[time-nuts] SRS PRS10 repair

Brian M brayniac at gmail.com
Wed Aug 26 00:28:34 EDT 2015

Hi -

So I took the time tonight to poke at things with the scope. Hopefully it
will be of interest.

First off, I probed the MCU (MC68HC11) TX line directly. And, it looks like
I misstated in my last mail. The MCU itself is 5V TX idle TTL Serial. On
the unit's output, it is inverted and 0V idle. Not sure why that's the

That said, I have lashed up some simple NPN inverters which are also
level-shifting to a BBB UART. And with that I've got serial comms
established. I get the power-on message and response from "ID ?" is "

Thanks again to all for their input. Always more to learn =)

- Brian

On Tue, Aug 25, 2015 at 7:50 PM Hal Murray <hmurray at megapathdsl.net> wrote:

> > Hang on a minute, polarity does not switch all of a sudden.
> The standard RS-232 interface chips include an inverter.  The normal output
> from serial pins on microprocessors or PCI/USB serial chips expects that
> inversion.
> For short runs where you are designing both ends, it's common to skip the
> RS-232 drivers.
> So if you are trying to talk to something like a GPSDO board without the
> typical 9 pin serial connector, there is a reasonable chance you may need
> to
> add an inverter.  (or maybe a real RS-232 interface chip)
> --------
> It's also possible to cheat on the RS-232 interface ship.  A TTL/CMOS
> driver
> will work with most RS-232 receivers and a resistor with maybe a pair of
> diodes will protect a CMOS receiver from RS-232 levels.  If you are doing
> that, you need an inverter in there someplace.  With a microprocessor, the
> inverter is often available (for free) in the pad driver.
> --
> These are my opinions.  I hate spam.
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