"Interesting that they don’t require resistors on this yet they do normally. Can anyone explain why that would be?"
carboncymbal,
Not sure if you say this post on a different thread so I added it here to ensure coverage.
All-
I’m an engineer at Teknic and came across your comment about step input timing errors and noise interference.
The ClearPath motor inputs are optically isolated and current limited. This design provides exceptional noise immunity even in the most difficult EMI environments like plasma CNC machines and applications that use a VFD. The optical isolation hardware of the ClearPath motors provide enough noise immunity that shielded and twisted pair cables between the CNC motion controller and the ClearPath motor are not required.
You might wonder, “if the noise immunity of the ClearPath motors is so good, then why is there a possibility of getting step input timing errors and why are the termination resistors required?” Great question!
The minimum length of time for a step to be “on” is 715ns. In addition, the minimum length of time for the “off” period between steps is also 715ns. The ClearPath “step input timing error” occurs when the “on” time of a step is shorter than 715ns, and/or the “off” time between steps is shorter than 715ns.
Typically this error is elicited when one or more of the following conditions occur:
- The controller has open collector outputs and the resistors specified in the ClearPath manual are not installed correctly, or not at all. Here is a link to the fractional hp ClearPath manual:(
https://www.teknic.com/files/downloads/ ... manual.pdf)
An open collector output “drives” the signal to a true logic state but then the return signal “floats” back to its false logic state. This floating nature of the open collector signal means that the signal might not be “off” for at least 715ns required between steps before the next step is sent. This behavior is often exacerbated by capacitance in the control system, whether due to long cable runs or the electrical design.
The Centroid Acorn uses 24VDC open collector outputs, so installing 1k ohm resistors across both the step and direction outputs for each axis of motion is appropriate. This resistor promptly drives the signal to an “off” state between steps. In contrast, a driven TTL signal (this is a different hardware architecture than open collector signals) is less prone to this behavior because the circuitry “drives” the signal to ”on” and then “drives” the signal back to ”off”. This type of signal is referred to as a “driven signal”.
- The controller output voltage is lower than the minimum 4VDC required for the ClearPath motor and is unable to reliably turn on the optical isolated inputs. For example, some controllers only output a 3.3VDC signal.
- The controller's maximum step pulse output frequency is higher than 700kHz, or the step pulse time is less than 715 nS. These settings are typically configurable in the controller.
(
https://www.teknic.com/files/downloads/ ... om=100,0,0)
- Shared return paths for outputs. The V+ and GND wires for each controller output should travel the entire distance from the controller to the motor and then back to the controller. Some systems have limited I/O connections so the output signals go individually out to the motor, but then they share a common return path back to the controller. For example, three output signal wires could go out to the motor but then the return wires are tied together and only one return wire comes all the way back to the controller. This can cause problems related to noise immunity.
As a side note, we’ve recently reviewed the Acorn schematics with Centroid and they will be updating their schematics to include this 10k termination resistor. This should address the issue that you mentioned.
I hope this helps you and other users. If you have any other questions regarding the ClearPath motors, please feel free to use Teknic’s "Contact Us' form (
https://www.teknic.com/contact) or give us a call at 585-784-7454.
Best regards,
Tom T. - Teknic Servo Systems Engineer