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Minimum Current Requirement for M-series Counter

I have a PCIe - 6259 board and a SC-2345 signal conditioning unit.  I am trying to connect encoder output to a SCC-CTR01 module but can only supply 0.5 mA.  I know that the module must have at least 2.2 mA to switch to high, so I'm wondering if the direct counter connections through the terminal block have the same type of restrictions.  I have tried connecting the encoder output to these terminals (37 and 3 on the terminal block) but am not able to get any counts.  So is there a minimum current restriction on the on-board counters themselves?

Also, what should happen theoretically if there was not enough current supplied?

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Hi jbailey,

Does the encoder supply a TTL (0-5V) signal? The card itself should be able to count edges from a source with at less than 250 micro amps, typically 100micro amps is enough. Check out the specs on page 6.  If you hook up your encoder to terminals 37 (PFI 8) and 3 (PFI 9) and run test pannels from Measurement & Automation Explorer (MAX) on the individual channels, can you count edges from there? How are you currently trying to count edges and what programming environment are you in?

If there wasn't enough current supplied, then the voltage would sag and depending on how far it would not be TTL compliant. If this was the case then the edges would not be counted.

Hope this helps, let us know if it doesn't count edges in MAX.

Andrew S.

National Instruments

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Yes, the encoder is supplying a TTL signal.  I looked at those specs and page six seems to apply to analog output and I don't see anything regarding minimum current.  Are these the same specs as the counter?

I should have mentioned that I'm using a Linux-based real-time control system so MAX is not available to me.  I have in the past used a third-party (Comedi) driver but that does not work with the counters on the DAQ card.  I am now trying to use the NI-DAQmx version 8 driver on a 2.6.15 kernel for Fedora Core 5.  I was able to get this driver to work based on another thread in this forum.  In order to count edges I am using an example script that was installed with the driver.  I believe it is /usr/local/natinst/examples/ansi_c/Counter/CntDigEvents/CntDigEvents.c (sorry I am not on the Linux machine right now to double-check the file, but I know the .c file was CntDigEvents).

I originally tested the counter by connecting a function generator to pin 37 and setting it to send 2V pulses and was able to count these with the example script very accurately.  I have since tried attaching both phase A and phase B of the motor encoder to pin 37 and running CntDigEvents with the motor turning and neither setup produced anything but a count of 0.  I have verified that the motor encoder is sending out 5V pulses with a digital oscilloscope and by using the digital inputs on the 6259.
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Hi jbailey,

Where are you connecting the ground signal of the encoder? It should be on a digital ground. Sorry about the incorrect page, the table I'm reffering to is the Electrical characteristics on page 8. When you say the encoder can only source .5ma, do you mean .5ma per line (A & B) or total?


Andrew S.

National Instruments

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