Hi,
On 08/26/2015 06:28 AM, Brian M wrote:
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
case...
It's because you go through a 74HC14 inverter, see sheet 3 of 7 in the
PRS-10 schematics. This is expected, as RS-232 has an inverted level.
However, when a serial interface get's garbled data like if it is
inverted, then I am wondering if the 0V recoil makes the receiver
confused, as 0V isn't a valid voltage in RS-232 unless you want to send
break.
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 "
PRS10_3.24_SN_[....]"
Good. The level issue that I suspected.
Thanks again to all for their input. Always more to learn =)
Indeed, always.
Cheers,
Magnus
- Brian
On Tue, Aug 25, 2015 at 7:50 PM Hal Murray <[email protected]> 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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