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               HOW TO page 2



We could connect a single LED light to the first data pin
of the printer cable. LED's generally use 3 volts, not 5
volts, so that a resistor is used to lower the electricity
to a smaller amount. A resistor, does just that - it RESISTS
electricity, the same way an element in a toaster gets red hot
as it RESISTS electricity.

Another example of a resistor is
the ordinary lightbulb. While a toaster element just gets red
or orange hot, the 'filament' or resistor in a lightbulb gets
white hot, and would burn itself up in a poof of smoke
instantly, if it were not in a sealed glass bulb with all the
oxygen removed. The heat from the wire makes light as
a by- product , so ordinary bulbs are very wasteful, since
the heat actually uses large amounts of electricity, but
only makes a little light for all the energy used.

     When a resistor is put on a wire, it
slows down the electricity, so that 5 volts can be turned into
3 volts. Resistors do get hot, but if tiny amounts of electricity
are used, you can not feel the heat.

The printer port CHIP is very VERY sensitive, so that
soldering wires and LEDs directly to the pins is a bit
dangerous - one mistake could " blow up " the chip, which
is often soldered on the computer motherboard, and this
would ruin the entire computer.

A safer method, is to wire the 8 pins of the printer connector,
to a " buffer " chip. This chip acts like a fuse,
and an amplifier - it looks at the incomming zero or one,
(zero volts or + 5 volts ) on the " input " side of the
chip, and puts out a zero or one on an output pin on the
other side of the chip. If there is a problem, such as two
wires touching on the output side, the electricity cannot
go through the chip, to damage the computer. The chip
might be blown up, but the computer is safe!
There are hundreds of digital chips to choose from, and if wired
as a simple buffer, most of these chips can be used. The chip used
here has 8 input pins, 8 output pins, and a control
pin to turn the chip on and off, which is handy if you want
your robot " off ", but want the computer "on" to write a

The " simple " wiring to connect an LED, is to use a resistor
of about 330 ohms to 470 ohms, to connect a single
LED to one pin of the 8 bit DATA pins, and to connect the
other side of the LED to PLUS 5 volts.

( OHMS are units measuring how MUCH resistance is in a resistor )
When the printer port pin is ZERO, or " ground ", then the
+ 5 volts of electricity flows through the LED from + 5 volts, to zero volts.
This wiring method has the LEDs " ON "
with a "ZERO", which is reverse logic. If the LED is turned around,
and the negative end wired to ground, then when the printer port pin
is + 5 volts, the LED will light up. ( Reverse logic is often used, since
many older printer port chips can handle [ "sink" ]
more electricity when grounding or shorting to Zero volts, than they
can handle in supplying ALL the plus 5 volts - many newer chips
do not have this limitation. Another way of looking at this would be
that if the chip goes to +5 volts, it must SUPPLY [" source" ] power,
to do work, such as lighting up an LED, or running a motor,and this
means that the chip USES more power. Chips are NOT designed to
supply large amounts of power - this would require them to SLOW
DOWN when they turn on and off !-- Instead, they are designed to
turn on and off millions of times a second, using only the smallest
amounts of electricity possible !! )

< picture of LED shorted to ground by a chip at ZERO volts
and another LED powered "BY" the chip, at +5 Volts >

As you can see from the diagram above, there are two ways to wire
any LED light to a chip -- either ON with a "1" (one) or +5 volts
or, by turning the LED around, "OFF" with a "1" (one) or +5 volts.
Also note that there is NO + 5 volt POWER supply on the printer
cable - just tiny + 5 volt DATA signals, which are designed ONLY
to connect to other computer chips. NOTE: IF you connect an LED
with BOTH sides to +5 volts, no electricity will move, and the LED
is OFF !, Similarly, If you Connect an LED with BOTH SIDES
to Zero Volts (ground), no electricity will move, and the LED is OFF !
The only way electrity will move is if one side is +5 Volts, and the
other side is Zero volts, or ground. If a CHIP PIN is putting out
+ 5 volts ( a LOGIC " ONE" ) and an LED is connected
to a + 5 Volt power Source on the other pin, NO POWER will
flow, and the LED is " OFF " even though +5 Volt power is being
supplied by the chip. THIS interesting fact can be used as a trick
later, when wiring in MOTORS to the circuit, to switch the direction
of the motor, using the least number of parts...

In early days of 80-86, 80-88, 80-186, 80-188, 80-286, 80-386, and
80-486 computers, there was no printer port standard, and
each manufacturer had different electrical values on how
the printer port chip worked. This means that connecting
LED lights and devices directly to any random old computer
is risky. It also explains why a simple Laser printer that worked
perfectly in the store, would take a week to get working when
you tried to connect it to your 286 computer at home...
When different OLD computers first turned on, each
one might turn on or off different pins on the parallel port, so
you can never assume a connected robot is "OFF". I have seen
computers turn on with all data bits as ONE, as ZERO, or
with the first 4 ON, or with the pattern on, off, on, off, etc.!
Many computers ran a PROGRAM that turned the printer
port pins ON and OFF in a series of programmed steps!
There is no way to tell what the engineers of the printer port
would start out with, other than to turn the computer ON and see!

The following wiring diagram is NOT safe nor recommended,
but usually works just fine for demonstration purposes.
Note that 99% of LEDs have a POSITIVE and Negative wires
( + 3 volt and ground / zero volts ) as shown:

Just to make life difficult, the 25 pin computer port is connected
to a 36 printer connector called a CENTRONIC connector, on most
printer cables. A printer cable is used here, since
wiring directly on the back of the computer is difficult,
and cannot be seen when using the computer in most situations.
The cable allows the connections to reach around to the
back of the computer for easy access. Modern computers (2009)
now do NOT even have a printer port, so that Printer Port cables
are becoming "clearance" items, very inexpensive, especially the
older, cheap ones that do NOT meet the new  " IEEE1284 "
standards. You should be able to find clearance cables easily.

Obviously, all 36 pins of a typical older CENTRONIC connector are
not used, typically with 6 pins not connected to anything,
and 13 pins all connected to ground (zero volts), instead of
the 8 ground pins used on the computer's printer connector.

Remove the CENTRONIC connector cover, and wire directly to
the pins. (why? - If you cut the connector off, and use the
wires, you have a mess of wires, and each cable manufacturer
uses different coloured wires! - if you use the connector
itself, each pin is always the same )

The cable shown above is the older, cheap type, with only 18
wires, and only one bare ground wire, wired to ALL the ground
pins inside the connector !
Here is a resistor of 330 Ohms and an LED, connected from
PIN 2 of the Centronic connector, which is wired to the
first data bit of the 8 bit " mailbox " memory location
# 378, which the computer uses as a printer output memory
NOTE: Since there thousands of different kinds of LED lights
and hundreds of different parallel port designs, you may have
a high power LED with a LOW power port, and not be able to see the
LED light up with the + positive LED wire going to pin 2 thru 9.
Some people on the web have had to use a lower resistance,
such as a 120 Ohm resistor, and/or put the LED +5 volt side to
an external +5 volt power source, before they could see anything
under normal lighting conditions. I will try to find some LEDs that
use tiny amounts of power, and list the part numbers and
prices. If you use a BUFFER chip, this should not be a problem
for most common LED lights.

As mentioned above, turning the LED light around will make it
turn ON with a ZERO or Ground on the PIN 2, and OFF when
Pin 2 is + 5 Volts. Many older parallel ports had 5 or 6 times
more power when GROUNDING ( sinking ) . If the LED
is still too dark to see under normal room lighting conditions,
then the port electronics are too weak, and/or the LED is a
HIGH POWER type that uses too much electricity - I have many
LEDs that will not turn on at all unless 9 Volts are used! For
DEMONSTRATION purposes, you can use an ordinary button
battery for power, such as the type now used for the BIOS - a
+ 3 VOLT 2032. Just connect + on the LED to the +, and the
Ground or " -" on the battery to any ground pin on the connector.
NOTE: 2032 indicates 2.0 Centimeters wide, and 32 Millimeters
thick, so that a 2025 ( 25 millimeters thick) or a 2016 ( 16 mm )
are the same voltage, just thinner. Many typical, cheap LED
pocket lights use a new generation of LED that is VERY bright,
but they can handle two 3 volt batteries for a total of +6 Volts--
most interestingly -- with NO resistor needed! Since these Dollar
store LED lights are 1, 2, 3, 4, or 5 for one dollar, WITH batteries,
they are the perfect, inexpensive way to QUICKLY test a port.
You can wire in an LED with a battery immediately and not need
an external +5 Volt power supply. You do not even need to
solder the wires! ( note: do NOT use two 3 volts to make 6 volts!)
    I purchase bulk, randomly packed, assorted LEDs at places
like the Electronics Market, set up in a parking lot every last
Saturday of the month, on Aviation Boulevard in Los Angeles.
I find that in a LARGE ziplock bag ( $2 ) of LEDs, are types
that have the power reversed, or that use 9 volts, or that flash,
or that use about 1.5 volts, or that shine invisible light, and some
are NOT designed to give off light at all, but are designed to
generate electricity -- all LEDs will generate electricity if you
shine light on them, typically with the Green LEDs making the
most power, but some LEDs are made deliberately to create
the most electricity, efficiently, and can be used as detectors.
So you may not be guaranteed that "any" LED will work
"BRIGHTLY" enough to see easily on " any " parallel port !


The best way to connect something to the Parallel
port is by using a BUFFER CHIP. The buffer chip is
connected BETWEEN the pins on the printer port of
the computer and the lights, switches, transistors,
or inputs that go from the " OUTSIDE" world. One
of the simplest chips to use is an 8 - pin IN to 8 - pin
OUT, since the main computer port, MEMORY
LOCATION 378 ( hex ), has 8 pins going out from the
computer. Since most computers use 8 lines at a time,
this chip is also common, and very inexpensive,
even compared to some simpler chips.


One of the most common series is the 7400 series,
(Pronounced " Seventy Four Hundred " )
known as TTL ( Transistor, Transistor, Logic ) which
has been used for 25 years. The original chips used
huge amounts of power compared to today's chips,
so newer versions are used. A chip appears as
7400 if is original, and there are letters often added
after the 74, to say which newer version it is.

A more modern chip series is the 4000 series, ( called
CMOS-- Complimentary, Metal Oxide Silicon ,
pronounced "Cee Moss " ). usually starting with 40,
and again there can be letters after the 40.

I use the 74 series, but with letters HC or HCT
after the 74 if possible. HCT chips " appear" to circuits
the same as the old standard 74 chips, but inside, they
are built with the newer 4000 series parts, so you
get the best of both types. HCT chips do not require
extra chips or resistors to connect "old" 7400 TTL
chips to "new" 4000 type CMOS chips, and thus
are simpler to work with.

Typical 8 bit IN / OUT chips would be the 74 240,
which takes 8 lines of data, ( Zero or One, which
in electricity is Zero Volts or +5 Volts ), and outputs
the opposite -- it inverts the values on all pins, which
can be handy. (Invert means all INput 0s are 1s on
the OUTput pins, and all 1's on the IN are 0's on OUT )
The 75 241, is identical, with 8 Input pins, and 8
Output pins, but the 8 outputs are IDENTICAL to the
Inputs, so this is called NON-Inverting.

Similar chips in the series would be the
74 540 Inverting buffer chip, and the
74 541 NON-Inverting 8 line buffer. These chips have the
advantage of being able to TURN OFF, or disconnect,
all 8 lines, which can be handy to have your
circuitboard connected, but more or less " OFF " to
test programming inputs and outputs, while NOT
sending signals to your robot or switches etc.
Other chips in the 74 HC series can buffer 2, 4, or 6
data lines so that if you only need a few lines to buffer,
many different chips can be used.

Lets start with the simplest chip, the 74HCT541.
The pins on the chip are in the " same" place as the
old 7400 series, the H is High Speed, the C means
that the chip is CMOS on the inside (not TTL ) and
the " T " indicates that it is wired so that it adjusts
to either 4000 series CMOS or 7400 "T" TL chips. The
541 is the number to identify that its the 8 bit IN/OUT
chip in the 7400 series. = 74 - HCT - 541
( Typically, the first 7400 chip started at 00, then came the
7401, the 7402, etc. until the series got to 74541 )
There are 8 inputs, and 8 outputs.

At mailbox, memory location, 378 (hex) there are
8 data bits. These data bits are copied onto the
8 DATA pins of the printer, parallel port. Now, we
connect the 74HCT541 to these pins. The output
8 pins of the chip can now be safely connected to
8 LED ( Light Emitting Diode ) lights, with 8 resistors,
to cut down the electricity from +5 volts, to about
+3 volts. Remember that a DATA One (1 ) is equal
to +5 Volts on the DATA pins, and a DATA Zero (0)
is equal to Zero ( 0 ) volts. ( Zero volts, is called a

If we look again at the STACK or pile of memory locations
of a typical 16 Thousand ( 16 K ) computer, as the IBM
PC was first shipped to the public, we can now, put in
a 74 HCT 541 on the 8 data lines.

Then we can add up to 8 LED lights to show if the
mailboxes or memory BITS are Zero s (0 s ) or Ones (1 s).
We will have to add resistors to limit the electricity
to the LEDs, and a typical value might be from 220 OHMS,
( an Ohm is a value of how much resistance is used )
to 470 Ohms, depending on which of the thousands of
different kinds of LED is used. It is usually " safe " to
pick a middle value like 330 Ohms, for most common
LEDs that you would find in a typical electrical store.
Some LEDs will be brighter or dimmer than others, but
as long as you can see them glow, that is all that is
needed for now.

Each LED is wired from GROUND or Zero Volts, through
a resistor ( 330 Ohms value ) to the DATA pins on the
Parallel, or Printer Port, again, through the 74 HCT 541.

A problem occurs here since the printer port only has
the DATA +5 volts, or " LOGIC" volts, and no +5 volt
supply for POWER, so that at this point we have to get
+5 volt POWER from somewhere to POWER the electronic
circuits INSIDE the Buffer Chip. In my original test
cardboard, I use +5 Volt power from a Joystick connector
on the back of the computer. The original IBM PC did not
have this port, but ALL computers have +5 Volt wiring
inside for the harddrives, the floppies, and the motherboard.
99% of standard IBMs and Clones had four wires,

Yellow, Black, Black, RED and these were
+12,       0,         0,        +5

The small white connector is a typical FLOPPY connector
of the modern 3 1/2 Inch type. The large white connector
will fit a typical harddrive or an OLD, 5 1/4 Inch floppy used
on the first IBM s and Clones.

We need to connect to the RED wire, and bring it out of the
computer case, to SUPPLY + 5 Volt power to our cardboard
circuit board. One easy way is to just SOLDER a wire to an
unused white connector, and tape up the connector with
black, electrical tape, usually found at a typical dollar
(SOLDER, is a very soft metal, usually wound on a roll
like shiny silver wire, and for electrical soldering, it has a
tiny hollow center filled with a yellow paste called " FLUX".
SOLDER will melt if you get a hot, pointed, soldering " IRON"
or a soldering " gun " and touch the solder to the hot tip.
The soldering iron TIP is hot enough to burn your skin,
just like an element on a stove. When the solder melts,
the FLUX paste melts, and coats the solder (pronounced
SAW - DER ), and also coats any copper , silver, or gold
plated metal, such as transistors, LED posts, resistor ends,
or Capacitor leads. This thin film of FLUX cleans the metal,
and stops air from combining with the hot metal to make
" metal oxide , or " rust ", which would stop the solder
from sticking to the heated metal you are working with.
If you touch two wires together when
hot solder is on them, and let them cool to room temperature,
- which only takes 2 seconds - they are permanently
" glued" together - but the glue in this case is METAL,
so electricity goes through it, from one wire to the other.
This is how most electronics are put together.
For a small project like this, you put all the
parts together, one- at- a- time. In a factory, the whole
circuit board is dipped into a pool of liquid solder, and
removed to cool - hundreds of parts are all soldered in
a few seconds. For this robot project, you will need a roll
of " electrical " solder, usually a small diameter wire,
with " electrical " Flux (( the other flux is used for putting
copper pipes to together, and has acid in it to quickly
eat away dirt and copper corrosion ! )). For many years
solder was called 60 / 40, and had the metal " Lead "
in it. Newer solder has no lead, (which is a poison if eaten)
but the newer solder is more difficult to work with.

You will need a soldering " iron " or " pencil " which is a
long metal post with a sharp tip, and a plastic handle
- much like a screwdriver in its shape.

< picture of a soldering iron, a soldering gun, electrical
solder with a flux center, a roll of 'wire wrap' fine wire,
and a roll of black electrical tape >

Both electrical solder and the soldering iron or gun can
be obtained from electrical hobby shops, and in Canada,
at places like Canadian Tire. Various HARDWARE stores
also would have some items. Radio Shack is now called
the Source and dropped all the small parts, but in the
United States, you can still get some of the parts you need.
FINDING the 74 HCT 541, the solder, extra wire, the
soldering gun, the LEDs and the Resistors, is a task in
it self. Every Province, State, and Country around the
world has different Stores, so you will have to use the
Internet to find a local electrical / electronic / store or
hobby store.

The 74 HCT 541 is actually an entire miniture circuit board,
filled with transistors, diodes, resistors and capacitors, so
this entire " circuitboard " must have its own +5 volts power
supply and a Ground. As you can see from the diagram
below, the circuit is very complicated !
( thanks guys, at NXP division of Philips Semiconductor )


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