Hi, I'm just wondering what are the hardships that are encountered by increasing the speed of a microprocessor bus? Do things like current leakage, PCB trace length and proximity affect how far you can push the speed of the bus? Also I read in a book about the 68000, that Floppy disk drives and interfaces need to have termination resistors to minimize signal deflection (that is if there is more than 1 FDD on the same cable), how bad is this problem? is it enough to actually affect the signal?

Also I read somewhere that it is impossible to design a high speed system using wires is this because of the resistance that the wires have?
Posted on 2005-01-24 15:49:21 by x86asm
The Crays bus was all wires (twisted pair) with ECL logic so there's a good example that you can create a bus with wires.

The termination resistors can be found on some backplanes on all the long lines. Sometimes it's required on the plugin board. It's just like an antenna and you're trying to match impedances to prevent reflections.

All the things you mentioned become more problematic as speed increases. I found a short article about terminating busses and I'll look it up later.
Posted on 2005-01-24 16:40:16 by drhowarddrfine
Also I read somewhere that it is impossible to design a high speed system using wires

Without a definition of "high speed", that statement ends up defining "high speed" as the speed where ordinary wires stop working. There are still "wires" in the form of PCB traces, and metal traces within chips.

Termination resistors are used to deal with transmission line effects. If the wires are long enough (it varies with speed of voltage/current change), they become transmission lines, and you can no longer treat them as pure DC.

In addition to transmission line effects, there is also a signal skew problem at higher speeds. No two signals will arrive at their destinations at the same time. For signals that change "at the same time", the differences in timing are the skew. At very high speeds, layouts for parallel and distributed signals must be carefully designed to keep skew at a low level.
Posted on 2005-01-26 00:32:20 by tenkey