Stephen H. Smith wa8lmf2 at aol.com
Sat Feb 5 21:25:46 EST 2011

On 2/5/2011 5:54 PM, Joe Della Barba wrote:
> Thanks for the help everyone.
> I looked up some NMEA specs and marine devices are supposed to have
> opto-isolators on the input side. The VHF must have one, because it reads
> the data fine.
> So I need one of those for the TT2 maybe? Is there a circuit around for
> this.
> I looked up this device you mentioned
> (http://www.datasheetcatalog.org/datasheet/maxim/MAX220-MAX249.pdf)
> Do these invert the GPS output? AFAIK, I don't need it inverted, just more
> voltage.
> Thanks!

The  MAX-232 type devices are intended to go to/from TTL  and RS232 -- not for 
buffering RS232-RS232.

There is nothing magic about optoisolators.  The main purpose is to isolate the 
device inputs from voltage surges and differing ground potentials caused by 
various boxes running off differing power sources.  The coupling between the 
input side of an opto (typically an LED) and the output (typically the 
collector and emitter of a transistor) is an infra-red light beam; not 
electrical conductors.  There can be hundreds (even thousands) of volts ground 
differential between the two sides.

My post responding to Andrew's query is what you need -- an ordinary 50-cent 
LM124 dual op-amp running in non-inverting mode.   Unlike the typical op-amps 
intended for split +/- supplies, the LM124 can run off a single-ended  
+volts-only supply  and respond to an input voltage that goes clear to the 
negative supply value; i.e. ground in the single-ended mode.

The LM124/LM224/LM324 (all are the same device but with differing temp ratings) 
running on a single-ended +12 VDC can easily drive several RS-232 and 
quasi-RS-232 (like the TT) loads with a full 0 to +10 VDC swing.

An LM124 can easily buffer the GPS signal without inverting.  You connect the 
inverting input of the op-amp to ground through a 2-meg resistor.   Drive the 
non-inverting input with the signal from the GPS-18 through another 2-meg 
resistor.     If you should somehow wind up with a device with TTL output (i.e. 
upside down data sense as some surplus OEM GPS devices are) , just reverse the 
two connections.   Connect the GPS signal to the inverting pin and ground the 
non-inverting pin through the 2-meg resistor.

Done with just an op-amp and two resistors!

This is essentially the same hookup I use in my home-brew tone-keyed sound card 
interface documented here:


Note that my response to Andrew, involving split power supplies assumed he 
wanted to reproduce a full +/-10 VDC true RS-232 signal at each of several 
outputs.   In your "half-RS-232" scenario, with the signal swinging between 0 
and +10 VDC, rather than between -10 and +10, you don't even need the negative 

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