Repair methods (was Cromemco 3101/Beehive B150 score)

[Tony Duell]

> 
> Tony wrote:
> > I haev never understood how you can understand digitial electronics
> > properly and not understand analogue electronics. I certaimly couldn't
> > understnad digital stuff until I understood things like transmision
> > lines, termination, etc.
> You can understand it well enough to use for many practical purposes by just
> knowing a few "rules of thumb"*. A better, but still not necessarily

True enough. I don't set up a TDR every time I wire up a simple logic 
circuit.. (I have been known to assume that a twisted pair of wire-wrap 
wire with 3 twists per inch has a chracteristic impedance of 120 Ohms, 
though. That is near enough for most circuits!).


> detailed, understanding of the analogue background can help considerably in
> some of the more tricky situations - especially if you are
> "stretching"/abusing the "rules". Too detailed a consideration of the
> background can actually get in the way [the best example I have of this is
> the realisation that one has much more tolerance in termination than
> analogue theorists tend to expect - half or double the normal termination

A good understanding includes an understanding of tolerances IMHO. I have 
no time at all for so-called engineers (and I've met plenty of them) who 
can calculate component values to 10 significant figures but have no idea 
how accurate they actually need to be (hint, for pull-up resistors a 
factor of 10 may make little difference). 

A good undersanding of the analogue side of 'digital eleectronics' 
includes, therefore, knowing when you need to terminate a signal, and how 
accurate that teminator needs to be.

-tony