 8500 has three slots, the 9500 has four. The card actually uses an enhanced version of PCI known as Apple RISC Bus (ARBus). Typical cards provide co-processors, multiple-processor arrays, video compression or graphics acceleration. Such cards come in 7 or 12 inch versions — not all models accept the larger size. In some instances the larger cards must be fitted into particular slots.
Some Mac clones have a riser to allow simultaneous operation of NuBus and PCI cards.
PC Cards (PCMCIA)
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This standardised expansion system, using a 68-pin connector, is used in the Newton Personal Digital Assistant (PDA), more recent PowerBooks and portable PCs.
The cards come in three types. The PowerBook 190 accepts all three types but has only one
slot. The PowerBook 5300 accommodates two Type II cards or one Type III card — the latter goes in the lower slot and stops you inserting a second card.
Varieties of card include:-
  Serial Port/Modem: a serial port or modem.
The serial port can be used with an extra
printer or external modem. Cellular modem
cards are also available.
Network: for interfaces such as Ethernet.
Memory: adds memory similar to RAM disk.
Mass Storage: a miniature hard disk.
  Multifunction: a combination card, such as a
video output plus Ethernet interface.
SCSI: interface used for external drives etc.
Video: for an external monitor screen.
Parallel Port: for Centronics printers etc.
AIMS: special memory for storing photos.
With the PowerBook 5300 any storage card can be ejected by trashing its icon — and can be replaced by another drive card whilst the Mac is running!
Special Slots
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Some computers have slots dedicated to a particular feature — they often vary between different models. PowerMacs can use a special slot for communications, video inputs or a TV tuner bay, whilst a PowerBook uses internal slots for a video output or Ethernet card.
