It looks like there are still some concerns about resistors and Teknic ClearPath servos.
Here's part of what Tom posted:
teknic_servo wrote: ↑Thu Feb 18, 2021 6:51 pm
. . .
The Centroid Acorn uses 24VDC open collector outputs, so installing 10k 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”.
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Tom T. - Teknic Servo Systems Engineer
Various resistors can be used. Ohm's law helps us decide the value of resistor to use.
Using a 1K resistor:
24V / 1000 = 0.024A and 0.024A X 24V = 0.57W which means that even a 1W resistor would be too hot to touch.
Using a 10K resistor:
24V / 10000 = 0.0024A and 0.0024A X 24V = 0.057W which means that a 1/4W resistor would stay near room temperature.
The function of the resistor is to snap the voltage HIGH after the Acorn sends an active LOW signal. Tom described the situation. Without the resistor, the voltage "floats" back to the HIGH level. The time it takes to float is determined by the capacitance of the wiring. Before microprocessors and even before the famous 555 timer chip, timing circuits were built using resistors and capacitors. Kodak used that principle for over a decade on its S-Printers. A 911 light sensor tube acted as a resistor, depending on the amount of light passing through a negative. The keyboard had pushbuttons to select a capacitor. Kodak used vacuum tubes to amplify the voltage. When the "shark fin" pattern reached a specified voltage, shutters and filters were activated. That behavior was essential for an S-Printer, but it is exactly what needs to be eliminated in a servo drive signal. An oscilloscope shows the "shark fin" or "shark tooth" timer caused by capacitance on a long cable. The oscilloscope also shows that the "shark fin" or "shark tooth" pattern is squared up when the specified resistor is used.
It was very common before 7-segment displays were used on desktop calculators to use PIXIE tubes to display numbers. PIXIE tubes required more than 5V, so Open Collector logic was used. In the case of the Acorn, an Open Collector allows the user to choose the voltage he uses to send pulse streams to the servo drivers. Using 24VDC is better than 5VDC in a electrically noisy environment. In my opinion, there is nothing wrong with the Acorn board or the ClearPath servos. The flexibility that an Open Collector circuit gives us comes with a possible requirement to "snap" the signal back to its OFF state (floating HIGH).