Electronic – Disadvantages of USB to RS232 dongles

Here is how I would suggest going about debugging this problem:-

1. Check the connections.. if you've mixed up signal ground and chassis ground (shield) you could get funny behavior just like you describe. Pin 7 on the DB-25 is the one you want to connect to DE-9 pin 5. I'll assume you're using a commercial adapter/cable or have one wired like this.

   

   Maybe check the transmit out line to ensure it's idling at -12 ( < -3 anyway).

2. Make sure the instrument is set to RS-232, and not current loop (Jumper 7,8 and 1,2).

3. Try the most common serial port setting (9600/8-N-1), 9600 baud, NO parity, 1 stop bit switch 1, 2, 3 OFF switch 4, 5 ON switch 6 ON switch 7, 8 ON

   Make SURE you understand which is 'OFF' and which is 'ON'. Don't laugh, I've seen this happen more than once, and some types of DIP switches are kind of ambiguously marked. If in doubt, an ohmmeter across the switch.. in fact you could check all of the switches to ensure they're working.

   I like Realterm as a terminal program. It's free, and easy to change the parameters. Try it on something known working first of course, and look for an online tutorial. Really worth it.

4. If still gibberish after that, get a scope screen cap or photo and post it.

5. If you have a continuing need for a logic analyzer, this one is very cheap and relatively useful. Not as good as my Tektronix boat anchor, but this one will fit in a coat pocket. But I'd get a scope first if you don't have one. Even the worst scope is better than nothing.

For USB, the easiest way it to monitor for the presence of VBUS.

I'm not sure what you mean by "overcurrent", you shouldn't need to load the USB power line much if at all. If you're really worried, stick a 10K resistor in series with the USB Vcc, and just run it to an input pin. That should protect you from any faults.

Unfortunately, for serial, it can be a lot more involved.

If you're lucky, your system uses either inverted TTL level signaling, which means the line level when there is no traffic is Vcc, or RS232 level signaling, which means the no-traffic line voltage is either ~+10v or ~-10v, depending on whether your bus is inverted.

Then, you can simply monitor for the voltage level. 0v means nothing connected.

If you're unlucky, your system uses TTL level signaling, with the quiescent bus state being 0V. In this case, really the only possible way to determine if a device is connected is to actually send out some traffic, and see if you get a response.