On Mon, Feb 04, 2008 at 08:25:31PM -0800, Kirk Wallace wrote: > > So the ground, will only be usable if all the transceivers are isolated, > so that the ground wire is actually a 0V network common?
As I understand it, the pinouts.ru circuit runs network ground and power to each node, making it a 4-wire bus. The node (e.g. PC or controller card) ground is isolated. (Two occurrences of different earth symbol on LHS of the diagram.) I'd use that method between machines. (Though I hadn't planned that far, until this discussion. ;-) It'll be necessary to source good optocouplers though, if high baudrates are planned. Common power ground should be OK, on the machine, without isolated transceivers, I expect, because it makes everything relatively equipotential, certainly within the common-mode tolerance of the receivers. The recommended 100 ohm resistors are "defeated" by being shorted out by a common ground, but that's OK. If one part of the machine runs off a separate plug pack (wall wart?), the 100 ohms will pull the network ground into line. > > I was given a link to RS485.com and found that they present schematics > for all the products I looked at. I found it very educational too. I'll have to have a look at that. > I didn't really want to build an adapter, but the adapters I had seen up > until recently were too expensive. I have been given links to some > inexpensive adapters, but now I am learning they may have deficiencies. > Now the question is, do I go with what may work well enough (especially > over the length of a CNC machine) or what is more technically correct. > Some of the literature I have read seems to indicate that in the real > RS485 world that it may be more practical to set the network up and then > tune the termination, bias and etc. as needed. This RS485 thing seems to > be a project in itself. Well put. I don't have practical experience using RS485 in noisy environments, so I'm also collecting field know-how to supplement my theoretical training. It is my intention to run cat5 cable around my machine, inside some plastic sheathed (oil-resistant) flexible steel conduit I bought on sale. The metal conduit, and the twisted pairs of the cat5, will reduce EMI susceptibility. (And the twisted pair should have a characteristic impedance close to 120 ohm, so there should be little need to tweak the termination.) To go off the machine, over unscreened cat5, I have to consider opto-isolation, as used in the pinouts.ru design, and a bus power supply to preserve the separation of the two grounds. Fortunately, I've only just started the design of the board which handles off-machine comms. > Thanks for the help Erik. No worries. :-) I'm hacking my way through the same jungle. Your clarity of thought has helped me stop to take a most timely look at the compass. Erik ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
