CC_1993_i0030.pdf

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CIRCUIT CELLAR ~)~~~~ ~

Q Same C

THE COMPUTER

APPLICATIONS

JOURNAL

d-eat Hrtlcles,

Twice as Often

’ A ” ’

ave you heard the news? If you’ve already read

FOUNDER/EDITORIAL DIRECTOR

Steve Ciarcia

PUBLISHER

Daniel Rodrigues

r Steve’s editorial, or you were particularly observant

r. when checking out the subscription cards in the last

Issue, then you know. If not, let me be the first to tell you:

the Computer Applications Journal is going monthly. That’s right. Now you

can look forward to finding a new issue in your mailbox each and every

month.

It was just about a year ago when we redesigned the look of the

Computer Applications Journal and I promised that the quality of our editorial

would remain as high as ever. Now, as we shift gears and go monthly, I

continue the same pledge of high-quality editorial. All of our regular

columnists will be in each issue, and we have some new features to go

along with the change. The fun starts with the next issue, February 1993,

and will continue each month thereafter.

In the last issue, I listed the themes for the upcoming year, but couldn’t

give any dates because we hadn’t released the news of going monthly, yet.

Now, I want to repeat those themes, but put some dates with them so you

know the time frame with which you’re working should you have expertise in

a particular field and want to write an article. The date following each theme

is the deadline for proposals, so be sure to get your ideas in early.

EDITOR-IN-CHIEF

Ken Davidson

PUBLISHER’S ASSISTANT

Susan McGill

ASSOCIATE EDITOR

Lisa Nadile

CIRCULATION COORDINATOR

Rose Mansella

TECHNICAL EDITOR

Dan Woods

CIRCULATION ASSISTANT

Barbara Maleski

ENGINEERING STAFF

Jeff Bachiochi & Ed Nisley

CIRCULATION CONSULTANT

Gregory Spitzfaden

WEST COAST EDITOR

Tom Cantrell

BUSINESS MANAGER

Jeannette Walters

CONTRIBUTING EDITOR

John Dybowski

ADVERTISING COORDINATOR

Dan Gorsky

NEW PRODUCTS EDITOR

Harv Weiner

CIRCUITCELLAR INK,THECOMPUTERAPPLICA-

TIONS JOURNAL (ISSN 0896.8985) is published

monthly by Cirwt Cellar Incorporated, 4 Park Street,

Suite 20. Vernon. CT 08066 (203) 875.2751. Second

class postage paid at Vernon, CT and additional

offices. One year(l2 issues) subscription rate U.S.A.

and possessions $21.95. Canada/Mexico $31.95, all

other countw $49.95 All subscription orders pay-

able in U.S. funds only, via international postal money

order or check drawn on U.S. bank. Direct subscrip

tion orders to The Computer Applications Journal

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recall (215) 830-1914.

POSTMASTER: PleasesendaddresschangestoThe

ComputerApplications Journal, Circulation Dept., P.O.

Box 3050, Southeastern, PA 19398.

ART DIRECTOR

Lisa Ferry

STAFF RESEARCHERS:

Northeast

John Dybowski

Midwest

Jon Elson &Tim McDonough

West Coast

Frank Kuechmann

February, issue #31:

March, issue #32:

April, issue #33:

May, issue #34:

June, issue #35:

July, issue #36:

August, issue #37:

September, issue #38:

October, issue #39:

November, issue #40:

December, issue #41:

Home and Building Automation

Embedded Interfacing

Data Acquisition

Graphics and Video

Communications

Real-Time Programming

Measurement and Control

Signal Processing

Power Control and Conversion

Programmable Devices

Embedded Control

(1011192)

(1 l/2/92)

(12/l/92)

(i/1/93)

(2/l 193)

(3/l/93)

(4/l /93)

(513193)

(6/l 193)

(7/i/93)

(8/2/93)

Cover Illustration by Robert Tinney

HAJAR ASSOCIATES NATIONAL ADVERTISING REPRESENTATIVES

NORTHEAST

SOUTHEAST

WEST COAST

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:617) 769-8950

-ax (617) 769-8982

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(305) 966-3939

Fax: (305) 985-8457

MIDWEST

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& Shelley Rainey

(714) 540-3554

Fax: (714) 540-7103

The big news for this issue is the results of the Fourth Annual Circuit

Cellar Design Contest. We get some really creative entries each year, and

this year was no exception. We hope to have full-length articles describing

each of the winning projects sometime in the upcoming year.

The first feature article in this issue, “Build the SmartROM EPROM

Emulator,” was a winner in last year’s contest and has been the source of

questions from more than a few of our readers. Other articles include more

hardware and software development tools, data collection and control

projects, and a nifty application by Ed that teaches both basic physics and

some tricky computer design tricks at the same time.

The theme for our next issue is Home and Building Automation, which

is always one of the most popular issues of the year. We have some great

articles lined up, so it’s one not to be missed.

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All programs and schematics in Circuit Cellar INK have been carefully reviewed to ensute their performance

isinaccordancewiththespecificatlonsdescribed, andprogramsarepostedantheCircuitCellarBBSlorelectronic

transfer by subscribers.

CircuitCellar INKmakes nowarrantiesand assumes no responsibility orliabilityofany kindforerrors in these

programs or schematics or for the consequences of any such errors. Fulthermore, because 01 possible variation

in the quality and condition of materials and workmanship of reader-assembled pro]ects. Circuit Cellar INK

disclaims any responsiblity for the safe and proper function of reader-assembled projects based upon or from

plans. descriptions, 01 information published in Circuit Cellar INK.

publication in whole or in part wilhout written consent from Circuit Cellar Inc. is prohibited.

Issue #30 December ‘92IJanuary ‘93

The Computer Applications Journal

Build the SmartROM EPROM Emulator

by Sanjaya Vatuk

68705 SLUSH/Not Quite an ICE, But Just as Useful

by Robin Brophy

Program 8748149s with the DAR-49

by Paul Hitchcock

LCD Lineup/Getting Graphic with the LM213B

by Tom Cantrell

¦ :

Winners in the Fourth Annual Circuit Cellar Design Contest

by Lisa Nadile

¦ Microvolt Measurements/Use a 20-bit A/D Converter in Your Next Design

by Russ Lindgren

Build a Computer-Controlled Multiswitch System

by Michael Swartzendruber

Editor’s INK/Ken Davidson

Same Great Articles, Twice as Often

Silicon Update/Tom Cantrell

Hot Chips IV/Silicon Sizzlers

Reader’s INK

Letters to the Editor

Practical Algorithms/John Dybowski

Denominations of Time

New Product News

edited by Harv Weiner

ConnecTime-Excerpts from the Circuit Cellar BBS

conducted by Ken Davidson

Firmware Furnace/Ed Nisley

Physical Constants & A Mini Interpreter

Steve’s Own INK/Steve Ciarcia

Evolution

From the Bench/Jeff Bachiochi

Entry-level Embedded Development/

On a Shoestring Budget

Advertiser’s Index

The Computer Applications Journal

Issue #30 December ‘92IJanuary ‘93

TIME DOMAIN QUESTIONS

cated than a normal TDR scope display, but since the

Thanks for the interesting article about the time-

unit doesn’t provide a display, I took the easy way out.

domain reflectometer. There were a few things that the

In response to your question about assuming the pulse

article did not mention, however, and I wonder if you

height is constant, this software uses two pulse heights:

might clarify them.

one for 50 and one for 75 ohms. This area of the software

The review of the theory of TDR and the description

could be improved by measuring the outgoing pulse. I

of the user interface were excellent, but you never

played with this somewhat, but it’s difficult; assuming it

mentioned in what form the results of the test are

is constant doesn’t usually produce significant errors.

presented to the user. I sort of gather that the instrument

The value of R31 is supposed to be 1000 ohms. I

looks for the (first? largest? last?) reflection and calcu-

noticed that myself. Its only purpose is to keep the

lates the distance to that point and the impedance

transistor tied when switching resistances. It could even

(resistance) there. How are these results displayed, and is

be eliminated with little effect.

it possible to search for additional reflections?

It seems that the hardware can answer the question,

“Was there a low-to-high and/or a high-to-low transition

across voltage V after time T!” Can you give a brief

IMPROVED BUFFERS

explanation how the software derives from this the

There appears to be a considerable amount of interest

delay, direction, and magnitude of reflected pulses?

in driving multiple VGA monitors from a single display

I’m curious about any theoretical and/or practical

adapter [judging from magazine ads and recent Circuit

reasons that you chose to use a pulse waveform instead

Cellar BBS discussions). I was recently hired to design just

of a step waveform for the measurement. Does the

such an eight-way VGA splitter, mainly because the

software calibrate itself by measuring the amplitude of

customer wanted to avoid the $900 price of a commercial

the outgoing pulse, or is it assumed to be constant?

unit. My design was similar to Michael Swartzendruber’s

I like the idea of using a latching relay to save power

splitter in the October/November ‘92 issue of the Com-

when selecting the cable termination, but it appears in

puter Applications Iournal (“Driving Multiple VGA

Figure 4 that R31 (100 ohms) is in parallel with the

Monitors”), but with, perhaps, a very important addition.

resistor (R8/R9) selected by Kl/S2. Does the software

Mr. Swartzendruber’s basic PNP emitter follower (buffer]

somehow take this into account!

design is shown below:

Dave Tweed

e +5 V

IOOR

Littleton, Mass.

VOIA

Vlll

fohn Wettroth responds:

-r 2N2907

75Q i

The TDR displays a distance to biggest (largest

absolute value reflection coefficient) discontinuity in

the cable and a termination resistance value. For large

The transfer characteristic of this buffer is Vout = Vin +

and small reflection coefficients, it displays open or

0.6 volts.

shorted; otherwise, a resistance (in ohms).

Considering the fact that the typical maximum

The TDR measures the amount of time required to

analog RGB output is about 1 volt, this 0.6-volt offset

get a pulse back and adjusts a DAC to a level corre-

may be undesirable (it does seem to work satisfactorily,

sponding to its level. Distance is cable velocity/time

however). The horizontal and vertical sync inputs are also

with a velocity factor for the cable type. Resistance is

driven in this manner and the 0.6-volt offset becomes a

calculated by reflection coefficient equation solution

main concern here because many monitors specify the

after a correction is made for loss in the cable for

sync inputs as TTL type, suggesting a maximum low

distance traveled.

input of 0.8 volts. With the input already at 0.6 volts [I

A pulse waveform was used for practical reasons.

actually measure over 0.7 volts!), trouble could be close.

Because of the specifics of how I search for the reflected

One monitor I tried wouldn’t sync at all due to the offset.

waveform using the DAC and a comparator, it was

Fortunately, the fix is no more complicated or expensive

easier. Step waveforms create return waveforms with

than the original circuit. If we add an NPN’emitter

steps and were more complicated to find in software.

follower onto the output of the PNP follower, the signal if

Looking at the waveforms on a scope is more compli-

faithfully reproduced. Consider the following:

Issue #30 December ‘921January ‘93

The Computer Applications Journal

The transfer characteristic of the NPN follower is

Vout = Vin - 0.6 volts. Combining both equations

yields Vout = Vin, and we have a true buffer. A

complete circuit is shown below:

+5 V

+5V

470R

2N2222

(NPN)

Vin-

VOUt

75R

2N2907

(PNP)

loon

Note the increase in the PNP emitter resistor

value due to the reduced load of the NPN base.

Caution: The PNP transistor must be first in line

because an NPN obliterates the lower 0.6 volts of the

desired signal.

To produce a nice steady DAC ramp on the

analog color lines for oscilloscope inspection, draw a

single horizontal line across the graphics screen,

increasing in intensity (0% to 100%) from left to

right. Such an image lets you easily observe offset and

scaling of your analog driving circuitry.

We feature a series of single board computers for

process control applications. Each is available as a

bare printed circuit board, or fully assembled and

tested. Optional development software is also

available. Please contact us to discuss your

requirements and receive a literature package

covering technical specs and pricing.

Dale Nassar

Amite, La.

CORRECTIONS

Issue #28, Page 36, Figure 4a

Pin 1 of U6 (MAX7224) should be grounded.

Tiii It

Issue #28, Page 37, Figure 4b

The zener diode (D3) should be labeled 5.1 V.

Issue #29, Page 24, Figure 2

The NPN transistor, Q12, should be labeled 2N4401.

Issue #29, Page 70, Figure 4

The photodiode and resistor should be connected as

follows:

vcc

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#103

The Computer Applications Journal

Issue #30 December ‘921January ‘93

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