[Uaflug] [OT] Looking for a serial level converter

[Orion Sky Lawlor]

On Mon, 13 Aug 2007, Joshua J. Kugler wrote:
> I'm about to buy one of these:
> 
> http://www.ftdichip.com/Products/EvaluationKits/TTL-232R-3V3.htm

I've been hacking away with USB-to-serial-to-microcontroller
stuff this summer for robotics, and I've been quite
surprised to find that standard RS-232 (the usual PC serial
protocol) actually both sends and receives from TTL
microcontrollers without any problems.

Officially, RS-232 serial voltages are -12V to +12V bipolar, 
and TTL is 0V to +5V, so you wouldn't think this would work.
But it *has* worked fine on the half-dozen PC serial ports, 
and three different USB-to-serial converters I've tried--
this makes me think some nonzero fraction of RS-232 
peripherals actually use TTL voltages, so ports secretly
all accept them.  The only gotcha to watch out for is to put 
a little resistor (like 1Kohm) on the big computer's TX line, 
since the -12V from there can freak out your TTL input port
(a few protection diodes shunting to ground and +5V wouldn't 
hurt either, but I haven't needed them on PIC microcontrollers).

So if you're willing to build a 6-pin harness yourself, you 
could just use your motherboard's builtin serial port, or 
buy a $9 USB-to-serial converter from newegg:
	http://www.newegg.com/Product/Product.aspx?Item=N82E16812188102

Locally, Geek City has a basically identical USB-to-serial converter, 
but it's $40.  Both of them just plug-and-play under Ubuntu 6, 
showing up as /dev/ttyACM0.  You set the baud rate, parity, 
and so on with stty:
	stty raw clocal 57600 cs8 -parenb -cstopb -echo < /dev/ttyACM0

Now you can fling bytes out the parallel port with:
	echo "Wazzup?" > /dev/ttyACM0
and read them back with:
	cat /dev/ttyACM0
(Replace ttyACM0 with ttyS0 to use your motherboard's serial port)


> http://www.futurlec.com/Connectors/IDCMH10.shtml  Got a good source for them?

I'd go ghetto-style and saw a 16-pin socket down to 10 pins:
	http://www.goldmine-elec-products.com/prodinfo.asp?number=G16033
(Electronic Goldmine is my favorite component supplier.  
They're insanely cheap, and ship USPS here in under a week.)
--
                                   -Orion Sky Lawlor

http://lawlor.cs.uaf.edu/~olawlor/     ffosl at uaf.edu