🚀 go-pugleaf

In article <MZb4w2600WB88M=4wv@andrew.cmu.edu>, 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."