>>>READER-TO-READER QUESTIONS AND ANSWERS
note set at 440.0 Hz and all other notes
are based off of that. This is a tempered
scale and each note is a multiple of the
note preceding it. The multiplication
factor is the 12th root of 2 or about
1.0595. To go up a note from 440 A,
you multiply 440 by 1.0595 to get
466.18 Hz or A#. To go down a note
from A, you divide 440 by 1.0595 to
get 415.29 Hz or G#. To go down still
another note from G# to G, you divide
415.29 by 1.0595 to get 391.96 Hz.
The 440.0 Hz note A on a guitar is
the first string, fifth fret. Holding the
first string down at the fifth fret and tuning this string to 440 Hz puts it at A.
When released, the string will be tuned
to 329.6 Hz E. Holding down the second string at the fifth fret is the same as
E on the open first string. Hold down
the second string at the fifth fret and
tune it to match the first open string.
Now it gets tricky. The fourth fret on
the third string is the same as the 246.9
Hz B on the second open string. Hold
down the third string at the fourth fret
and tune it to match the fifth open
sting. The fifth fret on the fifth string is
the same as the fourth open string. And
the fifth fret on the sixth string is the
same as the open fifth sting. Tuning a
guitar in this manner puts it in what is
called concert pitch.
[#11088 - November 2008]
Parallel Port
How does the parallel transfer
of data work? Does data flow out
from the computer alone, or can
data be read into the computer from
the port?
Tim Naami
Poplar Grove, IL
[#11085 - November 2008]
Using a PC Power Supply as a
Stand-Alone Supply
I would like to use a spare PC
power supply as a stand-alone source
for the 5V and 12V it provides. What is
the simplest way to provide the 5V
feedback for a power good signal? Is it
possible to use the PG signal to also
act as a power on light, as well?
.pdf. Section 4"ATX Power Supply"
discusses the power-supply interface.
The 5VSB output can be used to
operate a small pilot light (e.g., an LED
in series with a 1K ohm resistor); its
maximum output rating is 0.72
amperes. You must hold PS-ON low
(tie it to the COMmon conductor) in
order for the power supply to energize
the major outputs. PWR-OK is held
high (to the +5V level) by the power
supply whenever the + 3.3V and +5V
outputs are properly energized; you
should connect it to COM through a
1K ohm resistor so that it can be
pulled low to indicate power failure.
One of the + 3.3V contacts in the
main connector is used to sense the
voltage at the mating connector
(i.e., on the PC board in its normal
application) in order to compensate
for voltage loss in the power supply
cable. Remember that you are dealing
with low voltage at several tens of
amperes of current here. In any event,
it's very important to tie all of the
like-named contacts together at the
mating connector (or on the PC
board) -- COM to COM, +5V to +5V,
+ 3.3V to + 3.3V, etc.
If your power supply has the
optional connector with a 3.3V sense
line, tie the sense line to the + 3.3V rail
AT THE LOAD TERMINALS.
Also, read www.ibm.com/devel
operworks/power/library/pa-spec9
.html about some of the exceptions
that you may find across the universe
of PC power supplies. And beware of
Dell supplies which, in my experience,
do not conform to the ATX connector
wiring standard.
Peter A. Goodwin
Rockport, MA
A number of bidirectional parallel
port configurations have been
made available for the PC over the
years, but even the standard parallel
port (SPP) provides for TTL level
communication on eight output and
eight input lines.
The accompanying table shows
the DB25 pins which are used. Digital
output is carried on the printer data
lines. Digital input is read from status
and control lines. Note that three lines
are active low (0 VDC), while all
others are active high ( 5 VDC). It is
advisable to buffer the signals using
the LS74LS244 or similar, to protect
against shorts and other incorrect
wiring while experimenting.
Typically, LPT1 is assigned to I/O
base address 03BCh, LPT2 to 0378h,
and LPT3 to 0278h. To output data,
write the bit pattern directly to the
port. For example, outportb(0x03BC,
0xFF) to turn on all bits. The status
inputs are read at base address + 1
(bits 3-7), and the control inputs are
read at base address + 2 (bits 0-2).
So, a complete input byte is read
using inbyte = (inportb(0x03BC +1) &
0xF8) | (inportb(0x03BC +2) & 0x07),
and all bits are converted to represent
active high using inbyte = inbyte
^ 0x83. While my examples are
written in C language, other languages
provide similar port reading and
writing facilities.
The first thing to do is to
check out the article "How to Convert
a Computer ATX Power Supply to a
Lab Power Supply" at www.wikihow.
com/Convert-a-Computer-ATX-Power
-Supply-to-a-Lab-Power-Supply.
The principal reference for the
modern PC power supply is the
Intel ATX Specification; a copy of
which can be found at ftp://download
. intel.com/design/motherbd/atx_201
94 January 2009
Figure 1
