Pinout for SED9421

[Scott Stevens]

On Fri, 25 Nov 2005 09:59:54 -0500
Allison <ajp166 at bellatlantic.net> wrote:

> >Subject: Re: Pinout for SED9421
> >   From: shoppa_classiccmp at trailing-edge.com (Tim Shoppa)
> >   Date: Fri, 25 Nov 2005 09:31:03 -0500
> >     To: cctalk at classiccmp.org
> >
> >> FYI:the average PLL is a bear to build and debug, they required clean 
> >> power and good board layout with ample groundplanes.  The digital ones are
> >> very good, simple to layout and shift rates with only a mux.
> >>
> >>
> >> Allison
> >
> >Don't neglect: PLL's require analog components of rather tight tolerances
> >to give consistent behavior from time-to-time and unit-to-unit.
> 
> That falls in the catagory of a bear to build part.
> 
> >The digital data separator (I heard its designer once refer to it as
> >a "jerk-locked-loop") has no such tight tolerances and in fact is usually
> >driven from a crystal oscillator.
> 
> That is a good description.  It also depends on the number of bits used
> as to how locked it is.  Really only makes a difference when the peak shift
> is bad due to impropper media or bad write precomp or the drive speed is 
> really off.  Usually fixing the drive speed is easiest.  The upside is 
> predictable performance and repeatability.
> 
> >The one-shot-with-critical-RC-constant used in early FM data separators
> >is a good example.  With a little tweaking it really works pretty well.
> >But imagine mass-producing such a beast and training the assembly line
> >people to do the tweaking, as well as field service, as well as ...
> >(Of course us hackers don't mind!)
> 
> I hate analog oneshots unless they are timing uncritical.  I worked with 
> an engineer that did everything with onshots and his stuff was prone to 
> wandering off or plain quitting. 
> 
> The all time worst was the 1771 internal data sep.  Tandy initally did that
> to save parts. Really bad.
> 

I once worked for a guy whose approach to any problem was to slap in another one-shot:

Timing problem casuing a glitch on the chip enable?  Don't do an analysis.  Slap in a one shot!  He refused to allow me to modify the circuit to properly correct the problem.

That kind of design leads to a cascading disaster.  It's an analog kludge approach.  And the R/C of a one-shot circuit is prone to drift over time.

> 
> Allison
>