Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!sun-barr!lll-winken!tekbspa!optilink!elliott From: elliott@optilink.UUCP (Paul Elliott x225) Newsgroups: sci.electronics Subject: Re: Using MC1488 for TTL->RS-232 Summary: 1488 requires at least +/- 9V Message-ID: <2876@optilink.UUCP> Date: 2 Jan 90 17:32:16 GMT References: Organization: Optilink Corporation, Petaluma, CA Lines: 38 In article , kg19+@andrew.cmu.edu (Kurt A. Geisel) writes: > I am trying to convert a TTL signal into an RS-232 signal. I was > under the impression that this can be done simply with the MC1488 > monolithic quad serial line driver. Here is my set-up: > > 2 |----------| 3 > TTL ---------->| |---------> RS-232 > 1 | MC 1488 | 14 > -5 V --------->| |<--------- +5 V > 7 | | > GND ---------->|----------| > > What I get: > > When the TTL input is at 0 V, the RS-232 output is at 3.25 V. When > the TTL input is at +5 V, the RS-232 output is at 3.94 V. Needless to > say, this is useless. The MC1488 requires a split-supply of at least +/- 9V to yield RS-232 output levels. (with 9V, you get +/- 7V typ. out). I am puzzled by the output level you get with +5V TTL in, however. Looking at the internal schematic of the part, it seems that you should get a negative output of about -3.6V. Perhaps there is a wiring error? Anyway, if you don't have the higher Voltage supplies available, may I suggest trying the MAX-232 transceiver from Maxim (*). It has an on-board charge-pump (using external capacitors) that boosts a +5V supply to +/- 10V for driving the RS-232 line, and is very easy to use. This part is being second-sourced or cloned by other manufacturers. * Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 -- Paul M. Elliott Optilink Corporation (707) 795-9444 {pyramid,pixar,tekbspa}!optilink!elliott "I used to think I was indecisive, but now I'm not so sure."