My HyperTermnal program specifies 8 data bits, 1 stop, no parity, and there is no mention of a start bit. (Earlier in my career I saw examples of 7 or 8 data bits sandwiched between a start bit and a stop bit, plus a parity bit, so I get that it is possible.) In my current setup it isn't clear there is a start bit.
Nonetheless, are you pretty sure I have a start bit in front of each byte?
No, I'm not
pretty sure. I'm
absolutely and unequivocally positive.
Think about it a sec. You're sitting there Idling, with no characters being transmitted. Your voltage level is at, say, -5V. Then you send out a byte, let's say it's FF, which will also have all bits negative voltages. With no start bit, nothing happens. Then you have the Stop bit, which also goes negative. So nothing at all happened when you sent out an FF? Illogical.
RS232 isn't self-clocking, so you need to have a framework around the data-frame in the bitstream. The Start and Stop bit provide that framework. (And the reason you paid attention to the Stop bits is because there could be 1 or 2. And parity could be Even, Odd, or None. So you had to pick, and get those options right. But there's never anything to decide with the Start bit. It's always there, and always 1 bit long.)
Frankly, the only thing that I have really focused hard on is finding any combination of the variables that I have to work with that gets the waveform to synchronize with the 1s and 0s - which I have accomplished and which only seems to work with one combination of settings. (I'm pretty/very sure that I have 8 data bits, 1 stop bit and no parity. So mostly what is left is polarity and MSB/LSB settings.
Yes, I understand. Part of the reason that an 0x55 is a bad choice is it's symmetry. What happens if you mis-set the polarity, AND the endianism in your Config? Uh, it winds up being still an 0x55, sort of, due to the double inversion.
...to be square we've heard from users here who bailed out on Rigol scopes because they found errors in the decoder.
So I think you can assume that while there is a good chance I haven't got all the setting right, there is also a chance (until you get your hands on a 1000Z) that there might be something in need of a firmware adjustment.
I never suggested otherwise. I wasn't attempting to defend the Rigol... there could be numerous issues. I'm just saying that until you get your ducks in a row that the oddities you're seeing cannot be sorted out. If you do something wrong, the results are going to be wrong, even if the Rigol is right. If you do everything right, then things could still be wrong... but in that case you then know it's an issue with the Rigol. Comprendo?
...but again, look at the last image and you will see that the image shows what strikingly looks like a high degree of symmetry and alignment between the binary code and the waveform. Obviously the viewing is limited by what practically fits on the screen, but the last image has what looks like 3 full bytes.
Yes, but what I was trying to point out is that you achieved that by mis-setting the Config options.
I'll see if I can get some other characters going - or feel free to post a specific repeating string you like and I'll give it a try.
As I mentioned above, just a string of simple 0x01's (decimal 1's, binary 0b00000001's) would provide a distinctive and unambiguous pattern. There are certainly others, but that's one where I described the exact on screen voltage pattern that corresponded to it.
Thx again for the help.
No problemo. I'm glad to see you digging into it. Just sorry to see you getting hung up on RS232, which seems a lot simpler (at the logical level) than it really is (at the physical level). E.g., while TX and Rx are inverted, the Control lines (RTS, CTS, DTR, DSR, etc.) are not. Once you get on the other side of the line-driver, your levels aren't shifted any more, and the bit polarities aren't inverted. They're still LSB though.
Someone else suggested you read up on a reference book on RS232, but you may get enough from
Wikipedia to clarify a few things. Jump down to the section on Voltage Levels, and check out the picture there...