Finally.
I can't remember what led me to take another look at slow RS232 capability with an Arduino but lo and behold someone has written a variation of the go-to SoftSerial dubbed SlowSoftSerial. It even handles a true floating-point baud rate, the 1.5 stop bit and works just like any Arduino modem.
The catch is that it only runs on a Teensy board - the snack-cracker sized variety that requires soldering to utilize. But on this side of the investigation we end up with a very quiet printout - gone is the chatter, but, alas, some of the sensitivity we saw with the hardware serial port. It would seem the SlowSoftSerial library is pretty selective at what it determines to be a 5-bit word, evidently providing a lot more scrutiny than a standard UART.
And it doesn't seem to be a matter of latency, just sensitivity. The only other option is to have the Arduino simply spit raw bits into the host and bring modem code into the software. But at that, we'd be back to (what seems like) a higher degree of software timing - granted, the Teensy also has to derive its timing from a crystal clock but at least this is more akin to firmware/hardware doing all the hard work and let the terminal do what it does best.
And so, the BaudotConverter library has shifted from the SerialRTTY Qt modem into the Teensy world, and at some point we took care to align the character set with most modern standards.
Other notes:
- Worked two contests (Makrothen and CQWW) for about 200 contacts and had no busted calls for at least the latter.
- One welcome statistic is that I had no instances of someone reporting NEG COPY or such, as would happen with a UART/driver timing of a flat 45 bauds.
- Most recent tweak is the low-noise preamp (Q1) to drive the op amp at low signal levels. This may not be necessary for all rigs but with the Icom I would often have to cheat and read from the rig's decoder.
- Fun fact: recent experiments with a vintage guitar fuzz pedal have revealed that the 2N4401 is an ideal low-gain amp with a very low noise figure, hence its prominence.
- Q4 was tried with an n-channel MOSFET at one time and it performs exactly as the NPN, and of course a PNP reverses the polarity and the Teensy decodes nothing. And so, the main thing is that the device work with the voltage divider and work within the 3.2V Teensy logic.
- Word to the wise: I'm on my third Teensy. What blew them up was being powered from the USB host and operating in circuit with its own power. So, be sure to sever the host 5V power right away and set up a 5V regulator in the circuit to save on magic smoke.
- The resistor shown as R5 was formerly a trim pot but wiser times show that this value isn't terribly granular as long as R6 can be adjusted. For that matter, the space freed up by that trim pot can go to good use as a handy spot for the Q1 preamp.
My SerialRTTY (including Arduino) code base is here - for the moment this project is in the 'teensy' branch but could easily become main branch.
Many, many thanks to Paul Williamson for the fine work on SlowSoftSerial library which obviously made this all possible.
As always - this is intended as a starting point for experimentation, not a final solution - especially when it comes to adapting to fine rigs other than Icoms.
Simply put, if your approach to radio is to just buy, buy, buy then just remember you get what you pay for, and sweat buys a hell of a lot more than any other currency.
