[time-nuts] SRS PRS10 repair

[Magnus Danielson]

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 <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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