Trying to probe RS232 on an analog oscilloscope - need some help. Thx

[Electro Fan]

Ok, 1200 baud (roughly 1200 bits per second?) would mean 10 bits would take 1/120th of a second; not sure if I calculated that right but even if I did, how do I specify precisely that amount of hold off?  On my scope I can use the hold off knob by simply eye-balling when the pattern stabilizes, but I'm sure if I can key in a specific time value - or maybe I'm confused, some more 

I don't know your scope well enough to say what options you have for specifying a precise holdoff value.  But even if you have to adjust it only by turning an analog knob, you can achieve the right result by sending something that is NOT a "U" and adjusting holdoff to the lowest value that gives you a good stable pattern.   For example, a "Z" is hex 5A, which means that the second four bits are reverse of the first four.  It's got plenty of transitions, but you'll only get that sequence of two '1's to be stable in the right spot if your trigger holdoff is set properly.


Also, I'd recommend you make a simple RS-232 breakout box so that you can probe things while devices are talking to each other.  It can be as simple as taking male and female DE-9 connectors, wiring everything straight through, but expose the insulation on pins 2 & 3, as well as pin 5, so that you can easily connect scope probes to 2&3 and ground clips to pin 5.

When you have a DSO, you can use that kind of breakout box to really dig in to serial device protocols.  With a 2 channel scope, you put pins 2 & 3 each on a channel, and you can see the call and response as two devices talk to each other.

I get it - Very cool suggestions!
Thanks