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Interconnection Protocols
Berk ÜSTÜNDAĞ
Istanbul Technical University
Computer Engineering Department
[email protected]
http://berk.tc
Contents
1.Introduction
1.1 Goals of the lecture
1.2 OSI Reference Model
2. Wired communication techniques
2.1 Asynchronous Serial Communication
2.1.1 RS232
2.1.2 RS485 / RS422
2.2 Synchronous Serial Communication
2.2.1 I2C
2.2.2 Microwire
2.2.3 SPI
2.2.4 USB
2.2.5 IEEE1394 (Firewire)
3. NonWired communication techniques
3.1 Optical Communication
3.1.2 IRDA
3.1.2 Optical Fibers
3.1.3 Laser
3.2 Radio Frequency Communication
3.2.1 Frequency allocation (ETSI)
3.2.2 Radiomodems
3.2.3 Bluetooth
3.2.4 IEEE802.11
3.2.5 GPRS, 3G, UMTS
4. Mobile Data Transfer
4.1 Smart Cards
4.2 Contactless Smart Cards
4.3 Touch Memory
4.4 Magnetic Strips (Cards)
4.5 PCMCIA cards
5. Application Examples
5.1 GPS (NMEA protocol), vehicle tracking
5.2 Can Bus – automative applications
5.3 Virtual Money
5.4 Mobile officers, PDA
5.5 PC AT keyboard interface
5.6 Pay TV
5.7 Energy meter (PLC-Power line data collection)
1. Introduction
1.1 Goals of the lecture
Selection of digital communication environment
Designing the wired/nonwired interface
Interconnetion software
Application development
1.2 OSI Reference Model
As a first step in standardization, the International Standards Organization (ISO) developed a seven-layer
model known as the ISO Open Systems Interconnection (OSI) reference model.
1-Physical layer:
The lowest layer, the physical layer, is concerned with transmitting raw bits
over a communication channel. It is concerned with insuring that when one
side sends a ``1'' bit, the other side receives a ``1'' bit.
The physical layer is usually the focus of an electrical engineer and deals
with such questions as

How many volts represents a ``1'', how many a ``0''?

How to generate a ``1'' and a ``0''?

How long does a bit time last?

How many pins does the connector have

How many wires does the transmission media have?

Are pulses electrical or optical?
2-Data link layer:
The data link layer converts the raw transmission of bits into an error-free
data communciation channel. It deals with communication between two
machines sharing a common physical channel. It

Divides the bit stream of physical layer into frames, messages
that contain data and control information.

Handles lost, damaged, and duplicated frames (Why would it
be possible to duplate frame? - error control, timed out).

Handles slowing down a fast transmitter. The process is
known as flow-control. (Why flow-control? - much like water flow
control)
•Switches and bridges use MAC addressing to make networking decisions
and therefore these types of equipment function on the data link layer.
•IEEE 802 Standards
•The 802 Project defines 12-plus subcommittee standards groups. Some
are as follows:
•802 •Internetworking/LAN
.1
Protocols
•Defines routing, bridging, and internetwork
communications
•802 •Logical Link Control
.2
(LLC)
•Allows Network layer protocols to link to Physical
layer and MAC sublayer protocols
•802
•Ethernet
.3
•The Ethernet standard; defines CSMA/CD
•802
•Token Ring
.5
•Defines logical ring topology, media, and interfaces
•802
•High-speed networks
.12
•Defines 100 Mbps technologies
3-Network layer:
The network layer controls operation of the subnet
(communicaiton between hosts). It

Directs (routes) packets from source to destination
host (but may not guarantee that all packets are
delivered).

Worries about congestion - hosts sending data into
the network faster than the network can handle.
Deals with addressing: how do we specify which machine data
should be delivered to?
4-Transport layer:
The transport layer makes sure data gets delivered to a specific
process on a specific machine. It:

Is an end-to-end protocol because it deals with the
ultimate endpoints of communications, the sending and
receiving applications.

Deals with retransmitting data if the network layer
fails to deliver it.

Deals with suppressing duplicates. If it retransmits
messages, it may introduce a duplicate, if the
retransmission was unnecessary.

Also deals with addressing. Which process on a
particular machine?
5- Session layer:
A session can be considered as a one-run of a particular
applicaiton. The session layer

Provides a cleaner interface to the transport layer.
For example, one that is not operating system specific
(e.g. sockets).

Provides synchronization such as recovering from
transport layer failure. For example, a file transfer may
take two minutes, during which the network failed (power
outage, for example). As long as the host is still running
properly, the session layer should recover from this
network failure without much intervention of the users.
Similar case in a PC editing program such as Word, if a
bad disk is encountered, one shouldn't have to start from
scratch.
6-Presentation layer:
The presentation layer performs sevices that are requested often
enough to warrant development of a general solution. For
example,

Encoding data in a standard format, so that ASCII
systems can communicate with EBCDIC systems.

Compressing data to reduce communication costs.

Encrypting data for privacy.
7- Applicaiton layer:
The application layer refers to the user programs themselves.
2.1 Asynchronous Serial
Communication
Electronic data communications between elements
will generally fall into two broad categories: singleended and differential. RS232 (single-ended) was
introduced in 1962, and despite rumors for its early
demise, has remained widely used through the
industry.
2.1.1 RS232 Data Interface
•Independent channels are established for two-way (full-duplex)
communications
• The RS232 signals are represented by voltage levels with
respect to a system common (power / logic ground). The "idle"
state (MARK) has the signal level negative with respect to
common, and the "active" state (SPACE) has the signal level
positive with respect to common.
• RS232 has numerous handshaking lines (primarily used with
modems), and also specifies a communications protocol.
•The RS-232 interface presupposes a common ground between
the DTE and DCE. This is a reasonable assumption when a short
cable connects the DTE to the DCE, but with longer lines and
connections between devices that may be on different electrical
busses with different grounds, this may not be true.
RS232 data is bi-polar....
+3 TO +12 volts indicates an "ON or 0-state (SPACE)
condition" while A -3 to -12 volts indicates an "OFF" 1-state
(MARK) condition
Modern computer equipment ignores the negative level and
accepts a zero voltage level as the "OFF" state. In fact, the
"ON" state may be achieved with lesser positive potential. This
means circuits powered by 5 VDC are capable of driving RS232
circuits directly, however, the overall range that the RS232
signal may be transmitted/received may be dramatically
reduced.
The types of driver ICs used in serial ports can be
divided into three general categories:
•Drivers which require plus (+) and minus (-) voltage
power supplies such as the 1488 series of interface
integrated circuits. (Most desktop and tower PCs use
this type of driver.)
•Low power drivers which require one +5 volt power
supply. This type of driver has an internal charge
pump for voltage conversion. (Many industrial
microprocessor controls use this type of driver.)
•Low voltage (3.3 v) and low power drivers which meet
the EIA-562 Standard. (Used on notebooks and
laptops.)
Glossary of Abbreviations etc.
CTS
DCD
DCE
DSR
DSRS
DTE
DTR
FG
NC
RCk
RI
RTS
RxD
SG
SCTS
SDCD
SRTS
SRxD
STxD
TxD
Clear To Send [DCE --> DTE]
Data Carrier Detected (Tone from a modem) [DCE --> DTE]
Data Communications Equipment eg. modem
Data Set Ready [DCE --> DTE]
Data Signal Rate Selector [DCE --> DTE] (Not commonly
used)
Data Terminal Equipment eg. computer, printer
Data Terminal Ready [DTE --> DCE]
Frame Ground (screen or chassis)
No Connection
Receiver (external) Clock input
Ring Indicator (ringing tone detected)
Ready To Send [DTE --> DCE]
Received Data [DCE --> DTE]
Signal Ground
Secondary Clear To Send [DCE --> DTE]
Secondary Data Carrier Detected (Tone from a modem)
[DCE --> DTE]
Secondary Ready To Send [DTE --> DCE]
Secondary Received Data [DCE --> DTE]
Secondary Transmitted Data [DTE --> DTE]
Transmitted Data [DTE --> DTE]
Is Your Interface a DTE or a DCE?
Find out by following these steps: The point of reference for all
signals is the terminal (or PC).
1 ) Measure the DC voltages between (DB25) pins 2 & 7 and
between pins 3 & 7. Be sure the black lead is connected to pin 7
(Signal Ground) and the red lead to whichever pin you are
measuring.
2) If the voltage on pin 2 (TD) is more negative than -3 Volts, then it
is a DTE, otherwise it should be near zero volts.
3) If the voltage on pin 3 (RD) is more negative than -3 Volts, then it
is a DCE.
4) If both pins 2 & 3 have a voltage of at least 3 volts, then either
you are measuring incorrectly, or your device is not a standard
EIA-232 device. Call technical support.
5) In general, a DTE provides a voltage on TD, RTS, & DTR,
whereas a DCE provides voltage on RD, CTS, DSR, & CD.
PC Com Port - EIA-574
RS-232 pin out DB-9 pin used for
Asynchronous Data
Description
Signal
9-pin DTE
25-pin DCE
Source DTE or DEC
Carrier Detect
CD
1
8
from Modem
Receive Data
RD
2
3
from Modem
Transmit Data
TD
3
2
from Terminal/Computer
Data Terminal Ready
DTR
4
20
from Terminal/Computer
Signal Ground
SG
5
7
from Modem
Data Set Ready
DSR
6
6
from Modem
Request to Send
RTS
7
4
from Terminal/Computer
Clear to Send
CTS
8
5
from Modem
Ring Indicator
RI
9
22
from Modem
RS232 (25 pin) Tail Circuit CableNull
Modem Cable for Async or Sync data
Cross Pinned cables for Async data.
Pin out for local Async Data transfer
RS232D uses RJ45 type connectors (similar to telephone connectors)
Pin
No.
Signal Description
Abbr.
1
DCE Ready, Ring Indicator
DSR/R
I
2
Received Line Signal
Detector
DCD
3
DTE Ready
DTR
4
Signal Ground
SG
5
Received Data
RxD
6
Transmitted Data
TxD
7
Clear To Send
CTS
8
Request To Send
RTS
DT
E
DC
E
RS-232 Specs.
SPECIFICATIONS
RS232
SINGLE
Mode of Operation
-ENDED
Total Number of Drivers and Receivers on One
1 DRIVER
Line
1 RECVR
Maximum Cable Length
50 FT.
Maximum Data Rate
20kb/s
Maximum Driver Output Voltage
+/-25V
Driver Output Signal Level (Loaded
+/-5V to +/Loaded
Min.)
15V
Driver Output Signal Level (Unloaded
Unloaded +/-25V
Max)
Driver Load Impedance (Ohms)
3k to 7k
Power
Max. Driver Current in High Z State
N/A
On
Power
+/-6mA @ +/Max. Driver Current in High Z State
Off
2v
Slew Rate (Max.)
30V/uS
Receiver Input Voltage Range
+/-15V
Receiver Input Sensitivity
+/-3V
Receiver Input Resistance (Ohms)
3k to 7k
RS423
SINGLE
1
-ENDED
DRIVER
10
4000
FT.
RECVR
100kb/s
+/-6V
+/-3.6V
+/-6V
>=450
N/A
+/-100uA
Adjustabl
e
+/-12V
+/-200mV
4k min.
One byte of async data
•The RS-232 signal on a single cable is impossible to screen
effectively for noise.
• By screening the entire cable we can reduce the influence of
outside noise, but internally generated noise remains a problem.
As the baud rate and line length increase, the effect of
capacitance between the different lines introduces serious
crosstalk (this especially true on synchronous data - because of
the clock lines) until a point is reached where the data itself is
unreadable.
• Signal Crosstalk can be reduced by using low capacitance cable
and shielding each pair
How to Get power out of PC RS-232 port ...
Example: PC mouse and software protection dongle
Another choice is the same cable commonly used in the Twisted
pair Ethernet cabling. This cable, commonly referred to as
Category 5 cable, is defined by the ElA/TIA/ANSI 568 specification
The extremely high volume of Category 5 cable used makes it
widely available and very inexpensive, often less than half the
price of specialty RS422/485 cabling. The cable has a maximum
capacitance of 17 pF/ft (14.5 pF typical) and characteristic
impedance of 100 ohms.
Category 5 cable is available as shielded twisted pair (STP) as well
as unshielded twisted pair (UTP) and generally exceeds the
recommendations making it an excellent choice for RS232
systems.
Interfacing Example - Analog Sampling Via the RS-232 Port
The above circuit when in a working state, will wait for a byte to be sent to it before it starts the
analog conversion and sends data back to the computer using the 8N1 serial format at 9600 BPS.
The circuit is based on a CDP6402C or equivalent UART. This, if you want to call it, is the brains
of the operation and performs the conversion of Parallel data to a Serial format for transmission.
The Analog to Digital Conversion is done by the ADC0804, while the MAX232 is used to
convert TTL/CMOS voltage levels into RS-232 Voltage Levels. The 74HC4060 is a
Oscillator/Divider which is used to generate the UART's Clock.
The Analog to Digital Converter (ADC0804) starts it's conversion when the UART's Data
Received line becomes active. Many people at this stage will say that this circuit cannot work! The Data Received (DR) output is Active High, while the nWrite (WR) input to the ADC is a
Active Low. This circuit is quite correct. If we look at the ADC's operation, on a high to low
transition of the nWrite input the internal Successive Approximation and Shift Registers are reset.
Provided the nWrite line remains in this state the ADC will remain reset. The conversion process
will start when a low to high transition is made on the nWrite input.
Therefore getting back to this circuit, the Data Received output will remain low while there is no
data to be received, thus the ADC will remain in the reset mode. When data is received by the
UART, a low to high transition will result on the Data Received line and thus on the connected
nWrite pin of the ADC.
This low to high transition will cause the ADC to spring to life and make a digital conversion of
the analog voltage on it's pins. Once the conversion is finished, it's nINTR (Interrupt) line will
become active low. This signal is then used to tell the UART to send the data residing on it's
Transmitter Buffer Register inputs (TBR8:TBR1). nINTR is also connected to the UART's Data
Received Reset so that the Data Received line will be reset. The circuit is then ready to repeat the
entire process upon receiving the next byte.
ESD Considerations for RS-232 Drivers
For applications that suffer from the hazard of overvoltage due to lightning,
ESD potential, or accidental transient voltage streeses, a bi-directional zener
diode, such as a TranZorbTM, dissipates the external energy before it gets to
the silicon chip. Additional series resistors limit the maximum current that the
internal structures can withstand. Outputs usually have a low impedence and
require less attention.
For maximum safety, the approach in Figure requires the least
board space while protecting each individual terminal.
Circuit Diagram of Isolated RS232C Interface
1
2
3
4
5
6
7
2
2
2
2
1
1
1
C2,C1
C3,C4
D2,D1
D4,D3
K1
K2
R1
470nF
100nF
1N4148
LED RED 3mm
DB9 R/A PCB TYPE PLUG
PCB TERMINAL BLOCK 4 WAY
1K
8
9
10
11
12
13
14
15
1
1
2
2
1
1
1
1
R2
R3
R4,R7
R5,R8
R6
U1
U2
U3
1K5
100R
680R
4K7
270R
6N137
CNY17-3, 4N37
74HC14
Daisy-chain configuration
In a daisy-chain configuration, the RS-232 signal enters through one receiver, is
looped through to a transmitter, and then goes to the next unit. Cable breaks are a
major problem for this technique. A break between slave 1 and slave 2, for instance,
prevents all downstream units from transmitting or receiving data. Other multi-drop
RS232 techniques involve pre-buffering or boosting the RS-232 output drive (enabling
it to drive multiple 5k inputs in parallel) or switching out the input resistance.
How And What Do We Attach To Messages
One simplest way is to use parity check. Add an extra bit so that
the number of 1s in a message is even (or odd). Whether it is even
or odd is pre-determined, known as even parity check, or odd
parity check.
For example, if we decide to use even parity check, the message
being sent is 1001100 which is 7 bits. We would add an 1 to
the end so that the number of 1s are even. The actual
message being sent will be 10011001, last bit being a partiy
check bit.
This actually is a special case of a class of error-correcting code
based on what is called Hamming distance.
1.What is CRC?
CRC stands for Cyclic Redundancy Check. Which means
that is based on cyclic algorithm that generates
redundant information.
The CRC performs a mathematical calculation on a block of data
and returns information (number) about the contents and
organization of that data. So the resultant number uniquely
identifies that block of data. This unique number can be used to
check the validity of data or to compare two blocks. So this
approach is used in many communication and computer systems to
ensure the validity of the transmitted or stored data.
1.In general CRC codes are able to detect:
•All single- and double-bit errors.
•All odd numbers of errors.
•All burst errors less than or equal to the degree of the
polynomial used.
•Most burst errors greater than the degree of the polynomial
used.
Check sum concept
One approach in of error checking is to append the
sum value of all message bytes to the end of the
message. This sum can identify the message and
changes in its contents. On the other hand, if there
is more than one change one that adds up a value
and one subtracts one in a way that the sum
remains the same, so it can not be used to detect
errors. The same can happen if the check sum is
changed with the same value as the message.
CRC idea
The main idea of CRC is to treat the message as
binary numbers, and divide it by fixed binary
number. The remainder from this division is
considered the checksum. The recipient of the
message performs the same division and compare
the remainder with the "checksum" (transmitted
remainder).
Theory of operation:
The CRC is a simple binary division and subtraction. The
only difference is that these operations are done on
modulo arithmetic based on mod 2. For example the
addition and subtraction are replaced with XOR
operation that do the sum and subtraction without
carry.
Polynomial concept
The CRC algorithm uses the term polynomial to perform all of its
calculations. This polynomial is the same concept as the traditional
arithmetic polynomials. The divisor, dividend, quotient, and remainder
that are represented by numbers are represented as polynomials with
binary coefficients.
For example the number 23 (10111b) can be represented in the
polynomial form as:
1*x4 + 0*x3 + 1*x2 + 1*x1 + 1*x0
or
x4 + x2 + x1 + x0
Note the binary representation of the number (10111).
This representation simplifies the traditional arithmetic operations (addition, multiplication, etc…) that are all
done on normal algebraic polynomials.
If we can assume that X is 2, then the operations are simplified more and some because some terms can be
canceled. For example the term 3*x3 is represented as 24 in normal number representation and 24 = 16+8
which is x4+x3 in polynomial representation.
Generator polynomial:
In order to do the CRC calculation; a divisor must be selected which can
be any one. This divisor is called the generator polynomial. Even
though, some polynomials became standard for many applications.
Polynomial selection is behind the scope of this summary.
One of the most used terms in CRC is the width of the polynomial. This
width is represented by the order of the highest power in the polynomial.
The width of the polynomial in the previous example is 4, which has 5
bits in its binary representation.
Since CRC is used to detect errors, a suitable generator polynomial must
be selected for each application. This is because each polynomial has
different error detection capabilities.
CRC algorithms are commonly called after the generator polynomial
width, for example CRC-16 uses a generator polynomial of width 15 and
16-bit register and CRC-32 uses polynomial width of 31 and 32-bit
register.
CRC Example Number 1
M=1010001101 (k=10) and,
P=110101 (n+1=6)
Then the FCS to be calculated by the transmitter will be n=5 bits
in length. Lets assume that the transmitter has calculated the FCS
to be:
F=1110 (n=5)
Then the transmitted frame will be:
T=1010001101 1110
Following is a review of the CRC creation process:
1.Get the raw frame
2.Left shift the raw frame by n bits and the divide it by P.
3.The reminder of the last action is the FCS.
4.Append the FCS to the raw frame. The result is the frame to
transmit
And a review of the CRC check process:
1.Receive the frame.
2.Divide it by P.
3.Check the reminder. If not zero then there is an error in the frame.
The main idea behind the CRC algorithm is that the FCS is generated so that the
reminder of T/P is zero. Its clear that
(1) T= M * x^n + F
This is because by cascading F to M we have shifted T by n bits to the left and then
added F to the result. We want the transmitted frame, T, to be exactly divisible by the
pre-defined polynomial P, so we would have to find a suitable Frame Check
Sequence (F) for every raw message (M).
Suppose we divided only M*x^n by P, we would get:
(2) M*x^n / P = Q + R/P
There is a quotient and a reminder. We will use this reminder, R, as our FCS (F).
Returning to Eq. 1:
(3) T= M*x^n + R
We will now show that this selection of the FCS makes the transmitted frame (T)
exactly divisible by P:
(4) T/P = (M*x^n + R)/P = M*x^n / P +R/P = Q + R/P + R/P = Q + (R+R)/P
but any binary number added to itself in a modulo 2 field yields zero so:
(5) T/P = Q, With no reminder.