Mesa StepGen ouputs with Yaskawa servos

So I've been finally able to connect my Yaskawa servos to my 7i85s but I'm getting mixed results: it seems like the pulse train is not reliable and sometimes the servo drive does not recognize the pulses.

After taking a closer look at the 7i85s and assuming I can read correctly the datasheet of the ISL32174E used as differential driver, the diff. output voltage is not 5V but more like 4V. Not a huge issue, with a 120R series resistor there should still be about 10mA flowing through the opto-LED.

However I also noticed that the step/dir outputs from the fpga are terminated with a Thevenin termination (220R pullup to 3.3V and 470R pulldown to ground), which brings the voltage down to about 2.3V.
As per the ISL3217 datasheet, this is below the minimum 2.7V required for a logic HIGH.
Could this be the issue or am I wide of the mark?

The open circuit voltage of the terminator will be close to 2.3V but driven high ,
it will be closer to 2.7 or 2.8V (and about .4V when driven low) I don't think this is an
issue. What step length are you using? I would always start with at least 2X the drives
minimum step length value.
 

Right, I can indeed measure about 2.8V. Still, this seems way too close to the minimum threshold.

I'm currently using 1us wide pulses for steps and 5us for dir, which is well within spec:
 

2.8V is a legal high (the actual input thresholds are about 1.7V)
Its also about the best you can do into termination with 3.3V drive.

Note that the minimum VIH and VIL specifications are not the
input threshold values but the minimum high and low levels to
guarantee sufficient noise margin (typically 1.0V for TTL interface
levels like these)

As an experiment, you can get a bit higher drive by setting the
FPGA card I/O to 3.3V tolerant mode which reduces the output
resistance, but this is not the issue, its something else.

Thanks for the explanation.
I'll keep investigating and probing.

Hi

I am planning to use Yaskawa sgd7s with a mesa 7176e. Did you find a solution for the lost steps? Best jannik, Switzerland