An external modem is a modem that plugs into a serial port on the computer.
plug into an expansion slot on the computer. They have the serial port that they require to function, built in.
When using external
modems are 2 things you need to consider, the number of available serial ports
in the computer, and the UART type of each serial port.
IEEE Standard 1284.
10 feet max. Max speed is 150KBs (1200 Kbs)
Parellel ports are
SPP (Standard Parallel Port), EPP, and ECP. Enhanced Parallel Port. Enhanced Capability Paralle port. Degree of sophistication between ECP and
EPP corresponds to reverse alphabetical order. That is, ECP is more sophisticted
than EPP. EPP more sophisticated than SPP. EPP is for non-printer peripherals. ECP
is for printers and scanners. EPP-ECP standards are part of IEEE Standard 1284.
ECP can use direct memory access. Both have been displaced by usb, but
mostly only in consumer models.
PCI, PCIe, AGP, ISA, IrDA Busses, Ports
PCIe (PCI express): 1x, 2x, 4x, 8x, 12, 16, 32x. Higher number, faster speed, longer
slot. PCIe-16 is now commonly used for video. See chapter 2.
slots besides PCI (peripheral component interconnect) are AGP (accelerated
graphics), ISA (Industry Standard Architecture). Both are outdated.
Speed order slow
to fast: PCI, AGP, PCIe. PCI is 127 mBps. AGP is 508.6 mBps
IrDA port is an
Infrared Data Association Port. A kind of infrared port. Max 2 meters, line of sight! Really only practical at 1 meter or less.
Slow. About the same speed as parallel port.
IRQ and IO addresses for Serial and Parallel Ports (Section 4.1)
Memorize the following
port IRQ’s and IO’s (Input Output addresses)
Com 1 and 3 are
both IRQ 4.
IO for Com 1 is 03F8 – 03FF. IO for
Com 3 is 03E8 – 03EF. 4-1=3.
Com 2 and 4 are
both IRQ 3.
IO for Com 2 is 02F8 – 02FF. IO for Com 4 is 02E8 -02EF.
Odd com port number
with even irq number, even com with odd irq.
LPT1 is IRQ 7, IO
is 0378 - 037F.
LPT2 is IRQ 5, IO is 0278 - 027F
SCSI bus (Section 4.2)
SCSI bus types are
narrow 8-bit, wide 16 bit.
In addition to width
there is also speed. Fast, ultra.
Up to 320 megabytes
per second (MBps). Newer SATA tends to be faster and cheaper than scsi,
and also just as good for RAID.
7 generations of
SCSI, plus some variations within each generation
Used with pc servers
and high-end workstations. Tagged Command Queuing (TCQ) protocol helps
with servers accessing many files for many users, at once.
QAS: TCQ was not
the Holy Grail for SCSI. With Ultra320 (SPI-4) comes the draft proposal
for Quick Arbitrate and Select (QAS), which began appearing in drives and
controllers in the second half of 2001
IDE fought back
with RAM on IDE controller card, enabling IDE to have 90% the speed of
SCSI in multiaccess situations.
X SCSI-1, regular SCSI,
X SCSI-2, fast,
or fast wide SCSI, 1994
X SCSI-3, ultra
The first official
SCSI is now known as SCSI-1. It supported 5MB/second transfers, and
used an 8-bit bus.
next generation, was Fast SCSI, and supported 10MB/second transfer.
The release of CPU's
such as the 80286, made the option for sixteen-bit available. This is
known as Wide SCSI.
Combining Fast and
Wide yields Fast Wide SCSI. (Some versions of Fast-Wide are equal to
SCSI 3). Some references optionally will denote not-wide SCSI as Narrow
SCSI. If wide is not specified, narrow is assumed. Narrow is 8 bit.
8 bit allows for 8 devices (0-7). 16 bit allows for 16 devices.
The modifier Ultra
was used after Fast-Wide. Generally the SCSI-n designations are not used
anymore after SCSI-3, however some merchants may use numbers, albeit
incorrectly. There is SCSI Ultra, and then…
SCSI-1 uses DB-25
connectors or 50 pin Centronics (like that used for a parallel port to
attach at the printer end, however the cables are different - be careful not
to smoke your printer by plugging in to a scsi port on a computer instead
of into a parallel port, or by plugging your printer into parallel port but
using a scsi cable.
SCSI-3, Wide SCSI,
uses 68 pin connector.
Ultra2 SCSI uses
HD-80 connector. SCA. This is the only one that has power in addition
50 wire ribbon used
by SCSI-1, 2, and 3, internally. One end for scsi adapter card, multiple
ends for devices, much like IDE.
uses round cable. Daisy chained. Devices need power cable as well as
data cable. Except for 80 pin connector.
2 wires, 0 is represented by 0 v on both wires. 1 is represented by
one voltage on wire 1 and another voltage on wire 2, or vice versa.
Allowed for longer cable lengths. Later renamed High Voltage Differential,
HVD. As compared to SE, single-ended signaling.
Later LVD was introduced.
Combined SE and HVD. Low voltage. Allowed longer cables. 12 meters.
Watch out, lvd and hvd cables and connectors look the same.
abbrev MSE, called multimode or Multimode Single Ended, allows both
se and lvd drives to be attached. However SE cable length limits apply.
SE has larger distance.
All Ultra SCSI specifications
use differential SCSI:
only way to tell SE and LVD cables and connectors apart is to look at
the symbols or documentation. Failing to do so could result in damage
SCSI ID numbers
and assigning priorities: ID numbers are for device addressing and they
also assign priority.
By default, ID0
used for the device (such as the hard drive) containing an op sys. Adapter
card is ID 7. Adapter card has highest priority.
ID usually set with
is always the highest priority, regardless of bus width. 32-bit
bus (ultra2?), priority is 7,6,5,4,3,2,1,0; 15,14,13,12,11,10,9,8; 23,22,21,20,19,18,17,16;
Generally hard drive
is 0, cd drive is 3, floppy is 2.
settings, you will typically want to set the slower devices (scanners,
tape drives) to the higher-priority IDs, to ensure that they are not
crowded off the bus by the faster devices like hard disks.
with resistor packs, auto termination, jumpers.
works by using a small voltage regulator.
are usually integrated into the same type of connector used to daisy
chain the devices.
type must match the bus type (1, 2, or 3), as opposed to matching the
most advanced device in the chain. This bit of info is frequently needed to answer a test question correctly.
SCSI 1 has 6 m cable
length, rest are 1.5 to 3 meters.
5 mB/sec. Fast SCSI-2, and Wide SCSI-2, 10 mB/sec. Wide-Fast,
20 mB/sec. SCSI-2, Wide-Ultra, 40 mB/sec.
5, 10, 20, 40. 1; fast or wide; wide-fast; Ultra. ??
Compare with USB
and firewire. SCSI 5, 10, 20, 40. Newer versions have 80, 160, 320 and
USB 1.5 mB/sec and 60 mB/sec for vs 1, 2. The 60 mBps speed of USB is slower than PATA speed (EIDE). Ultra
ATA is the same as ATA/133, has speed of 133 mBps.