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ClearPath Servo and Masso Step/Direction Output?

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@zombieengineer

It just occurred to me that you’re likely talking about drawing the +S and +D signals off the TTL outputs available on the 20 pin connector and not from the +/- “4v” differential output shown in my crude drawing.

@safeairone

Your sketch is good and should work.

When I was referencing the S+ / D+ it could either be the terminals of the differential pair or the 20 pin connector (your choice). The DLN4003N will start conducting once the input voltage starts exceeds ∼1.2V (every 6 mV higher than the threshold the output will sink an additional 1 mA) - by the time the input reaches 1.3V the output is already sinking enough current for the ClearPath optocoupler. With a 3V input the output is sinking enough current to drive 25 optocouplers in parallel.

In an earlier post you mentioned that you wanted a more "off the shelf" solution - you could potentially use a L298 based H bridge driver board (4 channels per board, 46V maximum voltage). The maximum pulse rate according to the datasheet of 40 kHz. This is getting well into the "over engineered" zone as a single L298 output could drive >160 optocouplers in parallel. There is also a L9110 based H bridge driver board (12V maximum voltage).

This is very troubling.  I built two CNC machines using Teknic Clearpath servos (sd/sk model) controlled with Masso. One is G2 and is about 3 years old.  The other is G3.  I have had NO indication of missed steps

or any other problems with position control.  What is different?  I used Teknic power supplies and cables.

Ben

@zombieengineer

Before I order parts, just want to make sure I’ve got it right;  You referenced ULN4003 and DLN4003N. These numbers hardly exist on the internet and Google returns many sources for ULN2003, which I think will work (TTL level input, up to 48v output, DIP 16 format).

I have found a great DIN-mount DIP 40 breakout board that’ll make this project easy and neat (DIP 40 because I’ll need to use 2 of these chips). Nice thing about it is that it also has 2 lines that I can use as an onboard +24v bus and a ground bus.

@mr-ben

If your ClearPath servos are currently working with your Masso step/direction outputs, I think you’ll be just fine.  My case is kind of unique—let’s just say that I won’t be able to return my controller to Masso for a warranty claim anytime soon.

Quote from Mr. Ben on June 9, 2021, 11:04 pm

This is very troubling.  I built two CNC machines using Teknic Clearpath servos (sd/sk model) controlled with Masso. One is G2 and is about 3 years old.  The other is G3.  I have had NO indication of missed steps

or any other problems with position control.  What is different?  I used Teknic power supplies and cables.

@mr-ben

Short answer: Probably a different batch of optocouplers within the Teknic driver.

Optocouplers are good at high voltage isolation but poor in many other respects. One key parameter is the "Current Transfer Ratio" or CTR for short. The CTR is the ratio of current through the output transistor compared to the current through the input photodiode. Many optocouplers have an allowable range of 50% to 600% with an option to have devices from a narrower band of CTR (for example 80% to 160%). There is a massive amount of variability in these devices that is a real headache for circuit designers.

I suspect Teknic have not done themselves any favors with the current limiter component choices. The threshold voltage at which the current limiter kicks in is a little high (4.5V by graphical interpolation) when TTL logic allows for 2.7V. The optocoupler photodiode forward voltage is typically 1.5V leaving only 1.2V margin for the actual current limiter to be compliant. The simplest current limiter is a "N-channel JFET" where the "Gate-Source cutoff voltage" determines the voltage at which the current limit kicks in. The really cheap components have a threshold voltage of 3V ($0.184/device) while a device with a 1.2V threshold costs $1.21/device. The lower voltage threshold devices have less supply options (only one part number from one manufacturer vs 18 part numbers).

Back to the CTR issue - the optocoupler should trigger 100% of the time with a 8 mA input current, if Teknic have gone for a narrow CTR band device then the input should trigger somewhere between 3.0V (4mA) and 4.5V (8mA).

@safeairone

Quote from safeairone on June 10, 2021, 6:01 am

@zombieengineer

Before I order parts, just want to make sure I’ve got it right;  You referenced ULN4003 and DLN4003N. These numbers hardly exist on the internet and Google returns many sources for ULN2003, which I think will work (TTL level input, up to 48v output, DIP 16 format).

I have found a great DIN-mount DIP 40 breakout board that’ll make this project easy and neat (DIP 40 because I’ll need to use 2 of these chips). Nice thing about it is that it also has 2 lines that I can use as an onboard +24v bus and a ground bus.

My mistake - ULN2003 is the device I was looking at (apologies for the confusion - currently a little sleep deprived, truly a zombie engineer).

When I say the "DIP 40" my first reaction was "No" as typically a DIP 40 has wider spacing between the two rows of pins (0.4" vs 0.3").

However the breakout board you have quoted actually has holes for both row spacing so it will work for you. The extra "pins" (40 - 2 x 16 = 8) could also be used for a common rail for pull-up resistors.

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