ClearPath resistors?

[Richards]

I've been using the C86 board with my ClearPath motors, but decided to try direct wiring on a test-bench setup. Centroid released a new schematic showing how to use a +5VDC power supply and a resistor. I noticed that the resistor is in SERIES with the power supply. In Teknic's manual, it shows that the resistor is is PARALLEL.

Centroid schematic

Teknic schematic

[tblough]

Centroid's schematic uses the resistor to drop the voltage. Teknic uses the resistor as a termination resistor to suppress reflected signals on long wires.

[Richards]

Tom,
Thanks for the input. Right after I wrote that post, I left for a few hours. While I was gone, I realized that Centroid is using the resistor mostly as a "pull-up" resistor to sharpen the pulse of their open-collector circuit. Just as you said, Teknic advises the use of a resistor between the signal and +voltage to reduce noise on longer wire runs.

[Dave_C]

Centroid's schematic uses the resistor to drop the voltage.

I have a question as the change from 24 vdc to 5 vdc and using a resistor to "drop" the voltage leaves me a bit puzzled!

Isn't the recommended control voltage for the ClearPath Servos supposed to be between 5 and 24 vdc?

So if that is the case, why are we dropping the 5vdc with a resistor?

I'm still running the 24vdc supply with the resistors in parallel and I have no issues with 10 foot long cables from Clearpath.

What am I missing here?

Dave C.

[tblough]

Dave - That was my mistake. Centroid resistors are pull-up resistors as Richards points out, not dropping resistors. They insure that a strong high-level signal is available on the line until the active component pulls the line low. This helps reduce the effects of noise on the lines.

Dropping resistors are in-line with the signal and are used to change voltage. Pull-up/Pull-down resistors are between the signal and a voltage source or ground and are used as current limiting resistors to create a stable voltage on the line. Termination resistors are between differential signal lines are are used to "dampen" reflected signals.

I just gave the schematics in the OP a cursory glance and assumed the Centroid schematic used dropping resistors. My mistake.

[Dave_C]

Dave - That was my mistake. Centroid resistors are pull-up resistors as Richards points out, not dropping resistors. They insure that a strong high-level signal is available on the line until the active component pulls the line low. This helps reduce the effects of noise on the lines.

Dropping resistors are in-line with the signal and are used to change voltage. Pull-up/Pull-down resistors are between the signal and a voltage source or ground and are used as current limiting resistors to create a stable voltage on the line. Termination resistors are between differential signal lines are are used to "dampen" reflected signals.

I just gave the schematics in the OP a cursory glance and assumed the Centroid schematic used dropping resistors. My mistake.

No problem and thanks for clearing that up..

Well done!

Dave C.

[Richards]

I'm thinking about reconfiguring my Acorn/ClearPath connections. Right now, I'm using the C86ACCP board to interface the Acorn to the ClearPath motors. That board has worked perfectly; but, I have some new PLC toys to play with. What I'm planning on doing would not be practical for other Acorn users because the "solution" adds another $300 to $600 to the costs of using an Acorn but I have some Allen Bradley Micro820 and Micro850 PLCs on hand that a customer needs for another project.

I had not really looked at the Centroid revised schematic for ClearPath motors. The revision was introduced about the same time that Marty ran some experiments on the C96ACCP board that the manufacturer updated with noise suppression resistors. I had taken a quick glance at the new Acorn schematics at that time and somehow had the idea that the 100 ohm resistors and the 5VDC power were introduced for noise reduction. When I started this thread, I had just quickly reviewed the schematic again and I was still thinking noise suppression rather than pull-up resistors. It wasn't until I was away from the test bench for a few hours that I realized that the schematic from Centroid and the diagram in the Teknic manual were addressing two different issues.

The theory behind using a pull-up resistor is to sharpen the pulse on an open-collector circuit. Without the resistor, the corner(s) of the pulse tend to be rounded. If the rounding is severe, we have a situation described by the word "hysteresis". Hysteresis is just a fancy word meaning that the signal seen by the ClearPath motor lags behind the signal output by the Acorn. Good design creates signals that ttravel very rapidly through the hysteresis area. The pull-up resistor serves that purpose. (On the original TTL logic chips, a LOW signal was defined as 0.0V up to 0.4V and a HIGH signal was defined at 2.4V up to VCC. The signal rise time was to be 50ns or faster. Voltages between LOW and HIGH were undefined. The logic did not work properly if the rise times were greater than 50ns. Open-collector logic often used pull-up resistors to get proper rise times.)

As far as the value of the pull-up resistor goes, it is most easily determined by watching the signal on an oscilloscope. At one time designs for military spec products had to include a 2.2K pull-up resistor on all TTL (transistor transistor logic) devices. Boards like the Arduino use a chip that has built in 100K resistors. I've used 1.2K, 2.2K, 4.7K, and 10K resistors with 5VDC signals with good results. If the value is too far off the signal will still have a curve or it will "ring", meaning that the shoulder of the signals looks like it has ripples. If the ripples are high enough in amplitude, then they're seen as discrete signals meaning that the ClearPath motor might think that it received three step pulses when the Acorn only sent one step pulse. Frankly, I haven't looked at the 100 ohm circuit on an oscilloscope; but, if it is recommended by Centroid, then I'm confident that someone picked that value for a good reason.

Because the Acorn can output 400,000 pulses per second per axis, adding a simple resistor to the step and direction signals to keep the pulse shoulders square is an easy way to keep those servos moving reliably.

[Muzzer]

I don't think anything you've said is technically wrong as such. However, in this instance I seem to recall that Clearpath tried to do something "innovative" in the drive circuit for reasons that may be best left unexplored. Quite how you can screw up an (inherently low impedance) opto drive circuit is beyond me but these guys appear to have had a pretty good run at it. I rolled my eyes and moved on some time ago but IIRC it was something like a constant current driver they had in mind?

Most established drive manufacturers get away with a dirt simple unipolar drive and/or a differential pair but it seems somehow that wasn't sufficient for Clearpath. Now they are left with trying to present a convincing sounding solution to a pretty flaky design. You guys are doing verification testing of that attempted fix for them here. Hopefully this will help them to define a stable implementation - and encourage them to learn to walk before they try to run next time.

[Richards]

Muzzer,
The problem with opto-couplers is speed. None of the off-the-shelf opto-couplers that I've used can handle 400,000 pulses per second. I experimented with Gecko G203v drives several years ago. They use a fairly high speed opto-coupler, but it is limited to 250,000 pulses per second. For my DM542 stepper drivers, I use the 100,000 pulses per second setting, although they have tested fine at the 200,000 pulses per second setting.

In my experience, directly connecting the Acorn to an opto-coupler limits the possible steps per second to 200,000 or less. That is not the fault of the Acorn. The limitation is caused by the opto-coupler.

I've had no problems running my SDSK-2321S-RLN ClearPath motors using Acorn's 400,000 pulses per second setting. It looks like I'll have time to hook up a 'scope to my test bench in the next few days. I'll be watching the step wave forms using various resistors, starting with a 100 ohm resistor at 5VDC as recommended by Centroidcnc.