This issue seemed different enough from "multiple serial ports" thread that I decided to start afresh.

I'm looking to build a box that will take output from a GPS and send the output to 3-4 devices (Empeg, APRS-capable radio, laptop and ???). One-way only, so would seem I would need TX+GRND only, true?

This is not really a multiplexer in the way that I think of a mux or demux, maybe a "splitter". I found a commercial version of what I am after here but it is primarily RS422. I doubt that under the epoxy, that unit is very complicated. I found a few other units, too, but they were intended for automation use and were way too big and expensive.

Can anyone take a stab at what would be involved, circuitry-wise, in a home-brew solution? If this was TX-only, might is be possible to just physically split the RS-232 output or would some kind of amplification likely be required?

Serial ports shouldn't drain too much they work on the voltage differences and are generally pretty flexible as far as what voltages actually are ones and zeros. I once tried to convert a 0v/5v signal to -2.5v/2.5v and could actually receive it with a laptop, although admittedly there were a lot of bad bits in the reception that I ended up using a max232 anyways.

My guess is that it is possible to drive several receive lines with the output of a single tx line. And if that doesn't work you could use a couple of max233's if you really need the amplification, convert down to 5v, and then hook several of the 5v->12v steps off of that line. It can be done cheaper with max232's which need the additional external caps.

I, personally, have split a simplex RS232 signal to multiple sources simply by splicing. I don't know if it's up to RS-232 spec (obviously not, since RS232 is a duplex protocol, but you know what I mean), but it worked on mutiple disparate hardware recievers for the signal. In my particular app, it really was simplex, as it was receiving a feed from a satellite dish with no way to send data back, and it was connected to a black box provided by the feed source, and split off to also feed a PeeCee running QNX, a Sun Sparc machine running Solaris, and, it seems, something else, too, but I can't remember what now.

Regardless, it's possible and that's probably the way you should try first.

that device is a commercial marine solution - no doubt ruggedised and the outputs are optically isolated meaning its pretty rugged - [good in a marine situation].

For your use however, you're probably planning on building a homebrew solution/interface to your GPS and you presumably need to convert the GPS signals from TTL level to "RS232" level already.

This means you may already have most of the solution required to do this now.

I would probably change the existing MAX23X circuit design you have to feed the GPS NMEA (TTL Level) signal out to multiple of the TTL level inputs on [presumably] your MAX232 or 233 chip instead of just the one input as now.

I can't recall exactly which one, but one of the MAX2XX chips has 4 TTL inputs and hence 4 RS232 outputs "on board", so you could have 4 independant, short circuit protected serial lines from the one GPS with a little solder work and some additional wiring.

The MAX232 may not have enough TTL level inputs - its has 3 TTL outputs and 2 inputs I think [ but - I may be wrong and have it the other way way round].

In that case, two MAX chips may be required, simply wire them to power, ground, and seperate capacitors (if required) up the same way. then wire the circuit so that the TTL level output signal from the GPS goes to the first MAX chips pair of TTL level inputs in parallel, then do the same for the second MAX232 chips TTL inputs from the same GPs TTL input.

Of course two MAX chips like this doubles your power requirements, but in the wider schema of things, thats probably ok.

If you don't need the MAX chip in your GPS interface [because the GPS is already outputting RS232 levels], then a simple splice driving multiple serial ports might work - but it would depend on the loads imposed on the signal by the serial ports and the transceivers used in the GPS device itself (and possibly whether or not the GPS was running on batteries or not) as to how sucessful it was.

If it did not work and you already had RS232 level outputs you wanted to feed to multiple devices, you could have a small "black box" device that used a MAX23X chip that took the RS232 level signal from the GPS in, [to convert it to TTL], then send it out again via the TTL level inputs to the on chip TTL level outputs, thus creating a "buffered splitter" device - the only requirement being that you can power this black box from 5 volts somehow - possibly from the GPS itself.
The power drain won't be enourmous - probably 10-20 millamps.

You could potentially power this from a [one or other] of the serial ports you're pluggingthe outputs into by putting a small 7805 "low power" voltage regulator and some zener diodes to derive the 5 volts need to run the MAX chip(s) from your pc's serial port (connected to the RTS and DTR signals) - or even just using 4 AA cells will do - provided you have a manual on/off switch or the batteries will go flat over time.

You could use this balck box to get up to 3 serial ports out of the one serial output that way.

Also note, you may be able tomultiplex multiple "RS232 level" inputs (from your PCs transmit signals) (once converted to TTL by a MAX chip) to your GPS's serial TTL level input via a MAX23X chip and either a 7407 open collector hex buffer (or 2 circuits from a 7405 hex invertor) using a "weak" (e.g. 4k7) pullup to +5V. This would let anyone of the Pc's talk to the GPS, but only one at a time can "talk"

Folks who have been working with interfacing to ISO7816 compliant (i.e. smartcard) devices using their PC serial port will know this trick because its used to multiplex the PC's RS232 TX and RX signals on the single IO line on the smartcard.

that device is a commercial marine solution - no doubt ruggedised and the outputs are optically isolated meaning its pretty rugged - [good in a marine situation].

I have one of their NMEA MUX on my boat -- combines NMEA from depthsounder, knot log, GPS into a single stream for a PC. Works great, but I always wonder if that epoxy is there just to hide their simple circuitry!!

Once again, I am indebted to BBS denizens (jaharkes, Bitt, yourself) for your enthusiastic and informative replies. Your collective advice will help get me on my way. I have some time to work on this -- like a year of so! If I am still employed come Christmas and I pass my Ham exam, I may break down and buy this radio to start working with the APRS interface. I have Trimble and Encore GPS on hand and have some MAX233 from Jameco, and I finally broke down and bought a breadboard. Yes, heck, I was going to build a 232 interface anyway, so why not try to soup it up? One goal, given the nearly nonexistent space behind my dash, is to put GPS, 5V supply and RS232 splitter in a slim package that may fit under my rear seat. Another odd idea we have had is to see if we can do a stealth install of the GPS antenna inside the cheezy fiberglass spoiler on the wagon (pics to follow).

Well, standard time is almost here, and Seattle's 100 Days of Darkness create a great opportunity for thinks like soldering. Now to clean off the desk (spent most of the past day crimping/soldering up this ).

Thanks again!

I may break down and buy this radio

The friend who I gave a Trimble SVee6 to (and who built the TTL-RS232 pullup I'm using) installed one of these the week before the SVee6s came. He seems to like it reasonably well so far.

I have got the Kenwood TM-V7E and TM-733 which are basically the previous two versions of that TM-D700A. My sister has got the TH-D7A, and they have oubiously combined the best features ofTH- D7A and TM-V7A(E) into the TM-D700A. Ufortunatley, my budget is not the best these says... Maybe I should implement a software empeg packet/APRS decoder instead ;-)

The MIC-in port also works while playing? right?

Marius (LB4PE/OZ4PE)

Maybe I should implement a software empeg packet/APRS decoder instead

That would be enough to convince me to finally start doing APRS. Otherwise, I'll get to it one of these years

Derrick (KB3EGH)

OK, I've now managed to compile a program that will take a raw 22kHz mono stream and decode AX.25.

What next?
- Read from Mic-in
- Change the sample rate
- Decode APRS
- Make GUI
- Try to optimize the code if it uses too much CPU power (I think there is some points floaating around in there, which should be eliminated)

Might it be possible to change the frequency of the tuner to 144.800 MHz (APRS in Germany), even if it's wideband reception?
I have a tinytrak working (http://tracking.rowi.net) but would like to receive those APRS-Messages with the empeg and put it on the screen. But first I'm unable to change the frequency out of the normal european specs.

Rolf (DG8KBR)

hehe. I don't think that will be possible... but you can never be sure...
Decoding tinytrack-data and puting onscreen would not be difficult at all (a tinytrack is on my wishlist for christmas).
But for just receiving aprs, it should really be enough to connect the speaker-out from an FM receiver on 144.800MHz to the mic-in on the empeg and it should be possible to decode that even with music playing...
But my time is mostly occupied with the baby these days :-)

Marius LB4PE/OZ4PE

(What I'm saying here is not 100% correct, but..)

Serial ports (RS-232) is very nearly the same as TTL (but voltage difference), and the TTL standard speciffically says you can drive more than one input from an output (i seem to remember 4?) , and you can split it in several steps to get more by installing OpAmps or NOT-gates.

Marius