Hi, I'm a newb to the whole beagle/pi/arduino world, but from what I've
seen, beagle appears to have the best 'support system' so I'm going to
start here. I'm also not a 'hardware guy'.
I've been asked to design what is basically a data collection system, that
will have between 4 and 6 instruments connected to it. All of them are
'off the shelf' sensors that have, as their only means of interface, rs232/
db9 connectors (yes, I could change the db9's to something else 
In poking around it seems that the best way to get those inputs into the
system is with a 4 or 8 port USB hub. Is that a correct assumption on my
part or is there a way to get ports "directly" onto the system with some
sort of multi-port cape? If the answer is "you can do 4 but not 6", I
could probably live with that, as physical mounting space is somewhat
limited where all this has to go.
Thanks!
Bruce
What kind of sensors are these ? I am not exactly a hardware guy either, but rs232 if i recall correctly is serial. Whether or not this type of serial is compatible with the onboard UART serial . . . I am not sure. I am thinking that rs232 is 5v ttl where the uarts on the BBB are 3v3. Which would mean at minimum you’d need voltage level translation . . .
The point here would be perhaps a different interface type could be chosen if these two types of serial are not directly compatible. And still be cheaper.
Anyway as stated at the start I am not exactly a hardware guy, so I could very well be wrong.
RS232 is -13 to +13V.
Its NOT TTL signal
Hi Bruce
For RS-232 by far the easiest way to do it is order some of these: http://www.ebay.co.uk/itm/2PCS-High-Compatiablity-USB-to-232-RS232-Adaptor-Converter-PL-2303HX-MAX232-/271358227870. I’ve ordered a few, they work well.
There are other types but if it’s definitely RS-232 you’re using then make sure it has a MAX232 or similar chip on them, because (as iyaoshi says) it’s +/-13V so you need to convert that to levels that the USB comms chip can handle. The units I linked to above do, but most don’t - if in doubt, the board should have two chips on it. If it doesn’t it might work, or it might fry something.
If you’re not 100% sure, double check your requirements: the RS232 label is often incorrectly used as a catch all for any serial protocol. Google “RS232 RS485 RS422” and wade into the sea of standards. The big clue is RS232 has a single signal line, most of the others have two.
You can do it with a cape, I’m working with RS422 which is another variation of serial protocol so I’m using a stack of driver chips on a custom board and it works fine. I believe there are RS-232 capes you can buy, but given how easily you can get USB adapters like the one above, I’d use those with a powered hub and save yourself the hassle.
Cheers… Mike