JRD@cc.usu.edu (Joe Doupnik) (07/18/90)
Autotransformers to convert from 50 to 75 ohm Ethernet coax. Whoa! Direct coupled transceivers put D.C. on the wire, signaling levels are typically 0 to -2 volts. Short duration and d.c. are opposing concepts. What the transceivers really do is apply a given current to the wire (higher impedance means higher voltage, natch) and the receiver half is sensitive to the voltage levels of the bits (collision detection and all that, right?). A simple connection can be made directly with 75 ohm coax, but don't depend on three stations working properly, nor even two of them competing well for the wire. What will always work is a impedance matching pad made of resistors. A pie or tee section, say 6 dB, will do for a while. The loss means the cable must be kept short. What I would do if I could would be to tie 50 ohm coax to the tail ends of the 75 ohm stuff and pull the new wire to replace the old. The reason is that once people start networking then things keep growing. A flakey wiring plant ends up being a larger headache a year from now. Joe D.