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Measurement Cards

Towards the computer-driven test lab

J. Häuser

Computer systems in test environments are no longer a rarity. Many

applications fall back on the use of a PC as test equipment and end up

producing a system that would not be feasible with conventional test

equipment. With a suitable package consisting of sensors, test hardware, PC

and standard test software it is possible to build a highly ﬂexible system.

Finding suitable PC test hardware is

relatively easy, but tracking down a

user friendly program to capture test

data proved to be not so simple. The

previous article gave advice for

choosing test hardware. A future

article in Elektor Electronics will give

advice on choosing suitable software

products for the test environment. A

follow up article dealing with typical

sensors to take physical measure-

ments is also planned.

read the signal value at fixed time

intervals, or so called discrete time

intervals. The process of making a

discrete time signal from a continu-

ous time signal is called sampling.

Sampling occurs at equi-distant

points on the analogue waveform i.e.

at regular time intervals. The time

between any two samples is called

the sampling period. This is also

known by its inverse value, the sam-

ple frequency and is of critical impor-

tance when choosing test hardware.

To make a properly informed deci-

sion when choosing test hardware

for a particular test job it is impor-

tant to list all the important hard-

ware requirements.

This list should include the fol-

lowing criteria:

Figure 1. Analogue and digital scales.

Signal Conversion

To use any PC as a test instrument it must

ﬁrst be capable of inputting measured values

during the test procedure, working with

these values and then outputting the results

in a form that can be used by other laboratory

equipment, or if necessary to alter the test

procedure itself. To convert a PC into a com-

plete test laboratory three additional compo-

nents are necessary:

The world around us is characterised

by analogue behaviour. An analogue

signal is one that can have an infi-

nite number of positions between

two measurement points. A moving-

coil meter for example, is an instru-

ment whose pointer is deflected in

direct proportion to the current ﬂow-

ing in its coil ( Figure 1 ).

Using a digital computer, micro-

processor or DSP to work directly

with an analogue signal would in

theory require infinite computing

resources and is therefore not possi-

ble. Instead, it is only possible to

Interface choice for the test hard-

ware.

The type of converter.

The need for sample & hold

Signal resolution

speed (sampling rate)

A sensor that can take the measurement and

output it as a proportional voltage.

Suitable test hardware.

The measurement software.

After this process, your choices are

already becoming limited but there

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are other factors that also need to

be brought into the equation. For

example the number of analogue

channels needed, the output volt-

age range of the sensors, the num-

ber of any D/A converters required,

the necessary digital interface, the

type of stop and start signals for the

A/D converter, the need for galvanic

isolation (usually achieved by opto-

coupling), all need to be added to

the list of requirements. Last but

not least are the questions of bud-

get, anticipated equipment delivery

time and the reliability of the prod-

uct supplier.

Expansion Socket

Architecture

There are many manufacturers of PC

test cards that use standard expan-

sion cards architectures. So you can

take your pick between PCI, Com-

pact PCI, PCMCIA, ISA, USB, Paral-

lel-Port and COM-Port. But for which

application should you use which

architecture? The following explana-

tion should help to make things

clearer.

Figure 2. PCI Test card.

For use in stationary test equipment

using the standard operating

systems.

Wide range of industrial standard

connectors between the PC and

test hardware.

Price range £180 - £2500 approx.

One example of the fast analogue data cap-

ture possibilities of this interface is the data

acquisition card PCI-6110E from National

Instruments shown in Figure 2 along with its

block diagram in Figure 3 . This device

achieves a data capture rate of up to 5 MHz.

The main feature of this card is its ability to

PCI

The key factor governing the power

of any PC test environment is the

type of bus used. Despite the relent-

less increase in processor speed

and memory capacity the main

speed bottleneck occurs at the com-

munication channel with the expan-

sion card.

For many years the ISA bus was

one of the most common methods of

interfacing to the PC but today it

does not have the capacity to make

use of the power of current proces-

sors. The PCI (Peripheral Component

Interconnect) bus was developed to

address this problem. It offers a max-

imum data rate of 132 MB/s.

The specification for this bus

ensures that the interface can keep

pace with increased processor

speeds and upgrade investments

made to the PC will not be wasted.

This interface standard is quickly

becoming accepted by the industry,

its capacity to accept future proces-

sor enhancements means that it will

be an enduring standard.

The PCI solution is recommended

for use in the following situations:

Figure 3. Block diagram of a fast PCI test card.

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Figure 4. Compact PCI system.

architecture together with a robust,

modular and reliable computer hard-

ware built using the Eurocard rack

system and complying with EMC

standards.

This Industrial computer is built

using 3U and 6U height Eurocards,

unlike the cards you would normally

plug into an office PC the compact

PCI offers outstanding mechanical

integrity and simple installation. All

of the system cards are easily acces-

sible because they slide out from the

front of the computer. The operating

conditions of this system are more

precisely deﬁned than a standard PC

and include maximum shock load-

ing, vibration, air humidity and tem-

perature of its environment. It is

anticipated that this standard will be

in use for the next 10 to 15 years.

A Compact PCI-solution would be

recommended if the following fea-

tures are important:

sample the four channels simultaneously and

stream the measurement data directly to the

system hard drive. Each of the four differential

analogue input channels (CH0...CH3) has its

own 12 bit A/D converter. The sample values

are stored in a FIFO that is capable of storing

8192 values. This FIFO allows the four chan-

nels to measure simultaneously and then

transfer data direct to the PC memory or

stream to the hard disk. Also included on this

card are two 16 bit D/A converters (DAC0,

DAC1). Eight digital input /output lines (Digi-

tal I/O). Two 24 bit Counter/Timer

I/O, together with a versatile trigger-

ing system allowing either analogue

or digital signals to start and stop the

A/D converters (PFI/Trigger).

Reliable bus/slot connectors.

Vibration and impact resistant card

retention mechanism. Using

locking screws and card extrac-

tion levers.

Robust industrial housing suitable for

19” Rack Mounting

Cooling occurs by natural convection

or industrial fans.

Access to the system is always from

the front of the unit, including

connections to test or interface

cables.

This simpliﬁes monitoring of test sig-

nals changing cards.

Passive backplane, using an unplug-

gable CPU. (Compact PCI CPU

card) without changing the I/O-

Systems (Test card and connec-

tors, or Interfaces)

7 or 13 free Slots for Compact PCI

cards. Expandable using bus

bridges/bus-expanders.

Runs on Intel-Processors under Win-

dows 95 / 98 / NT / 2000 operat-

ing system.

Price range £650 - £3100 approx.

Compact PCI

Predominant in the ﬁeld of industrial

test and measurement is the com-

pact PCI system. This standard com-

bines the beneﬁts of the PCI system

PCMCIA

Figure 5. USB Test hardware.

The PCMCIA (Personal Computer

Memory Card International Associa-

tion) Standard specifies the size,

maximum permissible power, signal

paths and the programming con-

straints for cards using this inter-

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face. It was speciﬁcally designed to

allow simple plug-in memory expan-

sion or I/O for portable PC’s. There

are three different types of cards

available:

be industrial grade.

Capture of analogue signals with a

maximum sampling rate of

100 kHz with a resolution of 12

to 16 bits.

PCMCIA cards have an external slot

and can be connected without

opening the portable PC.

Price range £210 - £750 approx.

from this development.

The USB bus uses a four wire bus cable

with connectors of different type at either

end. This is because the USB uses a star

topology and a loop back connection is not

allowed and is not possible with this cable.

This interface has a continuous data rate of

12 MB/s with a guaranteed latency and is

especially useful for the test environment.

This data rate is more than sufficient for most

test applications and means that costly local

memory storage does not need to be

designed into the equipment to store fast test

data. The USB cable also supplies power to

the peripheral device. So with USB able to

supply the power to the device, all that is

needed to connect the test device to a PC is

a single cable. Using controller software on

the PC it is extremely quick to produce a test

environment to make the measurements

without the necessity of reconfiguring the

software or adding a special card to the PC.

If the device driver software has already been

loaded into the PC then it will automatically

recognise that the device has been connected

or disconnected to the bus. A USB solution

should be considered if the following points

are important:

Type I card with a maximum thick-

ness of 3.3 mm.(originally for

memory cards only, too narrow

for test equipment hardware).

Type II with a thickness of 5.0 mm.

Type III with a thickness of 10.5 mm.

In portable computers there are

often two PCMCIA Type II slots

implemented next to each other, this

gives space for two type I or type II

cards or one type III card. The inter-

face to the PC is well speciﬁed.

PCMCIA cards can however be

sensitive to other software running

on the system e.g. power saving

applications (“Power.exe”) or multi-

media drivers. This can have the

effect of slowing down data flow

handling between the card and the

PC and may cause interruption to

the measurement process. These

problems can be difficult to detect

and fix. Another problem that can

occur, especially if you are using

Windows 95/98 is that the card is

not properly recognised after instal-

lation and pops up under “other

devices” in the device manager.

What follows will probably be many

hours of tinkering before the system

ﬁnally accepts the card. Some note-

books are also very sensitive to the

presence or absence of these cards

and it is especially important to

deactivate the slot when the card is

removed otherwise it could lead to

system crash.

A PCMCIA solution can only be

recommended in the following situ-

ations. A USB system would be an

alternative solution:

Parallel Port

Many years ago the company Cen-

tronics developed an interface stan-

dard for printer control, over the

years this has come to be used as a

general purpose PC interface. This

works with parallel data and can

have a cable length of 8 m, cable

capacitance and crosstalk reduces

the usable speed/length of the cable.

These days each data line in the

cable is twisted with an earth wire

allowing a maximum cable length of

3 m. The maximum data rate is

largely dependant on the hardware

but it can handle up to 1 MByte/s

with a cable length of 1 m.

Using the parallel port is not nec-

essarily straightforward and many

PC’s can crash on boot up if the

peripheral device is powered up

before the PC is ready. In this case

the solution is to turn on the equip-

ment after the PC has booted. Often

it is possible to achieve a continuous

data transfer by reading the parallel

port through the system BIOS. There

seems little point recommending a

parallel port solution when the USB

is the better option.

Allows for portable test equipment using note-

books or portable PC-equipment running

the Windows 95 Rev. C / 98/2000 operat-

ing system.

Wide range of industrial standard connectors

between the PC and the USB device.

Capture analogue signals using a sample rate

of 100 kHz with a signal resolution of

around 12 – 16 Bit

USB Test hardware can be attached simply to

the PC no need for a screwdriver here,

just plug it in.

Price range £90 - £1500 approx.

ISA

USB

The ISA (Industrial Standard Architecture)

Bus allows a transfer rate up to 16.7 MB/s.

Test equipment using this interface are

becoming more rare largely due to its replace-

ment by the PCI. In most of today’s computer

systems it is becoming more difficult to ﬁnd

machines supplied with an ISA socket.

Investment in test equipment using this

interface standard could therefore be risky.

Price range approx. £100 - £5000

Virtually every PC based system

produced today comes complete

with a USB (Universal Serial Bus)

connector. This system has been

developed jointly by leading compa-

nies in the PC industry. This system

offers true Plug-and-Play capability

to peripheral devices. There is no

need to add any additional cards to

connect a peripheral device. While

this interface standard was origi-

nally designed for consumer orien-

tated applications today we find it

has a wide acceptance in many

other areas of application. The PC

based test equipment has beneﬁted

The test equipment supplier allows

the interested customer to try

the test equipment in the PC

system before committing to

purchase. This should ensure

that most of the problems

described above are avoided.

Portable test systems using note-

books running Windows 95 / 98

/ NT or 2000 operating system.

The connector between the PCMCIA-

test hardware and the sensor

connection box does not need to

The COM Port

Practically every computer system has an

RS232 interface and its not surprising that it

has been put to uses that were not antici-

pated when the interface was originally spec-

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or the smallest output voltage step

that can be produced by a D/A con-

verter. The resolution can be given

as a percent of the full-scale range or

more usefully as the number of bits

“n”, where 2 n equals the number of

levels that can be resolved.

Speed and Sampling Rate

Figure 6. COM port test hardware with DASYLab driver.

Two important measurement criteria

are speed and accuracy. There is

always a trade off between them.

Speed of measurement is the inverse

of the conversion time this is the time

that the A/D converter requires to

convert the analogue signal into its

digital equivalent. The greater the

resolution required, the greater this

time will be. The width of the data

output (e.g. 8, 10, 12 or 16 Bit) will

therefore be one factor that influ-

ences the conversion time. The con-

version process also influences the

conversion time choosing a converter

that uses a fast conversion technique

will obviously be more expensive

than one using a slower method.

Assuming there are no secondary

limiting factors (e.g. processing

speed) then the sampling rate of an

A/D converter will be inversely pro-

portional to its conversion time.

Sampling theory states that to mea-

sure a pure sinusoidal signal without

losing any information it is neces-

sary to take at least three samples

for each period of the wave. How-

ever for non ideal signals and to get

a better appreciation of what the sig-

nal is doing the rule of thumb states

that there should be 10 measure-

ment samples for each period of the

sampled signal. This means in prac-

tice that the fastest analogue signal

to be measured should be 1/10 of the

sampling rate of the converter.

A practical example will show the

constraints of typical test hardware.

A test card has an A/D converter

with a sample rate of 100 kHz. It has

16 input lines that can be switched

to the input of the A/D with a multi-

plexer. This gives an effective sam-

pling rate of 100/16 kHz = 6.25 kHz

per channel. Using the above rule of

thumb, this gives us a maximum

input signal of 625 Hz per channel.

The card in the above example would

therefore be suitable for measure-

ment of low frequency AF signals.

000093e

iﬁed. The advantage of this port is that equip-

ment developed for use with this interface

can be controlled by totally different com-

puter systems, the handshaking protocol can

however create some problems with this

interface. Test equipment using this port are

generally low cost and low speed. A COM

port interface can be recommended If these

criteria are important in your application:

ficult decision as to how the mea-

surements are to be made. The most

expensive option for fast measure-

ment is to capture each input signal

at the same time using a dedicated

high speed A/D converter on each

input line. The more economical path

is to use one A/D converter and

switch each channel in sequence

into the input of the A/D converter.

This last solution is more suited for

less critical testing.

Economical solution for measuring analogue

signals with a sampling rate of 1 Hz –

1 kHz and a signal resolution in the range

of 8 - 12 Bits Using the Windows

95/98/NT/2000 operating system.

Test equipment using the COM Port can be

connected without opening the PC case.

It uses a relatively slow data transfer rate.

This offers very good data protection and

generates very low interference over a rel-

atively long cable.

This uses a very simple cable assembly that

can if necessary, be provided with gal-

vanic isolation.

Price range approx. £20 - £200

Sample & Hold

In critical cases it may be necessary

to take a snapshot of the input sig-

nals and hold their values in a sam-

ple and hold circuit, until the con-

version (see the paragraph on speed)

has been completed on all input

channels. The rule applies here that

the effective sample rate of a chan-

nel is inversely proportional to the

number of channels that are being

measured.

A/D and D/A converters

Resolution

Another important criterion when choosing

the test card is the number of signals that

must be recorded at the same time. Together

with the question of speed, there is also a dif-

The resolution of a test system is

deﬁned as the smallest change that

can be detected by an A/D converter

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