Is stepper output from 7344 board truly a sinking output?

[Rob Wright]

It appears as though the stepper output needs to be referenced to ground (sourcing) as opposed to being referenced to +5Volts (sinking). The manual doesn't include a schematic for the stepper output circuit. Does anyone know where I can obtain a copy of the schematic for the 7344 board, or at least a simplified diagram of the output circuit? I am trying to interface the 7344 through a UMI to a Compumotor A series stepper motor driver. Any help would be greatly appreciated! Thanks in advance, Rob.

[Rob Wright]

Never Mind... Got it working. It truly is a sinking output. Had my scope hooked to it without a ground buster. (Who ya gonna call? GROUND BUSTERS!) Phneeeeee...

[Nihil]

Hey, Rob. I have a question: what's a ground booster? I'm trying to see my steps signal on the osciloscope, yet they seem to be a 2.4V signal (see image attached) instead of 5V.

the motor's driver works alright since i have a buffer that reads the voltage as a logical 1. But the signal I see on the osciloscope is my problem... do you have any idea why this could be?

[AnalogKid2DigitalMan]

Nihil:

Wow, I think you set a record on the forums for replying to a +10 year old message. Susprisingly, Rob has been here recently though.

A ground buster typically is a cheater cord or plug that does not connect the oscilloscope chassis and internal electronics to the AC mains earth ground. In my book, use of one is a no no- risk of personal or equipment damage. That is what the A-B differential channels mode is for- to take relative floating measurements between 2 points.

Regardless, do you by chance have your oscilloscope input configure to 50 ohms instead of 1Mohm? That would attenuate the signal.

-AK2DM

[Buechsenschuetz]

The waveform you registered seems a little odd. It's quite a long time since I checked the stepper output with a scope but I remember there were clear "rectangular" pulses with no significant slope. Also, the manual says that the stepper outputs have a 3k3 resistor connected to +5V. If there is no significant load on the stepper output, it should rise to almost 5V.

The manual also states that the outputs are O.C. with a maximum sink current of 64mA, I think this is clear enough.

[Nihil]

Thanks both for answering! I think I understand now what the problem must be. I connected the oscilloscope to the 7344 output, and the signal goes to the 5V as it should, yet it's not quite square (I'm attaching a picture).

So I suppose it must be the load connected to the output.

The output signal as it comes out of the 7344 travels through an 8-meter-long cable until it reaches a buffer (74HC541) that then squares the signal and then to the stepper driver. Maybe the loss through the cable is too much for the open collector + 3.3kohm pull up. (?)

So I'll try as it's recommended at this other post http://forums.ni.com/t5/Motion-Control-and-Motor-Drives/how-to-generate-CW-CWW-signal-using-NI-7340-...

thanks again!

[AnalogKid2DigitalMan]

Yes, I thought that was an odd shape too but was in a rush and forgot to call it out. It looked like an effect from capacitance, now that you mention 8 meters of cable, that could be the source of it. There are cables which specifically call out "low capacitance" which is made by paying careful attention to the construction method of the cable.

Also, scope probes used to have a trimmer cap on them which you adjust to show a true square wave when you connect the probe to the scope's built in frequency source.

-AK2DM

[Buechsenschuetz]

8 meters of (coaxial) cable will have a capacitance of 800...1000pF approx. (as a rule of thumb, you can assume 100pF/meter for most coax cables). This could well explain the rising slope. There is no falling slope as the output impedance of the 7344 probably is much smaller in low state and discharges the cable faster than it charges.

The adjustment cap on scope probes is used to correct the shape of the edges of rectangular pulses, there should be no slope and no overshoot. But a misalignment would never cause a slope as seen on the pics. The input capacitance of most scopes is below 50pF.

[Nihil]

Yes, i'm quite sure, the acutal problem is the capacitance. When using lower steps/sec (30k) the signal is ok, but when going to higher freq (130K) the amplitud drops to 2.7V so... yes.

Thank you guys