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Modern Instrumentation PHYS 533/CHEM 620. Lecture 9 Data Transmission & Data Acquisitions Amin Jazaeri Fall 2007. Transmission lines. Types of transmission lines parallel conductors coaxial cables transmission line wave propagation Losses

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Modern instrumentation phys 533 chem 620

Modern InstrumentationPHYS 533/CHEM 620

Lecture 9

Data Transmission & Data Acquisitions

Amin Jazaeri

Fall 2007

Transmission lines
Transmission lines

  • Types of transmission lines

  • parallel conductors

  • coaxial cables

  • transmission line wave propagation

  • Losses

  • incident and reflected wave and impedance matching

Transmission media
Transmission Media

  • Guided

    • some form of conductor that provide conduit in which signals are contained

    • the conductor directs the signal

    • examples: copper wire, optical fiber, wave guides

  • Unguided

    • wireless systems – without physical conductor

    • signals are radiated through air or vacuum

    • direction – depends on which direction the signal is emitted

    • examples: air, free space

Transmission media1
Transmission Media

  • Cable transmission media

    • guided transmission medium and can be any physical facility used to propagate EM signals between two locations

    • e.g.: metallic cables (open wire, twisted pair), optical cables (plastic, glass core)

Types of transmission lines
Types of Transmission Lines

  • Balanced Transmission line

    • 2 wire balanced line.

    • both conductors carry current. But only one conductor carry signals.

Types of transmission lines2
Types of Transmission Lines

  • Unbalanced Transmission line

    • One wire is at ground potential

    • the other wire is at signal potential

    • advantages – only one wire for each signal

    • disadvantages –reduced immunity to noises

Types of transmission lines4
Types of Transmission Lines

  • Baluns

    • Balanced transmission lines connected to unbalanced transmission lines

    • e.g.: coaxial cable to be connected to an antenna

Metallic transmission lines
Metallic Transmission Lines

  • Parallel conductors

  • Coaxial cable

Parallel conductors
Parallel Conductors

  • Consists of two or more metallic conductors (copper)

  • Separated by an insulator – air, rubber etc.

  • Most common

    • Open Wire

    • Twin lead

    • Twisted Pair (UTP & STP)

Parallel conductors1
Parallel Conductors

  • Open Wire

    • two-wire parallel conductors

    • Closely spaces by air

    • Non conductive spaces

      • support

      • constant distance between conductors (2-6 inches)

    • Pro – simple construction

    • Con – no shielding, high radiation loss, crosstalk

    • Application – standard voice grade telephone

Parallel conductors2
Parallel Conductors

  • Twin lead

    • Spacers between the two conductor are replaced with continuous dielectric – uniform spacing

    • Application – to connect TV to rooftop antennas

    • Material used for dielectric – Teflon, Polyethylene

Parallel conductors3
Parallel Conductors

  • Twisted pair

    • formed by twisting two insulated conductors around each other

    • Neighboring pairs is twisted each other to reduce EMI and RFI from external sources

    • reduce crosstalk between cable pairs

Parallel conductors4
Parallel Conductors

  • Unsheilded Twisted Pair

    • two copper wire encapsulated in PVC

    • twisted to reduce crosstalk and interference

    • improve the bandwidth significantly

    • Used for telephone systems and local area network

Parallel conductors5
Parallel Conductors

  • UTP – Cable Type

    • Category 1

      • ordinary thin cables

      • for voice grade telephone and low speed data

    • Category 2

      • Better than cat. 1

      • For token ring LAN at tx. rate of 4 Mbps

    • Category 3

      • more stringent requirement than level 1 and 2

      • more immunity than crosstalk

      • for token ring (16Mbps), 10Base T Ethernet (10Mbps)

Parallel conductors6
Parallel Conductors

  • UTP – Cable Type

    • Category 4

      • upgrade version of cat. 3

      • tighter constraints for attenuation and crosstalk

      • up to 100 Mbps

    • Category 5

      • better attenuation and crosstalk characteristics

      • used in modern LAN. Data up to 100Mbps

    • Category 5e

      • enhanced category 5

      • data speed up to 350 Mbps

Parallel conductors7
parallel conductors

  • UTP – Cable Type

    • Category 6

      • data speed up to 550 Mbps

      • fabricated with closer tolerances and use more advance connectors

Parallel conductors8
Parallel Conductors

  • Sheilded Twisted Pair (STP)

    • wires and dielectric are enclosed in a conductive metal sleeve called foil or mesh called braid

    • the sleeve connected to ground acts as shield

      – prevent the signal radiating beyond the boundaries

Parallel conductors9
Parallel Conductors

  • STP – Category

    • Category 7

      • 4 pairs

      • surrounded by common metallic foil shield and shielded foil twisted pair

      • 1Gbps

    • Foil twisted pair

      • > 1Gbps

    • shielded-foil twisted pair

      • > 1Gbps

Coaxial cable
Coaxial Cable

  • used for high data transmission

  • coaxial – reduce losses and isolate transmission path

  • basics

    • center conductor surrounded by insulation

    • shielded by foil or braid

Transmission line wave propagation
Transmission Line Wave Propagation

  • Velocity factor

    • The ratio of the actual velocity of propagation of EM wave through a given medium to the velocity of propagation through vacuum

    • Vf = velocity factor

    • Vp = actual velocity of propagation

    • c = velocity of propagation in vacuum

Transmission line wave propagation1
Transmission Line Wave Propagation

  • rearranged equation

  • the velocity of wave in transmission line depends on the dielectric constant of insulating material

    • ϵr = dielectric constant

  • The velocity along transmission line varies with inductance and capacitance of the cable

Transmission line wave propagation2
Transmission Line Wave Propagation

  • as

    • velocity x time = distance

    • therefore

    • normalized distance to 1 meter

      • Vp = velocity of propagation

      • √LC = seconds

      • L = inductance

      • C = capacitance


  • Conductor Losses

    • conductor heating loss - I2R power loss

    • the loss varies depends on the length of the transmission line.

  • Dielectric Heating Losses

    • difference of potential between two conductors of a metallic transmission lines

    • Negligible for air dielectric

    • increase with frequency for solid core transmission line


  • Radiation Losses

    • the energy of electrostatic and EM field radiated from the wire and transfer to the nearby conductive material

    • Reduced by shielding the cable


  • Coupling Losses

    • whenever connection is made between two tx line

    • discontinuities due to mechanical connection where dissimilar material meets

    • tend to heat up, radiate energy and dissipate power

  • Corona

    • luminous discharge that occurs between two conductors of transmission line

    • when the difference of potential between lines exceeds the breakdown voltage of dielectric insulator

Incident and reflected wave
Incident and Reflected Wave

  • Incident voltage

    • voltage that propagates from sources toward the load

  • Reflected wave

    • Voltage that propagates from the load toward the sources

Incident and reflected wave1
Incident and Reflected Wave

  • Resonant and non resonant transmission line

    • Flat @ non-resonant line

      • Transmission line with no reflected power

      • Infinite length transmission line

      • terminated with a resistive load equal in ohmic value to the characteristic impedance of transmission line

    • Resonant transmission line

      • When the load is not equal to the characteristic impedance of the transmission line, some incident power reflects back towards the source

      • energy present on the line would reflect back and forth (oscillate) between the source and load

Incident and reflected wave2
Incident and Reflected Wave

  • reflection coefficient

    • vector quantity that represents the ratio of reflected voltage to incident voltage (or current)

    • Γ = reflected coefficient

    • Ei/Ii= incident voltage/current

    • Er/Ir= reflected voltage/current

Data acquisition systems
Data Acquisition Systems

Fundamental Characteristics

  • Input properties

  • Gain and filtering capabilities

  • Sampling rate

  • Number of channels

  • Resolution of the digital converter

Data acquisition systems1
Data Acquisition Systems

Input options


Single-ended inputs Typically one channel is used for a single input line. The reference is a common ground to all the channels of the acquisition system

Differential Inputs Two separate line are used, generally 2 channels. The

Differential voltage is measured

Data acquisition systems2
Data Acquisition Systems

Input buffering

Buffer amplifier The buffer serves to present a specific impedance, generally

In the range of 1 M. The buffer also serves to AC couple or DC couple the input.

The buffer isolates the input from the data acquisition portion of the instruments

Data acquisition systems3
Data Acquisition Systems

Gain and filtering

Gain and filtering Gain is adjusted to optimize the range of the analog signal to the capability of the A/D converter.

The filtering serves to remove frequencies above the Nyquist frequency This is the first stage of anti-aliasing

Data acquisition systems4
Data Acquisition Systems

Sample and Hold

Sample and Hold Sample and hold circuits sample the input voltage and holds for digital conversion. This technique is use when simultaneous data is required for multiple channels

Sequential data acquisition generally bypasses the S/H circuitry (cost) to acquire data sequentially from channel to channel

Data acquisition systems5
Data Acquisition Systems

A/D converter

A/D converter The converter specifications include the resolution, bits, and the data rate. The rate of acquisition set the frequency range

Data acquisition systems6
Data Acquisition Systems

Assume that we collect a block of data N samples in length

This implies that T= N*t

Also, the fundamental frequency, f0=

The sample rate can be defined as S =

Since two samples are require to define a harmonic function Shannon Sampling Theorem

The maximum number of frequencies that can be calculated N/2

Fmax=f0 (N/2)= Nyquist frequency

Also Fmax=S/2

Data acquisition systems7
Data Acquisition Systems


Multiplexer The multiplexer serve to stream the digital data to the storage medium. The multiplexing operation is commonly the limiting component of speed in a data acquisition system

Data acquisition systems8
Data Acquisition Systems

Multirate systems

  • Data acquired at the highest sample rate.

  • Filtering(analog) performed at this point for the fixed sample rate

  • Data is decimated and digitally filtered for all other (lower) sample rates

Common implementations of interfaces
Common Implementations of Interfaces

  • Parallel port (8 bits per shot)

  • Serial (RS-232, RS-485)

    • usually asynchronous

  • GPIB (IEEE-488) parallel

    • General Purpose Interface (or Instrument) Bus

    • originally HPIB; Hewlett Packard

  • DAQ card (data acquisition)

    • like national instruments A/D, D/A, digital I/O


    • Computer Automated Measurement And Control

  • VME bus / VXI bus

    • modern CAMAC-like bus

Exchanging data
Exchanging Data

bit 0

Device A

Device B

  • Parallel: Fast and expensive

    • devices A, B simple, but cabling harder

    • strobe alerts to “data valid” state

bit 1

bit 2

bit 3

bit 4

bit 5

bit 6

bit 7


  • Serial: Slow and cheap

    • but devices A and must convert between serial/parallel

Device A

Device B


slide courtesy E. Michelsen

The parallel port
The Parallel Port

  • Primarily a printer port on the PC

    • goes by name LPTx: line printer

    • usually LPT1

  • 8 data bits

    • with strobe to signal valid data

    • can be fast (1 Mbit/sec)

  • Other control and status bits for (printer) communication

data valid

data held static for some interval


Parallel port access
Parallel Port Access

serial port

  • Most PCs have a DB-25 female connector for the parallel port

  • Usually at memory address 0x378

  • Windows 98 and before were easy to talk to

    • but after this, a hardware-abstraction layer (HAL) which makes access more difficult

    • one option is to fool computer into thinking you’re talking to a normal LPT (printer) device

    • involves tying pins 11 and 12 to ground

  • Straightforward on Linux

    • direct access to all pins

parallel port

Serial communications
Serial Communications

  • Most PCs have a DB9 male plug for RS-232 serial asynchronous communications

    • we’ll get to these definitions later

    • often COM1 on a PC

  • In most cases, it is sufficient to use a 2- or 3-wire connection

    • ground (pin 5) and either or both receive and transmit (pins 2 and 3)

  • Other controls available, but seldom used

  • Data transmitted one bit at a time, with protocols establishing how one represents data

  • Slow-ish (most common is 9600 bits/sec)

Time is of the essence
Time Is of the Essence

  • With separate clock and data, the transmitter gives the receiver timing on one signal, and data on another

  • Requires two signals (clock and data): can be expensive

  • Data values are arbitrary (no restrictions)

  • Used by local interfaces: V.35, (synchronous) EIA-232, HSSI, etc.

  • As distance and/or speed increase, clock/data skew destroys timing

sample on rising edge of clock


sample times centered in data bits



slide courtesy E. Michelsen

No clock do you know where your data is
No Clock: Do You Know Where Your Data Is?

  • Most long-distance, high speed, or cheap signaling is self timed: it has no separate clock; the receiver recovers timing from the signal itself

  • Receiver knows the nominal data rate, but requires transitions in the signal to locate the bits, and interpolate to the sample points

  • Two General Methods:

    • Asynchronous: data sent in short blocks called frames

    • Synchronous: continuous stream of bits

      • Receiver tracks the timing continuously, to stay in synch

      • Tracking requires sufficient transition density throughout the data stream

      • Used in all DSLs, DS1 (T1), DS3, SONET, all Ethernets, etc.

transitions locate data



interpolated sample times (bit centers)

slide courtesy E. Michelsen

Asynchronous up close and personal
Asynchronous: Up Close and Personal

  • Asynchronous

    • technical term meaning “whenever I feel like it”

  • Start bit is always 0. Stop bit is always 1.

  • The line “idles” between bytes in the “1” state.

  • This guarantees a 1 to 0 transition at the start of every byte

  • After the leading edge of the start bit, if you know the data rate, you can find all the bits in the byte

transition locates data

one byte






bit 0

bit 1

bit 2

bit 3

bit 4

bit 5

bit 6

bit 7



interpolated sample times (bit centers)

slide courtesy E. Michelsen

Can we talk
Can We Talk?

ASCII “A” = 0x41

9600, 8N1




bit 0

bit 1

bit 2

bit 3

bit 4

bit 5

bit 6

bit 7


  • If we agree on 4 asynchronous communication parameters:

    • Data rate: Speed at which bits are sent, in bits per seconds (bps)

    • Number of data bits: data bits in each byte; usually 8

      • old stuff often used 7

    • Parity: An error detecting method: None, Even, Odd, Mark, Space

    • Stop bits: number of stop bits on each byte; usually 1.

      • Rarely 2 or (more rarely) 1.5: just a minimum wait time: can be indefinite

1 bit @ 9600 bps = 1/9600th sec

9600, 7E2




bit 0

bit 1

bit 2

bit 3

bit 4

bit 5

bit 6


stop 1

stop 2

slide courtesy E. Michelsen

Rs 232 most common implementation
RS-232: most common implementation

  • RS-232 is an electrical (physical) specification for communication

    • idle, or “mark” state is logic 1;

      • 5 to 15 V (usually about 12 V) on transmit

      • 3 to 25 V on receive

    • “space” state is logic 0;

      • +5 to +15 V (usually ~12 V) on transmit

      • +3 to +25 V on receive

    • the dead zone is from 3 V to +3 V (indeterminate state)

  • Usually used in asynchronous mode

    • so idles at 12; start jumps to +12; stop bit at 12

    • since each packet is framed by start/stop bits, you are guaranteed a transition at start

    • parity (if used) works as follows:

      • even parity guarantees an even number of ones in the train

      • odd parity guarantees an odd number of ones in the train

Gpib ieee 488

  • An 8-bit parallel bus allowing up to 15 devices connected to the same computer port

    • addressing of each machine (either via menu or dip-switches) determines who’s who

    • can daisy-chain connectors, each cable 2 m or less in length

  • Extensive handshaking controls the bus

    • computer controls who can talk and who can listen

  • Many test-and-measurement devices equipped with GPIB

    • common means of controlling an experiment: positioning detectors, measuring or setting voltages/currents, etc.

  • Can be reasonably fast (1 Mbit/sec)

Data acquisition
Data Acquisition

  • A PCI-card for data acquisition is a very handy thing

  • The one pictured at right (National Instruments PCI-6031E) has:

    • 64 analog inputs, 16 bit

    • 2 DACs, 16 bit analog outputs

    • 8 digital input/output

    • 100,000 samples per second

    • on-board timers, counters

  • Breakout box/board recommended


  • This somewhat old interface provides a “crate” into which one slides modules that perform specific tasks

    • A/D conversion

    • time-to-digital converters

    • pulse generators

    • charge measurement

    • amplifiers

    • delay generators

  • Frequently used in timing experiments, like nuclear physics: catch events in detector, generate signal, measure strength, etc.

  • Often the modules are highly multiplexed (16 channels per card common)

CAMAC crate (above) and inhabitants

(right) including two custom modules,

two commercial time-to-digital converters

(TDCs) and the crate controller (note

interface cable (50-pin SCSI-2 style)

Camac features
CAMAC features

  • 16-bit (newer are 24-bit) data words

  • Full command cycle in 2 s  8 Mbit/sec

  • Look-At-Me (LAM) interrupts computer when some event happens

  • Commands follow N.A.F. sequence: slot number, address, function

    • so address specific modules by name/position

    • A and F values perform tasks that are defined by module

    • A often refers to channel number on multiplexed device

    • F might indicate a read, a write, a reset, or other action

Ieee 488 gpib hpib interfaces
IEEE 488 (GPIB, HPIB) interfaces

  • THE MAIN control and communication channel for research-grade instruments in physical/chemical/etc sciences

  • 16 bit parallel bus: 8 data or command lines, 8 control lines

    • control lines (8) = “data valid”, “not ready for data”, etc.

  • a device may be a “talker” and/or “listener” and/or “controller”

  • devices are uniquely addressed (0 ... 30  5 bits); 31 addresses all

  • data sent one byte at a time, managed by the control lines

  • high level generic instrument (voltmeter, oscilloscope, etc) control languages (e.g., SCPI = “skippy”) come and go; problem is that generic language cannot exploit unique features of any instrument

E merging dac c ommunications
Emerging DACcommunications

  • Enhanced Parallel Port (EPP): bidirectionally, as needed for DAC

  • Universal Serial Bus (USB): hot insertion of up to 127 devices attractive for research environment; can power net devices; in practice, relatively slow (100 Kwords/s?)

  • FireWire (IEEE 1394): fast enough for live video, can support 100s of devices

  • 1451.2-1997 PDF Standard for a Smart Transducer Interface for Sensors and Actuators - Transducer to Microprocessor Communication Protocols and Transducer Electronic Data Sheet (TEDS) Formats 1998 ($101)

Taste of the future ieee 1451 2
taste of the future?: IEEE 1451.2


    Abstract: The recently approved standard, IEEE1451.2 [IEEE], defines an interface between transducers and microprocessors useful in industrial automation and other fields. The standard defines a physical interface consisting of a data transport serial link, in addition to triggering, interrupt and hot swap signaling. The standard also defines a transducer electronic data sheet, TEDS, that describes the functionality of the transducer in machine-readable form. The interface supports as many as 255 independent transducer channels. These may be accessed individually or as a unit. This report describes the use of the correction and calibration features of this standard to implement a variety of measurement functions.