[Coco] DriveWire issues with Coco 1 F/285 Board

[Gene Heskett]

On Tuesday 02 July 2013 15:33:10 Torsten Dittel did opine:

> Gene Heskett <gheskett at ...> writes:
> > > > ...just enough for the max. 9.6 kBaud (POKE 150,1) the CoCo was
> > > > designed for...
> > 
> > Not really.  If you ever saw it on a scope, you would cringe in horror
> > that it even worked.
> 
> I must confess I've never seen the 9600 baud on a scope (I concluded
> from the 10kHz bandwith you mentioned that it would a least reach the

I didn't say 10KHz Torsten, I wrote 10Hz.  Yes, it really is that slow in 
the open loop mode.  The LM353N is perhaps 50x faster in open loop mode.  
That is with no feedback, so the gain is in the 100db range in either case.
A gain of perhaps 5, maybe 10 is what the coco uses.

Adding feedback, assuming internal phase lags aren't too bad, have the 
effect of moving the roll frequency knee to the right.  Internal phase 
lags, which the original 741 was justifiably famous for, can turn the 
feedback positive at the higher frequencies, so some op-amps will also have 
a minimum stable gain.  The limit at the HF end is hit when the slew rate 
limit is hit.  Thats another place where the 741 falls over as its slew 
rate is only a few volts per second.

FWIW, the well thought of op-amp for serious audio work, the NE5532, is not 
only a put a clipon heat sink and a fan on it power hog, but also hits its 
slew rate limit at 15 kilohertz at about the same time the output VU meter 
taps the full scale peg when 0 VU is the broadcast industry norm of +4db.  
Nowhere near the recommended 12 to 16 db of headroom recommended for good 
practice.  While the distortion products created are above the normal human 
ears range, it does if the speakers are good enough, make my ears tired of 
listening to it in a very short time.  I designed the audio distribution 
amps at WDTV, essentially replacing a home made design with another home 
made design, but one of my test criteria when it was being breadboarded the 
first time, was that it had to make 28 volts peak to peak with no visible 
slew rate, or crossover distortion visible in a 100 mhz scope when handling 
a 20 kilohertz sine wave.  The Texas Instrument TLO74 was able to do that.  
Thats 4 of the TLO71's shown in the list, but in a 14 pin dip. 

Its one weakness was mother natures fireworks.  When you have a 255 foot 
stack of Pirod tower being used for lightning target practice, located 
about 30 feet outside the back door, and its well tied to the house grounds 
but not well grounded itself as it is sitting on about 500 feet of 
sandstone, the EMP from the lightning strike, being picked up by an input 
or output cable perhaps 175 feet long (its a big building), can and is 
quite destructive, I found I needed to buy those op-amps in 2 or 3 stick 
quantities in order to get things back on the air after a storm.  In 
fairness to my design, the original home made design used nice big, ran hot 
to5 output transistors which also failed at the exact same rates.  And 
their slew rate was worse...

> corresponding voltage levels for each single bit, even if not having a
> rect pulse shape). However, I have seen on a scope what was left at
> 31250 baud, and that's why I can hardly imagine the 57600 baud working
> flawlessly for *any* bit pattern output as Robert was reporting, but
> who knows how "intelligent" modern serial interfaces are, maybe it is
> sampling the slew at very high speed and reconstructing the original
> data by considering the corresponding slew rates.
> 
> Regards,
> Torsten
> 
> 
> 
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Cheers, Gene
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