IO ports on a NI-700/900

bob

Hi there,

I've always been using the IO ports for input only, i.e. by connecting potential free output contacts of a relay and this works great. As the NI-700 and NI-900 don't have relay outputs, I thought I ask how people use them as outpus? There is already a +12v out with them, can a relay be connected? Anyone have a schematic handy?

TurnipTruck

To use them as outputs, connect you controlled device between the +12 and one of the four outputs. The four output terminal provide the ground to enable your device. In the off state, your device stays at +12 with respect to the power supply ground. Refer to the manual to see how much current is available. I have used this arrangement to control electronic relays with success. I wouldn't recommend using these ports to directly control a mechnical relay.

This configuration is often referred to as "open collector" if you would like to research it further.

bob

What is the difference between electronic and mechanic relay? Is using a very small "control" relay (like on the AXC-REL8 card or even smaller) which on its turn drives a larger relay for triggering the load? Is usng an UPC20 ok? Thanks much!

TurnipTruck

With a mechanical relay, your control voltage provides the power to an electromagnet that pulls in the contacts to energize your controlled load.

An electronic relay uses a semiconductor to to complete the circuit to your controlled load. Generally the control side of electronic relays is very high impedance, meaning it requires almost no current to activate the relay.

bob

I can remember I once tried to use a small (mechanical?) relay wired to an ouput port and it didn't work. The relay itself was ok.

DHawthorne

Not all relays are created equal either ... some do not have a diode across the coil, and when the relay releases, the inductive kickback can scramble your IO port. The current capacity of the IO port is 200mA (not very much, but below the coil draw of many small relays), so as long as you are under that and have that diode in place (your own or an integral one) you should be OK. Otherwise you need to buffer it with some solid state device.

bob

Thanks! What value should that diode be? Simply wired across the coil? And the coil to be wired to +12v and the respective number of the IO-port connector?

TurnipTruck

Diodes don't go by value, but by part number. A 1N4004 would be fine. It would go in parallel with your relay coil with the stripe end toward the +12 volts.

bob

Something like this?

Hedberg

Instead of using a relay and a diode, you might consider using one of the many little relay boards that are available. Every board I've seen is "diode protected." Makes for a cleaner installation, in my opinion.

A while back I went on a soldering kick and built a bunch of 2 relay boards from kits. Worked out to be relatively inexpensive -- $10 to $15 each. Also, someone here posted a link to a supplier who sold multiple relay boards on a big PC board and you could cut off as many relay boards as you wanted. Interesting concept and if I recall correctly, it came out to not much money per relay. These, if I'm not mistaken, also had "diode protection." I thought it was Dave Hawthorne who had recommended these multi board relay boards, but I haven't been able to find the post that mentioned them -- perhaps I'm mistaken.

EDIT:

Well, I searched a little more and found a previous thread about the use of IO ports. Two very nice posts (among others):

Vining posts about how to wire up discrete relays using a diode for protection. Note that in the diagram he posted that the "transistor" serves the purpose of the IO port. If you look at that diagram, it's essentially the same as the diagram that Bob posted in this thread. See: http://www.amxforums.com/showpost.php?p=23842&postcount=24

Also, see Dave Hawthorne's post about the Altronix RB610 which includes six separable relay boards. A quick google search indicates that these things are available for $30 or less, which comes out to not much per relay. See:http://www.amxforums.com/showpost.php?p=23859&postcount=28

I'll post a message to that thread to bring it to the surface.

jimmyw

Adding a transistor and a current limiting resistor does not add much cost (pennies) and much space, and it helps eliminate the chance of inductive flyback killing your io port, and possibly zapping the motorola chipset(bad news), whenever we do relay via io we do just that.

Jimmy

[Deleted User]

Possible relay options for I/O port.

The following is an ultra-low current relay that should work well with an I/O port. (Disclaimer: This is not an AMX endorsement for any product, just a personal favorite. Use at your own risk.)

http://www.elkproducts.com/products/elk-924.htm