Discussion in 'Electronic Basics' started by SME, Nov 11, 2003.

1. SMEGuest

Hello Group

I wish to reverse the logic from a parallel port output using transistors.
So when the output is high the line into the transistor is high

For example :-

Printport
Collector 12V

/
Base|/
D0 -----|
|\
\
Emitter
|
|
---------Gnd

As I understand it the above circuit when D0 is high (+5V) will sink the
collector bring the 12V input to Low (I understand there should be resisters
in the above circuit etc etc)

As mentioned above I which to have a circuit that when D0 is High the 12V
input is High and when D0 is Low it pulls the 12V input Low.

TIA
Steve.

2. Luhan MonatGuest

Steve,

Why not reverse the sense in the software?

Otherwise, you are actully describing 'non-inveted' output: High in =
High out.

3. John PopelishGuest

If you need less output pull down current than the parallel port can
sink, you can use a common base amplifier to increase the output
swing, without inversion. Tie the port pin to the transistor emitter,
and bias the base with a resistor to a voltage that is between port
high and port low voltage. For instance, if the port swings a full
five volts, you might tie the base to +5 with a 10k resistor. This
will limit the base current to less than a half milliamp when the
emitter is pulled low, while letting the collector sink 5 or 10 ma, if
the port output can sink that combination of collector and base
currents.

4. SMEGuest

The original controller board (not the stepper Drivers) has three dual opto
isolator's each one handle the Step and Dir.

Step is defaults to hi on the led side which sets hi transistor side these
then appear to go via two resister arrays which in turn connect to a
transistor (NPN) this then connects to a cable connector which is a 16way
(similar to an IDE cable but smaller) the ribbon cable from this connector
connects to a backplane that then connects to the X,Y,Z stepper drivers.

------ ResArray Connector
Step X (H) -| o |- | |..|
Step X (H) -| p |- StepX(H)--|-(TR)-|..|-StepX(H)---StepCtlr-(H)
Dir X (H/L)-| t |- DirX(H/L)-|-(TR)-|..|-DirX(H/L)-- StepCtlr(L/H)
Dir X (H/L)-| o |- | |..|
------

You made need to copy the above into a text editor!

The Step pin on the Stepper Controller is HI by default when the motors
moves a LOW appears for as long as movement is requested.
This LOW is witnessed all the way back to the frontside of the optoIsolator
when the motor completes its task the Step go's HI again.

The question is how is this done? (regarding the circuit above)

TIA
Steve

5. norm d.Guest

One way with transistors is to use 2 NPN transistors to get the logic you
want. Put a resistor in series between your +12VDC and the collector to keep
from frying the transistor. Use the junction of the resistor & collector as
the output to drive a second similar transistor circuit. A series resistor
going to the base of each transistor may be required also depending on the
transistors you use. The output of the 2nd transistor will be just the
opposite of the 1st transistor, but will follow the parallel output
directly.