Isolated Counter Input onSC2345 with FT01

[chefgps]

Hi folks,

newbie question: I need to count frequency, direction and volume from a flowmeter. The device supplies two

channels with square wave signals of 50% duty cycle, an Amplitude of 12-18 V. the two channels are behaving

much like the quadrature encoders of electric drives, i.e. the phases of the signals indicate the direction

of flow. I have a SC 2345 with FT01 modules available for the task which according to my understanding

could do the job, but requires TTL level for the signal. Could somebody provide me with hints on creating a

small circuitry (if possible inside the FT01) which provides optical isolation. I'm thinking of a NPN transistor

driving the input LED of a optocoupler device. However, if somebody has a working circuitry it wouldn't

mean less hassle to me creating and testing my own. TIA for any input.

[Buechsenschuetz]

Most optocoupler manufacturers have application examples on their data sheet. I would recommend using an optocoupler having a TTL compatible output. We are using the HCPL2630 (Agilent/HP) in many applications. I assume that your signals are rather low (i.e. in the kHz range), if the signals are very fast you should check the max transfer speed of the optocoupler.

If you want to drive the optocoupler with an npn transistor you will need an extra supply voltage for the LEDs. If the outputs of your sensor can supply around 5mA you can drive the LEDs directly with a 2.2kOhms resistor connected in series.

[chefgps]

Thanks for the explanation!

To save me some time as a "layman" do you have some suggestions what components would suit the purpose of

adapting a 250Hz - 250kHz signal at approx. 50% duty cycle with 8-12 Vpp. I need the "outside" npn to drive the optocoupler

leds and maintain a high impedance in order not to disturb my puls generating device. A secondary e.g. 5V Vcc for

this purpose is not a problem. TIA for any advice!!

[Buechsenschuetz]

I am not a consultant and therefore I can't supply you with ready-made solutions. However I think almost anyone with some electronics design experience will be able to help you.

You will need a driver for the input LED of the optocoupler of your choice (the HCPL2630 probably will work at 250kHz), and a supply for this circuit - not necessarily 5V. The driver should accept your input signals in the range of 12 to 18V and supply the input LED with a suitable current in the range of 5mA.

Concerning the output, the HCPL2630 has rather high leakage current which might cause problems when interfacing it to CMOS circuits. A pull-up resistor between the output of the HCPL2630 and the supply voltage of the next circuit will help.

[Buechsenschuetz]

You may check application note AN3001 on

http://www.fairchildsemi.com/apnotes/an_bydoc_d3.html

[AA Dan]

Hello chefgps, & Buechsenschuetz,

National Instruments has a beta version of a product called the SCC-CTR01 that you may be very interested in reviewing. If you are interested, please contact dan.hoar@ni.com for more details.

The SCC-CTR01 is an isolation signal conditioning module for the TTL general purpose counter timer signals from the MIO device which can be used in a SC-2345 carrier in the counter slots. The maximum I/O signal voltage levels it can handle are 0-48V. The maximum I/O frequencies are 400kHz. The conditioning provided by this module is very similar to the PCI/PXI-6624 isolated counter timer boards. I think that the SCC-CTR01 module will be a good solution for your application without the hassle of designing, constructing, testing, and debugging your own custom isolation and attenuation signal conditioning.

Alternatively if you need a released solution right away then please consider the PCI or PXI-6624.

Please let me know if you are interested.

Have a great day.

Dan Hoar

Digital & Timing Hardware Project Manager

National Instruments

[AA Dan]

The SCC-CTR01 has been released and can be used for isolated counter timer input/output up to 48 V and 400 kHz.  It is similar to the PCI/PXI-6624 in functionality.

[Kem]

I am in the same boat except when I placed an order with NI they sent me a SC-CTR01.  When I attempt to configure the SC-2345 there is no option for the SC-CTR01, would it be logically the same as the SC-FT01 where there is an option?

I am attempting to keep track of a stepper motor possition with the counter in a DAQPAD-6015, my thought was that I could count either a stream of pluses for up and down or have a direction and a stream of pluses line attached.  I have control of the stepper motor controller and can make the connection how ever it would work the best.  Can this be done and if so what input is the direction and what input is the count input?  Is there a safe way to read this counter asynchronously without getting ironious values while it is in transition?  Is the counter read path double buffered?

Ken or Kem

[AA Dan]

Hello Ken/Kem,

   The configuration of SCC-CTR01 in the SC-2345 is not necessary.  You can select SCC-FT01 or nothing and the product will still function correctly.  As long as your exteral connections to the front i/o connector screw terminals are connected correctly per manual pages 4-8, you should be fine. 

In MAX create a

[AA Dan]

Hello Ken/Kem,

   The configuration of SCC-CTR01 in the SC-2345 is not necessary.  You can select SCC-FT01 or nothing and the product will still function correctly as long as your exteral connections to the front i/o connector screw terminals are connected correctly per manual pages 4-8, you should be fine.    

In MAX, right click on data neighborhood and create a DAQmx task / global channel / or traditional DAQ virtual channel for the DAQPAD-6015 for a counter channel.  In DAQmx there is a position measurement channel that you may be able to use depending on the application of your stepper motor application.  Otherwise if you're going to write your own algorithm to figure out position based on count you'd configure the channel as a count edge channel and go from there.

I hope that helps out.  Have a great day.

Message Edited by Sparky on 02-28-2006 04:58 PM

Message Edited by Sparky on 02-28-2006 04:58 PM