[LinuxPPS] [Fwd: Re: Oncore]

[Hal V. Engel]

On Thursday 08 July 2010 01:23:17 am Paul wrote:
> No, I understand about the assert and clear, it is the data transmit and
> receive I was more concerned about - buffer the data or buffer and
> invert?
>
> On Thu, 2010-07-08 at 02:10 -0600, clemens at dwf.com wrote:
> > > I do notice on your schematic that you invert the transmit and
> > > receive data. Is this required? I just had it buffered with no
> > > inversion.
> >
> > Im not sure about the transmit side, mabe this is a difference in the
> > definition of 1/0 for tty vs ttl.
> > But for the receive side, this is just what I was pointing at when I
> > said that you have to test to see if you want to use ASSERT or CLEAR.
>
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RS232 is expecting a transition from high to low when there is a signal. 
TTL goes from low to high.  So at a minimum it needs to be inverted.

Technically RS232 has a voltage range of +-12 volts so going from high to
low it goes from +12v to -12v but most RS232 ports will see 0v as high and
-5v as low.    So just inverting the TTL signal will work with most ports
even though it is technically not correct.

I prefer to just get one of those ttl to RS232 converter kits (I get mine
on ebay).  These are cheap (about $12 including shipping) and easy to
build (about 10 minutes). They make the wiring required very simple and
you know when it is done that the RS232 side is going to get exactly what
it needs. All I do for the PPS is run a wire from the PPS ttl pin on the
Oncore to a 100 ohm resister and then to the DCD pin on the port.

If you search the list you will find a note from me with a schematic for
using one of these ttl to rs232 kits with an Oncore.  Since I just moved my 
email address has changed.  My old email address was hvengel at astound.net.

Hal