[Coco] TTL levels and the bit banger port.

[gene heskett]

On Saturday, July 23, 2011 01:50:16 PM Mark Ormond did opine:

> " The service manual for the Coco3 indicates that the serial output is
> +-5vDC"
> 
> 
> Doh...
> 
> Ok, now what would be the easy way to get this to +5 and 0v for signals?
> Throwing a max233 in there? (I know overkill, but the -5 is not
> something I want to feed a usb uart.)
 
If you are feeding the usb uart via its rs-232 connections, they are, or 
should be well protected, since rs-232 itself calls for at least 12 volts 
+-, and suitable surge protection to absorb any locally induced EMP's.  

At the tv station, where we had a 255 foot tall lightning magnet attached 
to the building with several pieces of copper 1 & 5/8" heliax, we commonly 
would lose several audio distribution amps when somebody "called that stuff 
butter" (an old butter commercial when Margarine was first affecting the 
butter sales), but we never had a single problem with equally long rs-232 
circuits or ethernet since both are designed from the ground up to survive 
'longitudinal' EMP's on their lines.  Unforch for me, I didn't think to 
consider that when I was redesigning a new audio DA card in about 1986 
because the old, transistorized model also suffered from the same blow all 
the transistors problem.  Since swapping a chip in a socket was a lot 
faster than replacing a 4 pack of TO-5 transistors, I used a TLO-84 per 
output, 9 per card because it was designed from the git-go as a dual 
channel, 4 outputs/channel for stereo, and which worked as well and had 
about a decade better response on the top end.  A slow, 741 equ op-amp 
(quad version=4432, used by the quad-jillions in commercial gear) can kill 
your audio quality in a heartbeat.  I put a lot of those in the trash can 
after watching it turn a 4 kilohertz sine wave at +6db output level into a 
slew rate limited sawtooth that grated on ones nerves in about 50 
milliseconds, like fingernails on a black board.  Then I said to heck with 
it and redesigned it from scratch, building about 30 cards so we had some 
ready spares.  I should have made 70 of them though because with 28 in 
service, and 27 on the shelf, Murphy's Law says you will be short one to 
fix them all after the next storm.  I would cheerfully have shot that SOB 
if I could have ever made a positive ID.  ;-)

My card could make a 25 volt p-p 50 kilohertz sine wave with no evidence of 
slew rate limits, but the EMP from a nearby lightning strike, coming back 
into the outputs would kill it almost every time.  Now we're digital, lots 
less trouble.

> Later,
> dabone
> 
> 
> --
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> Coco at maltedmedia.com
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Cheers, gene
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