1200 series DAQ boards digital I/O

[David Lewis]

I'm using an SCXI-1200 device and I am trying to understand how to use the
digital I/O ports. I guess I'm a software guy. To teach myself something
about hardware, I want to actually build a circuit to make four I/O pins on
the SCXI-1200 control the windings on a tiny stepper motor I have. I'd like
to optically isolate the SCXI-1200 from anything that happens on the motor
side of the circuit. I'm just going around in circles.

NI documents state the the digital I/O on 1200 series cards is the 82C55A
chip. I can't seem to understand or reconcile what NI says in the SCXI-1200
manual and what the datasheet for the 82C55A says, especially where there is
an apparent contradiction. I have three main questions. How much current
can the d
igital I/O pins on the SCXI-1200 source and sink? What is the
total current all pins at once can source and sink?

Can anyone suggest where I can look for information that goes beyond the one
page in the NI manual (SCXI-1200 manual 3-16, or in the PCI-1200 manual,
page 3-17) that illustrates how to connect the digital I/O lines to real
world devices?

Thanks

[Process_Systems]

I have an old VI that I used quite some time ago. It has some extra explanation in the "documentation" section of the VI Properties section. Hope it helps.

Dave

[Doug_Norman]

I don't have all the specs you are looking for, but I have used this board a lot in the past (distant past), so I can give you some basics. First of all, I wouldn't think of the digital lines as being either fast or as supplying power. Think of them as relatively slow TTL outputs. That means they wouldn't be my choice for running a stepper motor. They aren't going to sink much power and they are typically software timed. For specs you could search ni.com for "PCI-1200" and find the specs for that board. They should be the same as your SCXI-1200 since they share the same architecture. If I remember correctly, the digital lines are only going to be good for a couple mA each.

Regards,
Doug Norman

[AnujD]

David,

The source/sink current for the DIO lines on the SCXI-1200 is 2.5/-2.5 mA. This value should be per DIO pin. I am assuming the apparent contradiction that you found between the SCXI-1200 User Manual and the 82C55A is based on a typo on page A-9 of the User Manual. The digital logic levels chart should read .4 V for the Max Output low voltage. Additionally, the output logic low/high voltage (on 3-15 of the SCXI-1200 manual) cite the current at which the measurment was taken, but these are not max values.

I have also attached some other resources that might help you out.

This link dicusses how to use all 24 lines on your SCXI 1200. I know you only need to use 4 at the moment, but it may come in handy later.

http://digital.ni.com/public.nsf/websea
rch/862567530005F09C862565BF005086C1?OpenDocument

This link will show you how to increase the amount of current that the DAQ board can source or sink.
http://digital.ni.com/public.nsf/websearch/950EA9F3E05498748625629800585714?OpenDocument

This link shows you how to optically isolate your DIO lines:
http://zone.ni.com/devzone/learningcenter.nsf/webmain/9C97E62517E8184386256E21007A5310

Hope this helps,
Anuj

[David Lewis]

thanks for your reply.

I'm planning to control the position of an air vent or damper controlling
the forced air intake on a sawdust fired pottery kiln with the stepper
motor, so it will only have to move to known positions within less than one
revolution of the shaft, so I felt that even an SCXI-1200 that had to also
control a chassis with other modules measuring the temperature, oxygen
content of the atmosphere inside the kiln, and adding sawdust to the burner,
could put out four digital signals out its I/O lines to control the position
of a stepper. I guess I'm thinking of using a stepper for a job like what
they made servos to do.

I think I'm gaining confidence that the I/O lines on this board can sink or
source 2.5 mA per pin, and all pins could be used.

I reached a point where all the datasheets and manuals weren't making any
sense anymore when I put my question to this newsgroup, and I am grateful
for the replies. I got my SCXI chassis and modules communicating with MAX
and Labview, and I was having so much fun programming with Labview, then I
had to stop programming and figure out how to build some what seem like
completely elementary circuits and was not getting anywhere it seemed.
Everything seems more manageable now.

"Doug Norman" wrote in message
news:50650000000500000078BA0100-1079395200000@exchange.ni.com...
> I don't have all the specs you are looking for, but I have used this
> board a lot in the past (distant past), so I can give you some basics.
> First of all, I wouldn't think of the digital lines as being either
> fast or as supplying power. Think of them as relatively slow TTL
> outputs. That means they wouldn't be my choice for running a stepper
> motor. They aren't going to sink much power and they are typically
> software timed. For specs you could search ni.com for "PCI-1200" and
> find the specs for that board. They should be the same as your
> SCXI-1200 since they share the same architecture. If I remember
> correctly, the digital lines are only going to be good for a couple mA
> each.
>
> Regards,
> Doug Norman

[David Lewis]

Thanks for your reply.

I saw that page A-9 and just thought this makes no sense at all, so it
wasn't the contradiction I had in mind I was thinking about page 3-15
where it lists the "Darlington drive current" as -4.0mA maximum and thought
this seems to be somewhat greater than -2.5mA, and I was wondering what was
so special about current that was driving Darlingtons as opposed to other
devices. I've read enough bits of manuals and the knowledgebase and various
datasheets that I'm not sure exactly where I found apparent contradictions
anymore. I hear "them" say TTL can sink ten times what it can source, then
I see the 82C55A is CMOS designed for the 8008, then I read its TTL
compatible, then I become convinced it really only can source and sink 2.5mA
per pin, I'm wondering if the output is totem pole or open collector or
what, and I wonder what it all really means as I'm about to switch on the
power to a circuit connected to these pins..... Then, I don't turn the
circuit on, and I go back to the drawing board before I drive all the magic
smoke out of all the chips in my vicinity. .

The main problem is I only know enough about designing circuits to interface
a SCXI-1200 to the "real world" to be dangerous to my own equipment, and I
am trying to pull myself up by my own bootstraps surrounded by all the
reference material I can dig up.

Things are looking better now. I was wiring up a prototype with the idea of
running the SCXI-1200 I/O lines into an LS244 tristate buffer first, and I
was working with four TIP122s to handle the current draw of the proto
stepper motor I've got, and I was considering putting optoisolators in
between. I've got all these 4N25s in a drawer, and I only have one
SCXI-1200. I thought the SCXI pins could drive an LS244 which could drive
the 4n25 which could drive the TIP122 which could handle the windings on the
stepper.

I checked the links you suggested - the optoisolation link ends up at a .zip
file you download which then turns into a DOUT.wmf file that my machine and
myself don't know what to do to view. Do you know how to read this file?
It has a tantalizing and timely (for me) title, as I am working with the
optoisolators tomorrow.

thank you very much for your help.

"AnujD" wrote in message
news:50650000000500000098BA0100-1079395200000@exchange.ni.com...
> David,
>
> The source/sink current for the DIO lines on the SCXI-1200 is 2.5/-2.5
> mA. This value should be per DIO pin. I am assuming the apparent
> contradiction that you found between the SCXI-1200 User Manual and the
> 82C55A is based on a typo on page A-9 of the User Manual. The digital
> logic levels chart should read .4 V for the Max Output low voltage.
> Additionally, the output logic low/high voltage (on 3-15 of the
> SCXI-1200 manual) cite the current at which the measurment was taken,
> but these are not max values.
>
> I have also attached some other resources that might help you out.
>
> This link dicusses how to use all 24 lines on your SCXI 1200. I know
> you only need to use 4 at the moment, but it may come in handy later.
>
>
http://digital.ni.com/public.nsf/websearch/862567530005F09C862565BF005086C1?OpenDocument
>
> This link will show you how to increase the amount of current that the
> DAQ board can source or sink.
>
http://digital.ni.com/public.nsf/websearch/950EA9F3E05498748625629800585714?OpenDocument
>
> This link shows you how to optically isolate your DIO lines:
>
http://zone.ni.com/devzone/learningcenter.nsf/webmain/9C97E62517E8184386256E21007A5310
>
> Hope this helps,
> Anuj