Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!portal!cup.portal.com!ISW From: ISW@cup.portal.com (Isaac S Wingfield) Newsgroups: sci.electronics Subject: Re: Using MC1488 for TTL->RS-232 Message-ID: <25569@cup.portal.com> Date: 3 Jan 90 08:47:14 GMT References: <1990Jan1.001244.24596@utzoo.uucp>,<34032@mips.mips.COM> Distribution: usa Organization: The Portal System (TM) Lines: 28 Kurt Geisel writes: >Thanks for all the help. The consensus seems to be: it should work, >but you really should be using +/- 12v. Well, this is very temporary >and very hardware specific, so I think I'll stick with it for now. >The port I'm trying to drive seems to accept stuff well under the 232 >spec. In fact, the 1489 (RS-232 receiver, the most likely chip you're talking to), is usually biased so that the threshold voltages are about 1V and 3V (it's specs are rather loose). You have to add a negative bias in order to force the lower threshold below 0V, and most designs don't bother. The upshot of this is that *in most cases* you don't need a special driver at all - just use a CMOS inverter. I like to wire two in parallel for lower impedance. TTL thresholds are a little close to the 1489's for comfort. BTW, use another CMOS inverter for the receiver: RS-232 signal goes through 100k or so to limit current right into the inverter, and put a couple of diodes from GND to input and from input to VCC, both cathodes pointing up, to clamp the input for safety. For best noise immunity, use a Schmitt inverter. Not commercial quality, I know, but I've been doing this for years, and have yet to have a malfunction or part failure. Isaac isw@cup.portal.com