> 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.

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... 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_[....]" Thanks again to all for their input. Always more to learn =) - Brian On Tue, Aug 25, 2015 at 7:50 PM Hal Murray <hmurray@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. > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

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@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. >> >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

This was exactly the case on my Rb. It was suggested by the OEM that one could directly connect the 0/5V serial connection to a PC "RS232" serial port (but with the warning that reversing the TX/RX connection could destroy it). So the polarity was inverse from what you'd normally expect on a 0/5V interface, but the same as RS232. As a result of the caution from the OEM I decided it was much safer to use a real TTL level interface and the reprogramming of the FTDI chip solved the polarity issue. Easy and safe. Your NPN inverters/level shifters will work just as well of course. Congratulations on getting it talking. On 08/26/2015 12:28 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... > > 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_[....]" > > Thanks again to all for their input. Always more to learn =) > > - Brian >