Isolated Counter Input onSC2345 with FT01

[Logan_K]

I would like to add one caveat to Sparky's reply. Although you can configure a position measurement easily in software, the SCC-CTR01 only exposes the base counter signals (Gate, Source, Out). With only these signals you cannot use the up/down functionality of the counters, which is a requirement for most quadrature encoders. On the E-series device, this Up/Down functionality is exposed through the DIO lines P0.6 or P0.7, depending on which counter you use. If you need an isolated input for these lines, then you can use a SCC-DI01 isolated input module.

Refer to this knowledgebase for more information:
http://digital.ni.com/public.nsf/websearch/86EBA05BC4D586AD86257082007C5744?OpenDocument

Regards,
Logan K

[AA Dan]

I had a couple other notes after thinking about your note...

1) It sounds like you're driving the stepper motor and then looking for feedback on position.  Monitoring the control lines is not an optimal way of getting feedback on position, this is tyipcally done as Logan said with a quad encoder for angular position (and quad encoders can be monitored with SCC-CTR01 + SCC-DI01 for E-series like DAQPADs) or LVDT, string potentiometer, etc.,. for linear position.  My thought is that instead of monitoring the control lines with the SCC-CTR01 you could just drive the control lines with the SCC-CTR01 OUT signal and keep track of the pulses you've sent to the stepper motor in your LabVIEW VI.  I think that this would be just as effective as monitoring the control lines.  Either way, you're not getting an actual position back, but relying on everything to 'just work'.  If you really want the position feedback, then you should have separate sensors measuring that instead of relying on the control lines.  That way if say you send pulses to the stepper motor but the load is too heavy and so there is no actual rotation, a quad encoder would return no change in position... so you'd know that something was wrong. 

2) the DAQPAD-6015 counter / timer functionality over USB is all interrupt handled, so a little slow compared to the PCI/PXI E-Series and M-series.  You should just write up a quick LabVIEW VI and test performance to ensure that your application needs can be fulfilled with this device.  I think that the update rate is something like 500Hz for buffered input measurements like monitoring frequency of a pulse-train, or continuously counting edges, etc.,..  For operations like generating a pulse train, this doesn't require much I/O over the USB bus and so once the task is started there is little dependancy on interrupts (only if you change frequency or pulse width, etc.,. during operation for example).

Hope that helps out.

[Kem]

Guys

With regards to the counter operation I have solved it as follows:  I placed the SCC-CTR01 into slot 20 of a SC-2345 and configured it as a SCC-FT01 within the DAQ assistant.  Ctr0 is the count stream and is attached to the SouRCe (SRC) input.  A wire connection is made from green connector terminal block pins 41 to 46.  This connects the input called Gate on the SCC-CTR01 to the counter direction control.  This can’t be done in MAX because the SCC-CTR01 device is not selectable.  The direction is then selected as an external input.

This arrangement works well if I create a VI with the counter’s DAQ assistant and a display of the counter value.  If I add in other logic as in temperature current strain and digital IO with another DAQ assistant I then get errors from the counter display.  With this counter in hardware it is difficult for me to see how it can accumulate errors or how this other DAQ assistant can cause the counter to malfunction.  Does anyone have any ideas as to what to do?  Is there a way I can force the software to sample all the inputs is sequence?  I feel this may be the problem.

Ken 

[AA Dan]

Hello Ken,

I'm afraid that you may not be on the right track from the message you've sent below.  If you send me your email address, I'd be glad to take the discussion off line, perhaps give you a call.

I was concerned by several comments you made below.  For one, you definitely don't need to configure the device in MAX as an FT01.  It just isn't necessary to configure digital SCC devices at all.  Secondly connecting between pins 41 (PFI4/CTR1Gate) and 46 (PFI11/AI Hold COMP) on the SC-2345/2350 carrier's 42-position screw termial direct connections into the MIO device (see 2345/2350 manual pages 2-1, 2-2) doesn't seem necessary, and instead seems like an extraneous step that could be handled in a different, more elegant way.  If this routing is necessary than (which I can't think of the need) since these pins are PFI you can probably just use the signal routing properties for your task in your LabVIEW application.  The third thing sounds like a DAQmx programming question.  You cannot mix certain task types into a singal task.  If you're using separate tasks, then I think that probably what you're encountering is the issue with USB being an IRQ based bus.  You're probably attempting to do too much at the same time.  USB is great for streaming data through in one direction or the other.  For example sampling continuously with AI channels you can push lots of data from the MIO through the USB port to the mcu of the PC.  But latency of commands in USB is very high and so one point operations are very slow.  I'm not sure how you created your AI VIs but if you are doing 1 point software timed then you're using interrupts each time you get a point.  As you try to do that faster and faster, you're eventually going to exceed the rates (inverse of latency) that your PC can talk over the USB bus.  You should instead configure your tasks to be hardware timed continuous so that you can stream data faster w/o as many interrupts.

If you'd like to discuss offline, please let me know.  Or if you'd prefer you can probably contact NI's AE support group free of charge to get more assistance with your setup in MAX, VIs, wiring, etc.,.

[Kem]

Sparky

How can I send you my email etc in private?

Ken

[AA Dan]

I'm not sure how to keep things private, so please email me at graphicaltmllc@yahoo.com and then we'll pick up in that forum.