Joe Della Barba joe at dellabarba.com
Sun Feb 6 11:47:28 EST 2011

I ordered a couple last night from FleaBay.
Did you feed all 4 of the internal op-amps for 4 outputs, or was the output
from just one strong enough to do the trick?
73 de N3HGB 

Joe Della Barba
joe at dellabarba.com

-----Original Message-----
From: aprssig-bounces at tapr.org [mailto:aprssig-bounces at tapr.org] On Behalf
Of Stephen H. Smith
Sent: Saturday, February 05, 2011 9:26 PM
To: ,TAPR APRS Mailing List
Subject: Re: [aprssig] NMEA ISSUE HELP

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
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
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
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
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
inverting input of the op-amp to ground through a 2-meg resistor.   Drive
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
upside down data sense as some surplus OEM GPS devices are) , just reverse
two connections.   Connect the GPS signal to the inverting pin and ground
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
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
and +10 VDC, rather than between -10 and +10, you don't even need the

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