Hardware Overview Continued
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Serial Port
Serial DB9
connectors
• The serial port is used along with a local communication
standard that transmits 1 bit at a time.
• It is typically used for (external) modems but sometimes
used for digital cameras and other devices.
• The standard was used for keyboard and mouse but they
usually have special, designated connectors (unless they’re
USB).
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Parallel to Serial
• Most of the data in the computer moves around in
parallel, so to use the serial port the data must be
converter to serial format.
• This is done by the UART chip associated with the
serial port. The chip has a large buffer (some
memory) to hold data waiting to be put in serial
form.
UART
chip
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RS 232 and DB-9
• RS 232 is the old standard for local serial
communication – data is sent bit-by-bit but
grouped into bytes with a start bit and stop bit
announcing the beginning and end of the byte.
(There are newer standards, but the old name has
stuck.)
• The standard serial connector is called the DB-9.
Only two of the pins are used for data (receiving
and transmitting, full-duplex) the rest are for
control flow.
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Pin Purposes
1.
2.
3.
4.
5.
6.
7.
8.
9.
Carrier Detect - Determines if the modem is connected to a
working phone line.
Receive Data - Computer receives information sent from the
modem.
Transmit Data - Computer sends information to the modem.
Data Terminal Ready - Computer tells the modem that it is ready
to talk.
Signal Ground - Pin is grounded.
Data Set Ready - Modem tells the computer that it is ready to talk.
Request To Send - Computer asks the modem if it can send
information.
Clear To Send - Modem tells the computer that it can send
information.
Ring Indicator - Once a call has been placed, computer
acknowledges signal (sent from modem) that a ring is detected.
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COM Port
• The term COM (communications) port is
sometimes used instead of serial port.
• COM corresponds to a set of addresses
(software instead of hardware) that is
associated with the serial port.
– In particular COM1 corresponds to IRQ 4 and
I/O address 03F8
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Requesting an interrupt
• Devices cannot get the immediate attention of the
processor. Instead the device makes an interrupt
request.
• The processor only allows itself to be interrupted
at certain times.
• All devices interrupt the processor in the same
way, the processor must then trace back to find out
which device requested the interrupt.
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IRQ
• Devices that interrupt the processor are
assigned an IRQ (Interrupt Request Line).
• Typically a device has a set IRQ or finds an
available IRQ, occasionally two devices
want the same IRQ and a conflict arises.
• IRQs belong to a category known as System
Resources. This category also includes I/O
Addresses and DMA.
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To see the IRQ for the serial/COM port
Start/Settings/Control Panel
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System
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Hardware Tab/Device Manager
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Expand Ports (COM &
LPT)/Double click COM1
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Resources Tab
Resource info
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DIN connector
• A DIN connector is one that meets the DIN
standards.
• DIN stands for Deutsches Insitut für Normung eV,
the German version of ANSI, a organization that
determines standards.
• On PCs DIN connectors are often used for
keyboard and mouse. They may also been seen on
routers and switches.
• The DIN connector for keyboard and mouse are
actually mini-DIN connectors with 6 pins. The
port was developed by IBM and is called PS/2.
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Parallel Port
Female
parallel port
connector
• The parallel port is used along with a local
communication standard that transmits 8 bits (a
byte) at a time.
• It is typically used for printers or scanners but
sometimes used for external CD burner or zip drive.
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Pin purposes
• Pins 2-9 carry the data.
• The other pins have purposes such as indicating if one is
ready for data, indicating that data is being sent,
acknowledging that data was received, etc.
• Pins 18-25 can be used to have full-duplex communication
(ability to send data in both directions simultaneously).
• The parallel-port standards are known as Enhanced Parallel
Port (EPP) for non-printing devices and Extended
Capabilities Port (ECP) for improved printing.
– (I know, it sounds like it should be the other way
around.)
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LPT
• The term LPT (Line Printer Terminal) port
is sometimes used instead of parallel port.
• Like the term COM, LPT corresponds to a
set of addresses (software instead of
hardware) this time associated with the
parallel port.
– In particular LPT1 corresponds to IRQ 7 and
I/O address 0378
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USB
The USB A
connector which
plugs into the
computer
• Universal Serial Bus (USB) is an attempt to have a
universal port – a port which has reasonable
speed, which many different devices can use at the
same time, and which does not require expansion
cards.
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A USB socket in the back of the
computer. Many computers have
USB ports in the front. Some
users are confused about the
number of USB ports they have
– assuming the front and back
ports are distinct.
The USB B connector plugs into
the device. Not that it is distinct
from the USB A connector that
plugs into the computer.
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Connecting a USB device
• Connecting devices that use the serial port,
parallel port or expansion slot require the
system to be shut down, the device
connected and then the system restarted.
• USB devices can be added while the system
is up. This is known as hot plugging or hot
swapping.
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Up to 127 devices
• The USB standard supports up to 127
devices.
• A typical PC has one of two USB ports, one
can connect more USB devices than there
are USB ports by using USB hubs or daisy
chaining.
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The USB hub shown
allows four devices to
be connected to one
USB port.
One can plug a hub into
a hub for even more
connections.
USB hubs may or may
need their power
depending on the needs
of the devices plugged
into them.
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USB Cable
• A USB connection is simpler hardware than
a standard serial connection using a DB9
connector.
– It has A two wires for power : +5 volts and
ground. So devices that don’t require much
power (e.g. a webcam) do not need an
independent power source.
– It has a twisted pair of wires for carrying the
data.
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Enumeration
• The PC must detect all of the USB devices and
assign them addresses. This is known as
enumeration.
• The PC also determines the manner in which it
will exchange data with the device.
– Interrupt: small amounts of data at intermittent times
– Bulk: larger chunks of data with error detection
– Isochronous: real-time stream of data without eror
detection
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Fire Wire
• The IEEE 1394 standard is for a fast serial cable
that allows up to 63 devices to attach to a single
port.
• Its high speed and isochronous data transfer make
it useful for real-time data streaming.
• It supports Plug-and-play and hot plugging.
• Originally developed by Apple and known as
firewire. Other companies have other names
including Lynx and i.link.
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Fire Wire Connector
The standard supports a
set of cables that have
varying numbers of
pins.
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Networking Computers
• Two standard ways to make a computer part of a
network are to use an internal modem or an Ethernet
network interface card.
• Both use Unshielded Twisted Pair wires.
– Twisting reduces interference or cross-talk
– Shielding would reduce it more but is more expensive
• The plugs used are the RJ-11 (modem) and the RJ-45
(Ethernet NIC)
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RJ-45
• Registered Jack-45, an eight-wire connector
used to connect computers to an Ethernet
LAN using UTP wire.
• RJ-45 connectors look similar to the RJ-11
connectors that are used for phones and
modems, but RJ-45s are wider.
– Additionally there are two sockets associated
with an internal modem.
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RJ-45 (LAN) and RJ-11 (phone)
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Modem with socket
for two RJ-11
plugs.
UTP Ethernet connection
with one RJ-45 plug.
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System Resources
• System Resources are the logical/software way to
access various devices. It is a level above the
hardware but intimately connected to it as it helps
the processor to locate and interface with the
hardware.
• Data is transmitted between the processor and
devices using various shared communication
channels (the buses). Thus an addressing scheme
is required to determine where data is coming
from or where it is going, which device is seeking
the attention of the processor, and so on.
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Resource Conflicts
• Addresses must be unique if there are to be
used to distinguish various devices.
• If two devices claim the same resource,
there is said to be a conflict.
• Certain well established devices use a set
IRQ.
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IRQ Table
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IRQ Table
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No conflicts
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DMA
• Normally a device interacts with memory
(reads or writes data) via the processor.
• A Direct Memory Access (DMA) channel
allows a device to interact with memory
without going through the processor. This
speeds up data transfers.
• There are usually fewer conflicts over DMA
than over IRQ.
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Floppy Resource with DMA
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I/O addresses
• Every PC device has an Input/Output (I/O)
Address or port address.
• Standard devices have standard I/O addresses.
• These I/O addresses and the memory locations
they represent are like mail boxes for devices.
Data for a device is sent to that devices I/O
Address (mailbox).
• This scheme of associating memory locations with
devices is known as memory-mapped I/O.
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Keyboard’s I/O address range
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Logical device names
• The COM and LPT ports can be located using the
various addresses associated with them. However,
they can also be referred to by using their name
COM1, COM2, LPT1 or LPT2.
• This logically higher naming scheme (known as
the logical name) allows these ports to be found
even without knowing their assigned I/O addresses
and/or IRQ.
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Memory Addresses and BIOS shadowing
• There is some (ROM) BIOS associated with each device
that holds the lowest-level instructions that the PC cannot
forget in order to operate.
• But main memory (RAM) is faster than ROM, so
commonly used, but slow parts of the BIOS are placed in
memory to improve speed. This is known as BIOS
shadowing.
• A device’s Memory Range are memory locations
associated with a device to hold its shadowed BIOS. They
are placed in what is called “upper memory.”
• (Strictly speaking not a system resource.)
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Memory Range for NIC
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Plug-and-Play
• System configuration (adding cards,
assigning resources used to be technical and
tedious). Most modern operating systems
support Plug-and-Play (PnP).
• With PnP, system resources are assigned
automatically and in most cases, conflicts
are avoided.
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References
• http://computer.howstuffworks.com
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