On 6/7/2011 9:20 PM, David Christ wrote: > For ordinary RS-232 Tx would be on one pair and Rx on > another. the second wire in each pair would be connected to signal > ground. Yes, that meant that there were multiple wires running > signal ground.
Because RS232 is unbalanced and not impedance matched it is subject to IR drop on the grounded conductor adding noise to the signal. When that equipment is plugged into the power line, or when the chassis of one unit is ground at a different point from the other unit, AC power leakage currents cause noise voltage to exist between the two equipment chassis. This is true of ALL unbalanced circuits, and is why bonding from chassis to chassis of interconnected equipment is important. When you bond from chassis to chassis, you reduce that noise voltage to a very small value. When you use multiple conductors in parallel for the signal return, you also reduce the IR drop produced by the noise voltage. Because RS232 is not an impedance matched system, its bandwidth is limited by cable capacitance, and noise performance is limited by line loss, and noise voltage picked up on the signal pair and on the signal return (the grounded conductor). CAT5 (or 6 or 7) works VERY well for RS232 because it is very good twisted pair, which minimizes noise, AND because it has very low capacitance between the conductors of a pair, which increases bandwidth. The drain wire that is included with foil-shielded cable is provided so that a soldered connection can be made to the shield. Treat the drain wire as the shield connection -- wire it to the shielding enclosure at each end. Use one pair for each signal, and wire the signal return of each pair to pin 5. Finally, a reminder about the fundamental value of TWISTING. In the pro audio world, mic signals are VERY low voltage, and require VERY low noise pickup to achieve the dynamic range needed for digital audio. Twisting is FAR FAR more important than shielding in achieving this. Plain ordinary CAT5 makes excellent audio cable and excellent RS232 cable. I wouldn't lose any sleep looking for shielded CAT5 for my ham station.. 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
