Sharing NMEA 0183 Outputs

[ArVeeNiner]

Just a basic question, can a single NMEA 0183 output be shared by more than one device? I'm hooking up a Lowrance 600C and would like the single NMEA 0183 output to go to the Dynon, the Trio A/P, and the GTX327 transponder. Is this possible and can I just tap into the output wire?

Thanks

Kelly Johnson
RV-9A wiring

[hevansrv7a]

Originally Posted by ArVeeNiner

Just a basic question, can a single NMEA 0183 output be shared by more than one device? I'm hooking up a Lowrance 600C and would like the single NMEA 0183 output to go to the Dynon, the Trio A/P, and the GTX327 transponder. Is this possible and can I just tap into the output wire?

Thanks

Kelly Johnson
RV-9A wiring

At least with a Garmin, you can split it up to 3 ways with no problem. I'm running mine to the AP and the EFIS, for example. It's just a one-way serial connection.

[apatti]

With RS232 you can usually tie a single transmit output to multiple receive inputs without any trouble. This assumes the output driver has enough ummph (most do). The receivers are normally high enough impedance so that they don't draw too much current. Three shouldn't be any trouble. This will also be impacted somewhat by the baud rate and the length of the wires. The lower the baud rate and the shorter the wires, the less likely there will be a problem. Again, I can't imagine there would be any trouble with this in our applications.

Just don't tie multiple transmit outputs together. That is a no-no and could damage the circuitry.

Does anyone know if GPS outputs swing +12 to -12 or +5 to 0??

[az_gila]

Originally Posted by apatti

Does anyone know if GPS outputs swing +12 to -12 or +5 to 0??

Tony,

The outputs can have large swings, but the input switch levels are designed for typical 5 volt logic as in RS-232 levels.

Seems to be negative logic.... "1" < 2.0 volts and "0" > 3.0 volts ... at a receiver input...

From here...

http://www.actisense.com/Downloads/D...sue%201.10.pdf

gil A

[apatti]

Thanks, Gil. This is consistent with what I rember from when I did engineering work (aka "the old days"). Seems like we used to clamp the RS232 drivers with diodes to keep the output between -.3 and +5. Then we could use TTL circuits and didn't need a -12v or +12v supply voltage.

I noticed the product sheet you referred also had RS485 cables. I was surprised to learn that most aircraft serial comm is RS232. Even the communication between GRT's AHRS' and the PDF's is RS232. I really thought most folks would have gone to 485. I know 232 is fine for the speed and distances we use in our aircraft. However, 485 is much more noise tolerant. I figured that alone would make it more deisreable. There must be something I don't know. There are folks who are a lot smarter than me making these decisions.

[ArVeeNiner]

A thought occured to me, I know my GTX327 has a data output as well. Are these types of data outputs passthroughs or do they strip out some of the data from the stream? For instance, I assume that the GTX327 wants GPS input so that it can go active when the airplane is moving and go inactive when it stops. If that's the case, all it cares about is groundspeed I would assume. So, can I assume that ALL the data the GPS is spitting out goes through the transponder and is usable at the output end? I'm assuming the data that comes out is not altered in any way by the transponder.

So my thought is this regarding data:

Lowrance 600C to GTX327 input
GTX327 data output to a splice
One side of the splice goes to the Dynon 180 input
the other side goes to the Trio autopilot

I don't see a data output for the Dynon except for a PC output which, according to their directions, is used for PCs only. I don't see any other way to split this data with other units.

Thanks

[az_gila]

Originally Posted by ArVeeNiner

.......
So my thought is this regarding data:

Lowrance 600C to GTX327 input
GTX327 data output to a splice
One side of the splice goes to the Dynon 180 input
the other side goes to the Trio autopilot

I don't see a data output for the Dynon except for a PC output which, according to their directions, is used for PCs only. I don't see any other way to split this data with other units.

Thanks

Kelly... did you omit the RS-232 data from the Dynon to the GTX-327 that provides the Mode-C altitude data?

gil A

[jdeas]

Originally Posted by apatti

Thanks, Gil. This is consistent with what I rember from when I did engineering work (aka "the old days"). Seems like we used to clamp the RS232 drivers with diodes to keep the output between -.3 and +5. Then we could use TTL circuits and didn't need a -12v or +12v supply voltage.

With inexpensive rs-232 drivers I would not assume anything less that bi-polar voltages. I have not opened up a controllers in many years that did not have some sort of charge pump to make compliant RS-232. Even the 3v chips (like max-232a) use charge pumps to create the negative voltage.
These chips are very robust so I doubt you would damage one but you will decrease signal integraty.

[ArVeeNiner]

Gil:

I didn't mention the encoder input from the Dynon because that's an easy one. The Dynon has its own dedicated altitude output that doesn't have to be daisy chained so I understand that one.

My question is regarding the information required by the Dynon for winds aloft (I think the Dynon can calculate this based on heading vs. ground track) and mpg, miles to empty, etc, position information needed by the autopilot, and presumably ground speed (or movement) needed by the transponder. The question is how do I physically hook up a single data output from the GPS to three locations? Do I just splice the output wires themselves? Do I run the GPS output to first the transponder, then take the transponder output and T it off to the Dynon and the autopilot? Do I use a connector somewhere in line and jumper the information off to other pins within that connector and send the signals off on their merry way? I don't see another way of doing this.

Thanks again.

Kelly

[Sam Buchanan]

Originally Posted by ArVeeNiner

The question is how do I physically hook up a single data output from the GPS to three locations? Do I just splice the output wires themselves?

Yes, just splice the data leads together. The only glitch that may occur is if the data signal from the source isn't strong enough to send good data to all your devices. If that is the case, disconnect the least needed device to see if that will allow the other two to work properly.