Interfacing to the Parallel Port - N1II.pdf

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Interfacing to the Parallel Port
While there is a choice of ports on
the PC for amateur use and direct
interfacing, the parallel port is
probably the simplest. With eight
data wires, several control wires and
bidirectional capability, it offers a
convenient way to get information in
and out of a PC. The examples in
the next two sections use an older
software language,
BASIC
or
GWBASIC
to get information in and
out. Newer languages can be used,
but several varieties of
BASIC
are
available on the Internet at no cost.
The learning curve for someone who
has never used a programming
language is very short — usually a
matter of a few minutes. The two
examples that follow interface single
chip analog-to-digital and digital-to-
analog converters to the parallel port
of a PC.
Single-Chip Dual-Channel A/D
In this analog world, often there is
need to measure an analog voltage
and convert it to a digital value for
further processing in a PC. This
single chip converter and accompa-
nying software performs this task for
two analog voltages.
Circuit Description
The circuit consists of a single-chip
A/D converter, U2, and a
DB-25 male plug (Fig
A).
Pins 2 and
3 are identical voltage inputs, with a
range from 0 to slightly less than the
supply voltage V
CC
(+5 V). R1, R2,
C3 and C4 provide some input
isolation and RF bypass. There are
four signal leads on U2. DO is the
converted data from the A/D out to
the computer; DI and CS are control
signals from the computer, and CLK
is a computer-generated clock signal
sent to pin 7 of U2.
The +5-V supply is required. It
may be obtained from a +12-V
source and regulator U1. Current
drain is usually less than 20 mA, so
any 5-V regulator may be used for
U1. The power supply ground, the
circuit ground and the computer
ground are all tied together. If you
already have a source of regulated
5 V, U1 is not needed.
In this form the circuit will give you
two identical dc voltmeters. To
extend their range, connect voltage
dividers to the input points A and B.
A typical 2:1 divider, using 50-kΩ
5.68
Chapter 5
Fig A — Only two chips are used to provide a dual-channel voltmeter. PL1 is
connected through a standard 25-pin cable to your computer printer port. U2
requires an 8-pin IC socket. All resistors are ¼ W. You can use the A/D as an SWR
display by connecting it to a sensor such as the one shown in Chapter 19 of this
Handbook
(Tandem Match Wattmeter project). A few more resistors are all that are
needed to change the voltmeter scale. The 50-kΩ resistors from 2:1 voltage
Ω
dividers, extending the voltmeter scale on both channels to almost 10 V dc.
resistors, is shown in the figure.
Resistor accuracy is not important,
since the circuit is calibrated in the
accompanying software.
Software
The software,
A2D.BAS,
can be
found on the
Handbook
CD. It
includes a voltmeter function and an
SWR function. It is written in
GWBASIC
and saved as an ASCII
file. Therefore, you can read it on
any word processor, but if you
modify it, make sure you resave it as
an ASCII file. It can be imported into
QBasic
and most other
BASIC
dialects.
The program was written to be
understandable rather than to be
highly efficient. Each line of basic code
has a comment or explanation. It can
be modified for most PCs. The printer
port used is LPT1, which is at a hex
address of 378h. If you wish to use
LPT2 (printer port 2), try changing the
address to 278h. To find the ad-
dresses of your printer ports, run
FINDLPT.BAS
(also included on the
Handbook
CD).
A2D.BAS
was written to run on
computers as slow as 4.7-MHz PC/
XTs. If you get erratic results with a
much faster computer, set line
1020(CD=1) to a higher value to
increase the width of the computer-
generated clock pulses.
The software is set up to act as an
SWR meter. Connecting points A
and B to the forward and reverse
voltage points on any conventional
SWR bridge will result in the pro-
gram calculating the value of SWR.
Initially the software reads the
value of voltage at point A into the
computer, followed by the voltage at
point B. It then prints these two
values on the screen, and computes
their sum and difference to derive
the SWR. If you use the project as a
voltmeter, simply ignore the SWR
reading on the screen or suppress it
by deleting lines 2150, 2160 and
2170. If the two voltages are very
close to each other (within 1 mV),
the program declares a bad reading
for SWR.
Calibration
Lines 120 and 130 in the program
independently set the calibration for
the two voltage inputs. To calibrate a
channel, apply a known voltage to
input point A. Read the value on the
PC screen. Now multiply the con-
stant in line 120 by the correct value
and divide the result by the value
you previously saw on the screen.
Enter this constant on line 120.
Repeat the procedure for input point
B and line 130.
D to A CONVERTERS —
CONTROLLING ANALOG DEVICES
The complement to A/D convert-
ers is D/A (digital-to-analog) convert-
ers. Once there is a digital value in
your PC, a D/A will provide an
analog voltage proportional to the
digital value. Normally the actual
value is scaled. As an example, an
8-bit converter allows a maximum
count of 255. If the converter is set
up with a +5 V dc reference voltage,
a maximum value digital value of
255 would result in a D/A output
value of 5 V. Lower digital inputs
would give proportionally lower
voltages.
Fig B — Only three wires and a ground lead are needed to connect the
converter to your PC.
Circuit Description
This project is the complement of
the parallel port A/D converter
described earlier. It takes a digital
number from the computer, and
converts it to a voltage from 0 to 5
V dc. Only one chip, the MAX 512,
is required. It operates from a 5-V
supply and is connected to the
computer by a standard DB-25
parallel port connector. The chip
may be ordered from Digi-Key,
Allied Electronics and other ham
suppliers as MAX512CPD-ND. The
voltage regulator in
Fig B
provides
the 5 V source required to power
the chip.
Software
The software needed to run
the chip,
D2A.BAS,
can be found
on the
Handbook
CD. It is about
60 lines long, fully commented
and written in
GWBASIC,
so it
may be readily modified. The
parallel port address is defined on
line 105 as PORTO=&H378. Your
computer may use a different
address. To find the correct
address, run
FINDLPT.BAS.
The program takes the value
AIN from the keyboard (line 230),
converts it to a number between
0 and 255, and then sends it out
as a serial word to the DIA chip. If
you would like to use the project
with another program, use your
other program to set AIN to the
value you want to generate, and
then run this program as a sub-
routine.
At the end of the program is the
clock pulse subroutine. In the event
your computer is too fast for the
converter chip, you can stretch the
clock pulses by changing CD in line
5010 to a value greater than the
default value of 1.
Applications
This circuit provides the capability
of setting a voltage under computer
control. It can be calibrated to
match the power supply and the
actual chip used. Tests with several
chips showed an error of 25 mV or
less over the range of 0 to 5 V dc
output. —
Paul Danzer, N1II
5.69
Electrical Signals and Components

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