Thanks again guys - meanwhile another (related) question: When we talk about noise - we're talking about induced voltages right? Any ideas on what levels we could be talking about? i.e. what sort of voltages can be induced into a screen?
It's not hardly that simple.... There are many types of noise, and many ways for it to cause problems. One of the purposes of a shielded cable is to provide a constant impedance over the length of the cable. A break in the shield causes a change in impedance, which can cause the signal to be reflected back to the source, even in the absence of any external noise sources. Noise can couple capacitively, magnetically, or electrically. It's a very complex business. Working with 5V signals, it's not terribly diffcult to get induced noise of the same magnitude, or greater, than the signal itself.
That said, you really should not havve any problems it you follow some pretty basic good grounding, and routing, practices, and keep the signal impedances as low as possible, by using stiff pull-up resistors. A good shield, connected at the source end only, should work just fine, except in unusual cases. Connecting them at both ends *can* work better, but only if the entire system is well-understood. In the typical hobby CNC case, it's not, beause you don't know how things are connected inside the power supplies, motor drivers, BOBs, etc. Connecting at one end will avoid a lot of potential ground loops.
My machines are wired with shields connected at the source end only. All my limit and home switches, and other signals are 5V only, but with 300 ohm pull-ups. I even run the servo motor cables and their encoder cables side-by-side, actually Zip-tied to each other about every 12 inches, for almost 20 feet, and I've never had a single noise-related problem.
Regards,
Ray L.
Guest
Show Posts