[Coco] USB cable powered CoCo 3 ???

[Gene Heskett]

On Wednesday 26 May 2010, Roger Taylor wrote:
>What'ya make of this one...
>
>I built the first CoCoNet USB Serial Pak a few days ago which is the
>Drive Pak board with a USB board attached in the same place.  Run a
>cable to your PC, fire the server up, and you've got 115200 bps or
>230400 bps virtual drives and NitrOS-9, etc. the same as you'd get
>over a bitbanger cable or an RS-232 Pak or bluetooth pak.
>
>At first I connected the USB board's 5v to my serial board's 5v pin,
>and lo and behold when I plugged the USB cable into the PC... the
>CoCo came on.  The monitor lit up and there she was, flashing cursor
>and everything.
>I remember having two thoughts:  Oh Crap, and Hey, this might be
>useful!  The video was a little dimmer on Roy's adaptor and my LCD
>monitor, but everything seemed to function.  Turning the CoCo on
>brightened up the video.  Turning the power off dimmed the video back
>a little, and of course unplugging the USB cable turned the CoCo off.
>
>So.... my question is: what did I stumble across here and has anyone
>had any experience with powering the CoCo through the expansion slot,
>and if so, what are the power requirements and how stable is it?
>
>I've got a 512k upgrade in the 3 so I assume the power is enough to
>feed all these chips.
>
You have discovered the bane of anyone trying to make scsi work _right_.  For 
this exact reason there is an isolation diode in the scsi's power circuitry 
that lets the power through to the scsi loads, but prevents it from flowing 
back into the host computer if its turned off.

The problem that creates is due to most so call engineers, or the bean 
counters to go over the parts list, deciding to use an Si diode in place of 
the much more expensive Schotky diode the _good_ engineer used.

Si diodes have a .7 volt loss, so the terminations, designed to hold the 
noise margins of a scsi interface to something reasonable for a logic one, 
aka 3.0 volts, giving a noise margin for echos and such of about .6 volts.  
Drop the 5 volts to 4.3, and all the voltages scale down accordingly, but not 
the integrated circuits guaranteed noise margins.  So you have very little 
logic one noise margin  left and the scsi circuit upchucks all over itself 
from the errors.  I have replaced that cheap Si diode with a power schotcky 
on several scsi cards and made a card with a reputation for bitchiness into 
totally bulletproof setups.  The power schotcky, at the current drain it 
takes to power a couple of the old style resistor pack terminators, has a fwd 
drop of only about .12 volts.

Folks who do not understand that, are doom to be always advertising for more 
virgins to sacrifice in a vain attempt to make it work.

For your circuit, add a power schotky to the 5 volt line so external usb 
power cannot feed back and do that, it is considered a very dangerous thing 
to allow.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Girls really do know just what they want -- you to figure it out for 
yourself!