[time-nuts] Trimble Thunderbolt Rs-232 Levels

Didier Juges didier at cox.net
Thu Feb 14 09:05:22 UTC 2008


RS-232 specs are just about totally obsolete, but at the time they meant
anything, they required that RS-232 receivers work with as little as +/- 3V
and that they accept +/- 18V at least before failing. Signals between +3V
and -3V are unspecified and therefore may generate an undetermined state.
However, most recent (<10 years old) computers will work with unipolar TTL
levels because the receivers have a bias point set slightly positive. 

Didier KO4BB

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Hal Murray
> Sent: Wednesday, February 13, 2008 12:05 AM
> To: Darrell Robinson
> Cc: time-nuts at febo.com
> Subject: Re: [time-nuts] Trimble Thunderbolt Rs-232 Levels
> 
> 
> > I think I will apply power to the module and measure the 
> levels from 
> > the port.  If it is TTL out, I will need to do a level 
> shift anyway to 
> > give the computer proper voltages to run it's serial port.
> 
> Most RS-232 level shifters also include an inverter.
> 
> I'd just plug it in and see if it works.  Especially if it has a DB-9.
> 
> Ages ago, RS-232 specs called for 12 V.  I think they updated 
> it to work at 6V.
> 
> In practice TTL signals actually work, at least if you aren't 
> using long cables.  All the receiver chips I've checked have 
> had an input threshold at roughly the TTL switching level - 2 
> diode drops above ground.
> 
> 
> 
> --
> These are my opinions, not necessarily my employer's.  I hate spam.
> 
> 
> 
> 
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