1. Lecture 15 Computer Data Acquisition Systems
Contents of this Lecture:
Outlines: Basics of computer data acquisition system.
Learn from examples: PBL presentation
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2. General Computer DAQ System
Transducer
Signal
Conditioning
A/D
Conversion
Computer
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3. Computer data Acquisition System
A data acquisition (DAQ) system is a collection of add-on hardware and software components that allow your computer to receive real-world information from sensors. It consists of
Sensors
I/O terminal panel(s)
DAQ board(s)
Software
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4. Example of Computer DAQ System
DAQ Board
Computer
Timer
Digital Control
Circuit
Trigger
Interrupt
Parallel/Series
Input Port
Output Port
A/D
D/A
Filter + -
S/H
Sensor
Bridge
Instrumentation
Amplifier
Input
Strobe
Display
Control
Output Strobe
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5. Inside the Case of a Computer
It contains a system board (mother board). This board hosts most of the circuits connecting :
CPU,
memory,
I/O ports.
buses,
expansion slots,
storage
Power supplier
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6. Elementary Bus Structure
Micro-
processor
Monitor
ROM
Optional
User ROM
System
& User
RAM
User
Input/Output
Address Bus
Data Bus
Keyboard
System
Input/Output
Display
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7. Analog-to-Digital Conversion
An ADC converts an analog voltage to a digital number.
The digital number represents the input voltage in discrete steps with finite resolution.
ADC resolution is determined by the number of bits that represent the digital number.
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8. Signal Transmission in DAQ
Analog
input
voltage
Sample &
Hold
A/D
Conversion
Parallel
to Serial
Transmission Link
Signal Transmitter
Signal Receiver
Sample and
hold output
Analog output
Hold
Hold
Voltage
Sampled
Sampled
Sampled
PCM signal
Time
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9. Bits
The smallest unit in digital signal is the bit, a contraction of the more descriptive phase of binary digit.
A bit is a single element in digital signal, having only two possible states: on (indicating 1) or off (indicating 0).
Off On 1
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10. Bytes
Bits are organized into larger units called bytes, the basic unit of information in a computer system.
A basic byte contains 8 bits. The total amount of information it can convey is 28 (=256) possible combinations.
Off On 1
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11. How Many Bytes?
Since each byte can represent only one single character or command, to do anything useful requires a lot of bytes.
The common sizes of bytes groups are KB, MB, GB, and TB.
Name
Shorthand
Size
Byte
bit 20 1
Kilobyte
Kilobit
1 KB
1 Kb
210
Megabyte
Megabit
1 MB
1 Mb
220
1 GB
1 Gb
Gigabyte
Gigabit
230
Terabyte
Terabit
1 TB
1 Tb
240
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12. Quantization
Quantization is to converter the input voltage range into Q=2N-1 bands to encode a continuous analog signal to discrete digital levels.
Volt
Time
Quantization Interval:
Quantization Error:
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13. 1-bit Analog-to-Digital Conversion
A comparator is the most basic ADC
This is a 1-bit Flash ADC C
Comparator
Vin
Vref
Out
0 Volt
1 Volt
Vref =. 5 V 1
VOut
Vin
Out = 0 if Vin < Vref
= 1 if Vin > Vref
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14. Analog-to-Digital Converter Theory
N-bit
ADC
Analog
Input
Digital Output
0 Volt
1 Volt
Digital Output Code
. 5 V
. 25 V
. 75 Volt
000
001
010
011
100
101
110
111
3-bit ADC Scale
. 125
. 375
. 625
. 875
Step Size =
= V
Analog Input Signal
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15. 256 distinct possible values
ADC Types
There many different types of analog-to-digital converters (ADC) available for DAQ systems. Different ADC types offer varying resolution, accuracy, and speed specifications
The most popular ADC types are the parallel (flash) converter, the successive approximation, and the voltage-to-frequency ADCs.
Data Range 0-10 volts
8-bit A/D
256 distinct possible values
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16. Example of Encoding (8-bit system)
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17. Encoding (8-bit Bus, 0-5 V Input)
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18. Some Commonly Used ADC Types
ADC Comparisons
Some Commonly Used ADC Types
ADC Type
Typical
Resolution
Speed
Parallel Converter
4-8 bit
100 kHz – 500 MHz
Successive Approximation
8-16 bit
10 kHz – 1 MHz
Voltage-to-frequency
8-12 bit
1 – 60 Hz
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19. Parallel Data Communication
Parallel data communication is usually used in high speed and short distance communication in a laboratory type environment (with relatively low electrical interference)
IEE 488/IEC 625 standard: 16-line bus: 8 lines for data (7-bit ASCII + parity check bit) 3 lines for “handshaking” 5 lines for bus activity control
Up to 15 devices can be connected onto the bus.
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20. Serial Data Transmission
Data bits of serial digital signals are transmitted one bit at a time in a chain along a single path. It is often referred as pulse code modulation (PCM).
Typically, serial is used to transmit ASCII data using 3 transmission lines: (1) ground, (2) transmit, and (3) receive. The important serial characteristics are baud rate, data bits, stop bits, and parity.
baud rate: a speed measurement for communication.
data bits: the actual data bits in a transmission.
stop bits: used to signal the end of a data communication.
parity: a simple form of error checking in serial communication.
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21. Signal Characteristics
RS-232 is the most commonly used serial interface specification.
RS-232 specifies 25 interchange circuits that govern the data flow between the data terminal equipment (DTE) and data communication equipment (DCE).
A signal is considered to be ON when the voltage is between +3 and +15V, to be OFF when it is between -3 and -15V. The range between -3 and +3V is a TRANSITION region
TTL (Transient-Transistor Logic)
5 V
0 V
Time
Voltage
Space
15 V
Positive range ON function
3 V
Transition region
0 V
Time
-3 V
Negative range OFF function
-15 V
Mark
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22. Baud Rate
When we refer to the clock speed of serial transmission, we mean the baud rate.
If the protocol calls for a 4800 baud rate, then the clock is running at 4800Hz. It means that the serial port is sampling the data line at 4800Hz.
Common baud rates for telephone lines are 14,400; 28,800; and 33,600. Baud rates greater than these are possible, but these rates reduce the distance by which devices can be separated.
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23. Digital Transmission Links
Simplex: One way communication from A to B where B is not capable of transmitting back to A.
Half Duplex: Transmission from A to B and B to A but not simultaneously. The sample rate for m multipexed signals can be estimated:
Full Duplex: Simultaneously transmission from A to B and B to A.
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24. Transmission Bandwidth
To estimate the bandwidth required for selecting a transmission link, it is necessary to find the extent of the frequency spectrum of PCM signal.
bit rate of the PCM signal: 1 baud = 1 bit/s
for a single signal: R = nfs
for m multiplexed signal: R = nmfs
Where: fs is the sampling rate (samples/sec.) n is the size of data bits
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25. Transmission Bandwidth Calculation
In a general case of a single signal, sampled at fS, and encoded into an n-bit code, the minimum PCM bandwidth is determined by
For m multiplexed signals, each sampled as fS=1/t, the time in the HOLD state is m/ t. This means that an n bits of information must be transmitted during time t /m, the corresponding PCM bandwidth in this case is
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26. Example Serial-Com System Design
16 analog signals, each with a frequency spectrum between 0 and 5 Hz, are input to a time division multipexer. The multiplexed signal passes to a serial digital (PCM) transmitter consisting of a sample/hold device, 12-bit binary ADC and a parallel to serial converter.
(1) Suggest a suitable sampling frequency for each input signal.
(2) What is the corresponding number of samples per second for the multiplexed signal?
(3) What is the maximum length of time the sample hold device can spend in the HOLD state?
(4) Find the bit rate and minimum transmission bandwidth for the PCM signal.
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27. Example Serial-Com System Design
(1) Sampling rate should satisfy: (2) Sampling rate for the multiplexed signal (3) Maximum length of time in the HOLD state (4) Bit rate and bandwidth
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28. Plug-In Data Acquisition Boards
Plug in data acquisition board is a good solution for systems that don't need portability.
Key Benefits
Lowest cost
Large selection to choose from
Established platform
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29. Computer Data Acquisition Board
A plug-in data acquisition board is inserted directly into computer’s bus and transfer data directly to computer’s memory.
It utilizes computer hardware:
cables & buses
power supply
back panel, etc.
It is designed for particular bus structure, and unaffected by computer’s internal electrical noise.
CPU
transfers
to RAM
Display
CPU
retrieves
from RAM
Display
Display
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30. Bus Structures of Plug-in Board
The are several types of local bus structures commonly used for plug-in boards : AT
NEC PC-9800
Parallel Port
PCI
PCMCIA
PXI
USB
Others
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31. DAS-8 Series Major Models
The DAS-8 family is a series of 4 kHz analog and digital boards for IBM compatible computers.
Major Models
DAS-8
base model, 8-channel, 12-bit
DAS-8/LT
modified model for laptop
DAS-8PGA
programmable inputs, clock
DAS-8/AO
include 2 analog output channels
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32. Specifications of DAS-8PGA
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33. Preliminary Settings Base Address Switch
DAS-8 family boards need eight consecutive address locations in I/O space.
To avoid conflict with other devices, DAS-8 boards base address may be set by the base address DIP switch to any 8-bit boundary anywhere in the IBM-PC-decoded I/O space.
DAS-8/PGA Board
Base Address
Recommended I/O address choice for
the DAS-8 boards are Hex &H300,
&H308, &H310 (Decimal 768, 776, & 784).
Usually, the DAS-8 boards are shipped
with &H300 address.
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34. Programming I/O Address Map
Since the A/D provides 12 bits of data, it requires two bits to handle each word of data.
Data is held as Low Byte first, the High Byte.
This sequence is a convenience for Assembly Language programmers.
Address
BA +0 +1 +2 +3 +4 +5 +6 +7
Read
A/D Lo Byte
A/D Hi Byte
Status reg.
Status & gain
Read Ctr 0
Read Ctr 1
Read Ctr 2 -
Write
8-bit A/D conv.
12-bit A/D conv.
Control register
Gain cntrl reg.
Load Counter 0
Load Counter 1
Load Counter 2
Counter control
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35. Programming Driver Installation
A Device Interface Driver is required to pass the Base Address and corresponding Interrupt Level of a DAS-8 family board to a user program.
Driver installs automatically at boot-up and can handle up to five boards in a computer system.
Example of syntax for device
definition of a single board
located at 300 (hex):
DEVICE = DAS8DI.SYS/B:&h300
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36. Preliminary Settings SE/DIFF DIP Switch
The DAS-8/PGA uses an 8-position DIP switch foe selecting between Single-Ended (SE) and Differential (DIFF) operation.
The eight switch position allow each channel to be individually set. S2 Ch
DAS-8/PGA Board SE
DIFF
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37. Mode Descriptions (incomplete)
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38. Connector Pin Assignment1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 19 37
DAS-8 family boards use a standard 37-pin D male connector for analog/digital I/O.
The connector diagram of DAS-8/PGA is shown on left.
IN 0 -
IN 1 -
IN 2 -
IN 3 -
IN 4 -
IN 5 -
IN 6 -
IN 7 -
DIG COM
OP 4
OP 3
OP 2
OP 1
OUT 2
OUT 1
CLK 1
OUT 0
CLK 0
+12 V
IN 0 +
IN 1 +
IN 2 +
IN 3 +
IN 4 +
IN 5 +
IN 6 +
IN 7 +
+5 V
DIG GND
IP 3
IP 2
IP 1
INT IN
GATE 2
GATE 1
GATE 0
-12 V
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