On 09.02.16 11:23, andy pugh wrote: > On 9 February 2016 at 09:10, Erik Christiansen <dva...@internode.on.net> > wrote: > > The two-terminal dongle would be used for half-duplex, with separate > > grounds, relying on less than 7v float. > > This is where things get iffy. Imagine that I have the 2-terminal > dongle plugged into a laptop. What do I use for a gnd? Stuff a wire > down the side of the USB connector? trap the wire in the hinge? :-)
If the question has an element of seriousness, I'd suggest plugging a naked USB connector into a spare USB port, with a soldered connection to the shroud. That's less invasive than digging in to the earth wire in the charger cable and then covering the violation with heatshrink tubing. In either case, I'd interpose a 100 ohm resistor in the GND link. > Looking at Modbus specs,even the "two wire" version has a common. My > dongle doesn't. If twice the intended bus bias R values appear at both Tx and Rx ends, then there will be a high resistance (e.g. 2720 ohm) DC connection between GNDs and VCCs end-to-end, I figure. As the signal is differential, that will have otherwise floating GNDs bobbing up and down a bit, but held within common mode limits if there is no low impedance source involved, I expect. It should work, at least some of the time. But a 1999 "Circuit Cellar" "Designing RS485 Circuits" article says: "To comply with the specification, all of the nodes must share a common ground connection. This ground may be isolated from earth ground. The ground wire provides a path for the current that results from small imbalances in the balanced line. If the A and B outputs balance exactly with equal, opposite currents, the two currents in the ground wire can- cel each other out and the wire carries no current at all. In real life, components don't balance perfectly; one driver will be a little stronger and one receiver will have a slightly larger input impedance. Without a common ground, the circuit may work, but the energy from the imbalance has to go somewhere and may dissipate as electromagnetic radiation. The RS-485 specification recommends connecting a 100-ohm resistor of at least 0.5 W in series between each node's signal ground and the network's ground wire, as Figure 1 shows. This way, if the ground potentials of two nodes vary, the resistors limit the current in the ground wire." That seems to cover it. Erik ------------------------------------------------------------------------------ Site24x7 APM Insight: Get Deep Visibility into Application Performance APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month Monitor end-to-end web transactions and take corrective actions now Troubleshoot faster and improve end-user experience. Signup Now! http://pubads.g.doubleclick.net/gampad/clk?id=272487151&iu=/4140 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
