Trouble positioning a stepper motor accurately

I'm using a PXI - 7344 with an MID7064 motor driver for XYMR Microscope Stage(Danaher Motion) which has encoder of resolution 2000 (counts/rev).
The motor is in 200 steps/rev configuration and microstepping by 10 (closed loop).
So the motor and encoder are effectively in same resolution. So i expect it to be accurate to +/- 1 step(with three pull in moves ) because the motor and the encoder may not be in phase.
But i get accuracy of +/- 3-4 steps. I didn't have this problem previously (till a week ago), I had accuracy of +/- 1 step.
Any ideas on what could be wrong?
Thank you
Prathyush

Hy Prathyush,
Here is another thing to consider. You may be trying to stop the move in between physical poles on the stepper motor. Since the motor has 200 steps per revolution, that means that there are 200 physical poles on the motor to step with. You are microstepping at 10 microsteps/step. So, if you try to stop a move after a number of steps that is not divisible by 10, then you are essentially trying to hold the rotor between two poles which are competing to magnetically attract it. Essentially, the rotor is in an unstable "tug-of-war" so to speak. Now your encoder has a resolution of 2000 counts/rev, each count being evenly spaced out. If the motor is being held between two poles, there might be slight vibrations which cause the encoder feedback to vary slightly, thus giving you a 3-4 count error. If you set up your moves where you never land in between poles, I expect you should have better accuracy.
Wes P
Applications Engineer
National Instruments
Wes P
Certified LabVIEW Developer

Similar Messages

Trouble positioning a stepper motor accurately (to one step)

I'm using a PCI-7344 with an MD7506 motor driver from RS. I'm testing angle sensors whose voltage output is proportional to (you've guessed it...) angle.
I'm stepping the motor round by one step, checking that it's got there by reading the encoder, measuring the sensor voltage and repeating.
My problem is that the motor sometimes 'hunts' around the target position. In closed loop (with an encoder associated with a motor) this results in sometimes not being able to reach the target exactly with the pull-in moves.
Using open-loop and reading the encoder directly I get better results because I can loop more times, but the motor will still 'hunt' around a target point, changing direction a few times as it over
shoots by a step.
As there are a million MAX settings for this I won't list them all here, but I'm checking for a move complete when the InPos flag is set and I have a deadband of 0 steps too. I'm only rotating at 120 step/sec with a 10ms pause between steps.
Do I have to accept that stepper motors are only accurate to +/- 1 step or am I doing something wrong?
Thanks,
Mike
Mike Evans
TRW Conekt
N.I. Alliance Member, UK
http://www.Conekt.net

Mike,
Maybe other people will have better suggestions, but in my case I can advise to either:
1. increase the number of pull-in moves,
2. increase the microstepping of your drive,
2a... together with a higher precision encoder...
Other than that, the accuracy is advertised to +/- 1 step, so maybe you would like to consider a stepper with more steps/rev.
Good luck!, I hope other people will give other suggestions.
Nestor.
Nestor
National Instruments

Position drift during the stepper motor controlled back and forth motion

Hi everyone there,
I get a headache position drift problem during the stepper motor controlled back and forth motion. I have been wroking on the problem for the last two weeks, and still can not figure it out. So I write down here my problem in hope of getting some clues from those have some experiences in stepper motor motion control.
My goal is to use a stepper motor to drive a beam flage fixed onto the shaft to go back and forth for a certain angle, and this back and forth motion needs to be stable and reproducible within hours. With the SMD1 stepper motor driver, I only need to send in a pulse train to control the number of steps(the pulse frequency defines the stepping rate), and another pulse to controle the direction (i.e., cw or ccw). To do this, in LabView, I used the counter of the DAQ board to produce a finite pulse train to step the motor, and used one digital line to produce a pulse to switch the direction every 500ms, the timing is contolled by the software.(I used the oscilloscope to monitor these two pulses, the pulse train produced by the counter looks very stable, and delay between the pulse edges of the direction pulse and the pulse train is also very stable within 500ms. There seems no software timing problems at such a long delay.) And this labview controlled motor appears to work perfectly as I required, the motor steps a defined number of steps at one direction, and then steps back at the origin position. But after running for a few minutes, the back position shifts gradually(I can not observe a obvious shift at the first few minutes) with repect to the origin position(the position I start running). I looked it up at the manual and also the website, I got a clue that this might because the motor steps at the resonace frequency of the motor and hence misses or takes extral steps sometimes. So I changed the steping rate from 1Hz to 1000Hz to see if the shift disappears or gets smaller at certain frequency range, but the shift always exists.
I did observe the shift got smaller within the same running time when change the step model from full step (1.8degree/step)to half and quarter step model. According to this, the motor did miss steps (or pulses) during running, the quarter step model has smaller shift due to the smaller steps. But I also noticed that the motor always shift at one direction, no matter I started with clockwise or counter-clockwise, this seems really strange to me. Because the motor should shift at each direction with the same probability if we assume the motor missis steps randomly.
There is something must be wrong, either the labveiw program or the motor, but I don't even know a clue of this weird shift problem. And because of this, I haven't made any progress in my lab for the last three weeks. So any suggestions could be very helpful for solving this headache and will be really appreciated.
Thanks a million in advance and Merry Christmas!
Li

Hey, Brian Beal,
Thanks for your quick response. "accel" and "decel" mean acceleration and deceleration, am I understanding right?
If yes, then I don't have these in my LabView program, I just generate one TTL pulse train and one TTL pulse with LabView and send them into the SMD1 stepper motor driver.
The pulse train steps the shaft, and the other switchs the direction.
Yesterday, I confirmed that if the motor steer always at one direction either CW or CCW, then the stepping were very stable, no missing or extra steps happen for hours.
And I also observed that the motor took one extra step from time to time when the direction changes. And this only happens when the direction switch from CW to CCW. (bizarre!)
I guess this indicates that some pulse noise happens when the motor switch the direction too often. But I just don't have any clue for irregular extra steps.

Stepper motor positioning control system

hello
Iam building positioning control system using a stepper motor, and I need to know what are some position sensors available to feedback the position of the stepper motor.. Also I need to know how can use a C code in the labview...For example, if I have an algorithm developed in C; How can I use that code in the labview..
Thanks
Shaham

Shaham,
Below is a link to several encoders that NI provides specifically for stepper motors.
Quadrature Encoder:
http://sine.ni.com/nips/cds/view/p/lang/en/nid/205321
As far as the C code is concerned, this can be accomplished in two different ways, provided that the code is first placed in a dll file.
First, you can import the dll into a project library, by selecting Tools»Import»Shared Library. This option is good for code that will be used often, as it creates wrapper VIs for each function that can be reused.
Secondly, a "Call Library Function" node can be used to call a C function from a dll. This procedure is shown in the following tutorial, and is useful when code only needs to be called a few times.
http://decibel.ni.com/content/docs/DOC-9069
I hope this helps!
Drew
Drew T.
NIC AE Specialist

Stepper motor 8SMC1-USBhF and power meter PM100D: how to merge the two VIs into one and make a XY graph of the current position and the power

Hi there,
I have a stepper motor (8SMC1-USBhF) and a power meter (PM100D). I want to measure the power while the stepper motor is moving, and obtain an XY graph with the current position on x axis and the power on y axis. So I just merged the two VIs together by copying and pasting, and created local variables for current position and readout value, and changed them to read, and then linked them to the XY graph, but apparently it doesnt work.
So could you please instruct me how to merge them and obtain an XY graph appropriately?
Attached are the two VIs.
Thank you!
Attachments:
PM100D Simple Example.vi ‏42 KB
StepperMotor.vi ‏97 KB

Hello,
The XY graph requires that each datapoint on the X axis corresponds to a datapoint on the Y axis. To create XY data you will need to creat two arrays of X data and Y data with the same amount of elements then cluster the two arrays together to wire into the XY graph. As an example take a look at the VI snippet below. Is there a specific error being thrown?
Regards,
Isaac S.
Applications Engineer
National Instruments

Stepper Motor with Linear Stage (Position Control and Data Acquisition)

Hey All,
So.. I've attached a stepper motor to a linear stage and so far it's working pretty well. Using a stepper motor driver from Pololu, I've simplified the control of the motor by just using output pulses from the counter output of a PXI-6143. I've tested the motor using the Pulse Train examples in LabVIEW and all is working well.
My goal now is to allow the user to collect data from a pressure sensor, attached to the linear stage, every X number of steps. From what I can tell so far, and please correct me if I'm wrong, the motor step movement is synchronised with the pulse train input. That is to say, if I give the motor a 200 step pulse train, the pulse train ouput is completed at the exact time the motor has moved 200 steps. From this, I've created a VI that moves the motor X steps, aquires the data point, and then repeats this process for the required amount of data points. The problem with this is that the motor movement is not continuous; it stops for a split second to take the data point.
How can I have labview ouput a pulse train of say 1000 steps and record a data point every 50 steps?
Two ideas that came to mind were:
1. Use the counter input port on the card to count the pulses being sent to the motor.
2. Use an encoder connected to the motor shaft.
I wanted to stay away from theses ideas though since they require resources from the DAQ card.
Thanks,
Ryan

Hi Ryan,
Just to cover all the bases, what version of LabVIEW are you using and can you attach your VI? Initially your ideas sound like they should work, do you expect to be nearly maxing out the DAQ?
Thank you,
Deborah Y.
LabVIEW Real-Time Product Marketing Manager
Certified LabVIEW Architect
National Instruments

Difference between open loop and closed loop mode when finding the index on a stepper motor axis.

I have a test system which uses a PXI-7352 motion controller with a MID-7602 stepper motor drive to position a 200 step/revolution stepper motor with an attached 1000 line encoder. The MID-7602 is configured for 64 microsteps per step, giving 12,800 microsteps and 4000 encoder counts per revolution. The system is controlled using LabView 7.1 and NI-Motion 6.13. To home the motor, I've defined a sequence in MAX which first finds the reverse limit switch and then moves forward to the encoder index pulse.
When finding its reference in open loop mode, the system its reference at the same microstep each time. When findings its reference in closed loop mode, the system finds its reference somewhere within about a 20 microstep wide range.
I hope somebody out there with more experience with this controller can describe the difference between finding the encoder index in open loop mode and finding the encoder index in closed loop mode.
Thanks,
Mark Moss

Hello Mark,
I suspect that the behavior you are experiencing has today
with the resolution difference between your encoder and your
microstepping. Let me illustrate with an
example:
Example Setup
Stepper
Motor
NI
Motion Controller
10
steps per encoder pulse (Every 10 steps the controller gets an encoder
pulse, therefore positions 0-9 all appear to be 0 to the controller)
The
system has a maximum of 3 pull in moves set
Open Loop Scenario
The
system is commanded to go to position 9.
The
controller sends out 9 pulses.
The
motor moves to position 9.
Closed Loop Scenario
The
system is commanded to go to position 9.
The
controller sends out 9 pulses.
The
motor moves to position 9.
The
controller checks its position and sees that it is still at position 0
because it has not received an encoder pulse.
Because
the controller thinks it is still at position 0, it sends another 9 pulses
as its first pull in move.
The
motor moves to position 18.
The
controller checks its position and sees that it is at position 10 because
it has only received one encoder pulse.
Because
the controller thinks it is still at position 10, it sends1 reverse pulse
as its second pull in move.
The
motor moves to position 17.
The
controller checks its position and sees that it is at position 10 because
it has only received one encoder pulse.
Because
the controller thinks it is still at position 10, it sends and additional
single reverse pulse as its third pull in move.
The
motor moves to its final position of 16.
I believe something similar is happening with your
application as it looks for the index pulse.
Because the controller does not compensate for what it perceives as
position error in open loop mode, the motor always goes to the same commanded
position. In closed loop mode, the motor
is bouncing around inside the single index encoder pulse trying to find a
certain position.
There are several recommendations I can make towards
correcting this behavior. These
recommendations are in order of effectiveness:
Setup
your system so that there are more encoder counts per revolution than
steps per revolution.
Turn
off pull-in moves by setting them to 0 in MAX.
Use an
open loop configuration.
Play
around with the pull in window in MAX.
Regards,
Luke H

Stepper motor closed loop control using 7344

I have some question about the closed loop control of stepper motor using 7344. From the NI website I got that closed loop steppers work differently from closed loop servos. Instead of adjusting the output on each PID iteration like a servo system, closed loop steppers will do a pull-in move at the end of a move to adjust for any difference between the target position and the encoder feedback. By default, it will attempt the pull-in move three times.
Now for example I got five points: (0,0) (10,20) (30 40) ( 31 60) (50 65). The request time interval I set as 10ms. This mean that in the first 10ms, the machine should move from (0 0) to (10 20). But for some reasons the machine can not move to (10, 20), it moves to (8,16) and the first 10ms passed. In the second 10ms, how it move? Move from (8 16) to (30 40), If like this, the following error will accumulated. In practise, it is not accumulated. Is there anyone can explain something about the closed loop control of stepp motor? Thanks a lot!

Requested Interval is additional data for the Position buffer type. The Position buffer type requires a Requested Interval parameter, and indicates the time between contouring data points in milliseconds. For all other buffer types, the Requested Interval parameter is ignored.
The controller will use the closest value it can that is greater than or equal to the interval value you requested. Your time interval must be an even multiple of the PID rate.
For example, given a PID rate of 250 ms, a time interval of 11.2 ms between points is physically impossible, so a call to configure buffer with an interval of 11.2 causes the buffer to actually be configured for 11.25 ms, since 11.25 is the nearest possible interval greater than 11.2. The following table shows valid intervals for each PID rate.
My question is like this. For example I got five points: (0,0) (10,20) (30 40) ( 31 60) (50 65). The request time interval I set as 10ms. This mean that in the first 10ms, the machine should move from (0 0) to (10 20). If it is impossible for the machine to move from (0 0) to (10 20), there are two ways to solve this problem. First is the time interval is 10ms and assume it move to (8 16) within 10ms and take (8 16) as the start point and (30 40) as the end point of the second move. Second is extend the time interval, assume it takes 14ms to move from (0 0) to (10 20). After 14ms, the (10 20) is the new start point for the second move. I think it works as the second ways.If not, the following error will accumulated. Is there anyone can explain something about the closed loop control of stepp motor in detail? Thanks a lot!

Error -70006 when using stepper motor in Open Loop Mode

We have a LabView program that runs a test in which a table is rotated. At the end of the test, the table is supposed to rewind. Instead, the following message appears:
Error -7006 occurred at Read Trajectory status.flx
Possible reasons: For FlexMotion, an invalid axis number or other resource ID (Vector Space, Encoder, I/O Port, and so on) was used.
We have the PCI-7344 interfaced with a UMI-7764 and a Zeta4 that is controlling a ZETA83-93 stepper motor. There is no encoder in the system for this motor. The stepper loop mode was originally set to closed loop but the motor behaved erratically (moving multiple times when instructed by MAX to move once). The stepper loop mode was changed to open loop and the m
otor started behaving properly, but that is when the error message starting appearing. Are there any other configuration settings, such as Primary Feedback (which is now set to Encoder) or ? that I may need to correct to resolve this error?

First of all I would check a couple of things. The reason why your motor was not behaving properly in closed loop stepper configuration is most probably related to the fact that you don't have the encoder counts per revolution adn the stepper steps per revolution properly configured for your motor. Please follow the instructions found in the following knowledgebase:
http://digital.ni.com/public.nsf/websearch/29edb56146f274a386256a1c00624310?OpenDocument
When you configure an axis in closed loop stepper mode, we use the information of the encoder to generate the adequate velocity profile. If the encoder counts and steps per revolution are not properly matched, then the motor will not execute the moves properly. What you need to do is the following:
1.-Configure your axis as closed loop stepper in the Axis Conifguration Section in MAX.
2.- In thbe 1-D Interactive Basic panel, kill the motor and click on reset position.
3.- Manually rotate the motor one revolution.
4.- In the advanced panel on 1-D interactive, read the Encoder Value. This is the value you should enter on the Axis Encoder Counts per Revolution under Axis Configuration. Press Apply.
5.- Go back to the 1-D interactive Basic Panel. Change the bullet selector unbder Stepper Loop Mode from Closed to Open. Run a 10000 step move.
6.- When the move is finished, read the encoder counts from the advanced panel. This will tell you how many counts you have in 10000 steps. You can now calculate the stepper steps per revolution by the following formula:
(# of steps per count)*(# counts per revolution) = # steps per revolution.
Your motor should work fine in closed loop stepper configuration now and the error should not appear. Please make sure you are using the latest version of our driver (Ni-Motion 6.0.1)to avoid older firmware errors and fixed bugs from affecting your system. You can get it from:
http://digital.ni.com/softlib.nsf/webcategories/85256410006C055586256BB9002C16DF?opendocument&node=132070_US

What's the type of control for stepper motor?

Hi,
I'm using PXI-7358 controller, UMI-7774, Industrial Device NextStep third part microstepping drive, stepper motor SANYO-DENKI (type 103-8932-6421, NEMA42) and incremental encoder 3600 ppr (14400 count/round). I want to say what's the NI onboard type of control used to control motor step position? In the case of servomotor the type of onboard control is a PID, in the case of stepper motor what's the type of control? What's the MAX parameters to set for the project specific (overshoot, settling time, rise time etc..)? What does it means "Pull-in Window" e "Pull-in Tries" in MAX? So, it's possible control the stepper motor with user's algorithm control, excluding the onboard control?
Thanks for your patience,
Best regards
Lorenzo

>
Matt wrote:
> Go to SE24. Type in cl_dd_document and press enter. Select the methods tab. Look for the method "CONSTRUCTOR". Double click on it. Click on SIGNATURE button. The types of the parameters are clearly seen.
>
> matt
TYPE and VALUES OF TYPE -- different things. For example, TYPE C -- CHAR. VALUES of this: A, B, C, ..., 1, 2, 3.
Thus, return to question.
TYPES: sdydo_attribute(50) TYPE c
TYPE: sdydo_attribute
VALUES: ???
May be, value ABRAKADABRA correct?

How to run a stepper motor for an amount of time determined by a feedback?

Hi,
I'm using labview 8.6 and I'm trying to use a ORIEL sterppermike with a 20025 controller.
I have implemented a VI that has a stack of commands in a for loop. Every loop has the following sequence of commands:
command 1) start the stepper
command 2) wait for feedback to reach a certain number
command 3) stop the stepper
command 4) wait a certain time before starting again
To start the stepper I use a subvi from the example folder:
C:\Program Files\National Instruments\LabVIEW 8.6\examples\DAQmx\Counter\generate pulse.llb
--> Gen Dig Pulse Train-Continuous.vi
I don't understand why the sequence get stuck at command 1 and never gets to command 2.
I attach the vi for completeness.
Thanks to anyone that will help,
Matteo
Attachments:
SRL Matteo2.vi ‏121 KB

Hi, I'm not sure whether this is the right place for asking this question or not and if not, then please guide me to the right place for asking such question. Thanks.
I have a stepper motor with the Vector Network Analyzer Anritsu MS4623B. I got it up and running with the LabWindows/CVI. I need to sync the motion of the stepper motor with my data acquisition. Basically, I have to acquire the data in a way that in the start the motor will come back to home position wherever it was and then I want to move it to the specific position to do the measurements (For Example, I want to measure the 40 GHz channel between 1m to 3m movement of a stepper motor on a linear positioning system (ball-screw)). I've interfaced the VNA with the PC using LabWindows/CVI and I've controlled the clockwise and anti-clockwise movements of the stepper motor using LabWindows/CVI but I'm unable to sync the system in a way that whenever I run the code, the motor will run back to the home position and then move to a specific measurement start position and move till the end position and in between do the increment and stop and acquire the data. Is there any one to help me please? It's really urgent please and I'm doing this thing first time so please guide me in a proper way. I shall really apprecaite that and will be highly obliged with you. Thanks for your time and concern.
Thanks and Regards.
Attachments:
VNA & Motor Control User Interface.jpg ‏199 KB
VNA & Motor Control User Interface.jpg ‏199 KB
VNA & Motor Control User Interface.jpg ‏199 KB

Shared variable engine modbus - Getting a stepper motor to work

Hello,
I am new to Labview. I need to trigger the stepper motor to start by allowing current to run through the motor. This can be done by turning bit3 on the16 bit register 007Dh. The register address has been attached below as 007Dh.png so please view that before you go any further to have a better sense of what i'm trying to accomplish. Please also note that the decimal conversion of 007Dh is 125.
So my question is, how do you turn on bit3 of that 16 bit address? How would you translate the holding registers from the manual
into a 5-6 digit data item that labview can understand?
(Please refer to this page for the types of data items: http://zone.ni.com/reference/en-XX/help/371618J-01/lvmve/dsc_modbus_using/)
Attachments:
007Dh.png ‏17 KB

Hello Frank,
Thank you for the help with the modbus slave simulator, everything worked just fine. However I still can't get the real device to work with labview. I am wonding if you can answer a question for me as I am not sure I can do what I want with the equipment I have.
What I need to do is turn a stepping motor on and off using Labview as well as get Labview to record the speed and position of the stepping motor over time. I have an AED-AD stepping motor driver made by Oriental Motors. I have attached the user manual in case you need to see it. I am conecting the motor driver device to a computer using an RS485 converter, which oriental motors suggested I use for the purpose of using Labview to collect data.
My question is, will this setup work for what I need to do, or do I need some kind of data aquisition device as well?
After I solve this, I will be adding three more motors to the Labview program monitoring the same thing as well as some pressure sensors.
I have just finished my second year in the engineering program and have been hired as a research assistant to solve this problem so I am completely new to Labview. If there is anything else you can suggest to read or someone I can talk to to get more information about this type of control systems I would be very greatful. So far, I have been reading as much stuff as I can find about Labview on the internet but I have not come across anything that helps me figure out if my setup is appropriate for collecting data in Labview.
Thank you again for your help and advice! I hope you are having a great weekend!
Kind Regards,
Meesh
Attachments:
HM-60065-3E.pdf ‏1787 KB

Stepper motors to move antenna

Hello !
My name is Paul, I am from Romania.
I need some ideas on how to make a project.
I need to control a fake antenna (I am saying fake because it won't be attached to anything, just for display purposes) using LabView.
The antenna will be very small (about 15cm in diameter and made from light materials) so I won't need powerfull motors. I was thinking about hooking up 2 stepper motors (maybe from a printer) to some sort of controll board and to a PC with LabView. The antenna needs to move 360 degrees on a round base and 180 degrees of elevation.
I found a video on youtube that is a lot more complicated and a lot heavier than what I need, but should give you a better perspective of what I am talking about.
http://www.youtube.com/watch?v=vihdaseWNQw
I need the cheapest way possible (because I am on my own budget).
Thank you !
Solved!
Go to Solution.
Attachments:
antenna_base.jpg ‏201 KB

I drew a quick sketch hoping you will understand better what I'm trying to do. I only connected one stepper motor for a test run. I opened the "Arduino Stepper Motor.vi" in LabView and tested on stepper motor 1. It didn't move (but maybe I didn't set it up correctly in labview). The setup was made exactly as the picture I attached.
The stepper motor 1 (which controls elevation) has to move the antenna 90 degrees. So I'm guesing I have to divide the 90 degrees to 1.8 degrees (the motors deg/step) to find out how many steps I need in total. And the same for stepper motor 2 (360 / 1.8).
So I need a .vi that automaticaly transforms the degrees which you set the elevation and azimuth to steps for the stepper motors. And when I press "Start" the two motors move to the desired position. I also need a "Reset" button to automaticaly come back to the original position (which will be 0 degrees azimuth and 0 degrees elevation - let's say the magnetic north) each time I run the program.
Attachments:
setup.jpg ‏1120 KB
setup_drawing.jpg ‏1102 KB
EasyDriver-Stepper-Motor-Driver2.png ‏50 KB

Can I get a stepper motor to work useing an AO channel on a DAQ card?

Can I get a stepper motor to work useing an AO channel on a DAQ card? I know its a very low currentl coming out, but what Im going to be using the motor for is very weak. Has anyone tried this before? I certinaly dont want to spend 2,000+ on somthing that will be basicly moving a string on some pullys back and forth.
Thanks,
-Mark

Greetings, Mark
You can certainly use the A0 as a driving signal, but you need to amplify the current considerably to run a stepper motor. Additionally, most stepper motors require multiple phase driving signals to work properly. If I understand your application, I believe a stepper motor is a bit of overkill...you only want to use them when precise positioning and repeatability is an issue. You might look at some of the linear actuators available.
Eric
Eric P. Nichols
P.O. Box 56235
North Pole, AK 99705

Parallel port stepper motor

Hi All,
I followed the instructions on this link
http://zone.ni.com/devzone/conceptd.nsf/webmain/72C6FC6CE4AD4D1386256B1800794596,
in an attempt to drive a stepper motor. I used the VISA method and grounded pins 12 and 13 and tried to obtained some output with no success... Is there any other pins that I should take care of other than the data pins 2-9 and pins 12,13. I am using a mulitmeter to measure the voltage of the output data pins. I also tried the register method by using the outport.vi but also without any success. Both vis I written does not give me any errors. I tested the parallel port on a printer and it seem to be working fine. Perhaps I had missed out something? An early thanks for anyone who could enlighten me on parallel
port output.

Hello;
You don't need to worry about any other pins than the ones you described. The best way to trouble shoot that is by using a Scope to catch the output line of the parallel port and to have a really simple VI just writting a high level state on that line.
Regards
Filipe A.
Applications Engineer
National Instruments

Trouble positioning a stepper motor accurately

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