tkbot47 wrote: ↑Sun Mar 07, 2021 12:55 am
I'm still with Dave C's original concern about this. Many of us were wired with 1K resistors going to 24v, per both Centroid schematics and Teknic recommendations. Then, in recent posts, Teknic said that 1K was intended for 5v pullup, and that 10K should be used for 24v, mostly because of current draw and heating of the resistor, and their SDSK manuals were being changed to reflect that. Now we are told to run 5V with 100 instead of 1K ohm, but use a big enough wattage resistor to minimize heating. Why is 1K no longer OK for 5v as originally stated, and why is 24v with 10K no longer OK? For those of us who just changed to 10K at 24V, are we OK leaving it there? If one runs other than this latest recommendation, is there a danger to the Acorn board or it's warranty?
I'll provide my opinion till a response comes along from Centroid or Teknic.
To me it seems like you are asking 2 questions.
1. Why has the manufacturer recommended solutions changed over time? Including multiple changes in the last several weeks?
2. Are systems operating with any of the prior configurations at risk?
Here are my thoughts on those 2 questions.
1. Teknic's guidance on this has been incorrect in the past. Why? The only thing is I can say is that people and companies make mistakes or don't have the ability to consider all use cases. Fortunately for us, Centroid and Teknic have apparently discussed this particular issue and tested it now, which is great.
2. The purpose of the pull-up resistor is to drive the signal to the "high" state, in the case of the old advice, this would be +24v. Acorn's open collector system essentially acts as a switch to provide a path to common (~0v). without a pull-up resistor, the signal may not be a proper square wave. That basically means that the transition from low to high may be unpredictable and inconsistent, likely contributing to the step input timing error many acorn users have experienced with the Clearpath motors. You can think of a pull-up resistor as a constant "leak" that makes the resting state of the control circuit high. The leak is slow enough that when the acorn flips the switch to connect the signal to common, it overcomes the leak and still sends a "low" signal. Acorn open collector outputs are rated at 50ma. If you had the lowest suggested resistor of 1K for a 24v system, it would have provided a current of 24 mA, Clearpath takes another 12 mA, that puts you at 36 mA, still well within the capability of the Acorn outputs. The current is even lower with a 10K resistor.
Every individual system can be different, at the end of the day, as long as you are within the voltage and current limits of all components, you don't need to change anything if your system is reliable. There have been reports of systems degrading over time for some reason (systems that have been in place for 6-12 months before experiencing step input timing errors), so if you ever have issues you know have a plan to address them.