Limitation on RS-232 OUT signal to more than 3 devices?

[Plevyakh]

Guys,
In wiring up my Ack Avionics E-04, I need to tap into my GPS to provide GPS position to the ELT. Ack only recognizes the standard RS-232 Data, so I can't feed it ARINC429.

In checking my avionics wiring harness I had built up by Stark Avionics...they wired my P1001 GTN650 connector Pin 8 (RS-232 OUT 1) to 4 different outputs (see table below):

1) T22 Xpnder
2) GRT PFD 1 (EFIS display 1)
3) GRT MFD 2 (EFIS display 2)
4) DigiFlight II

On the P1001 connector the other RS-232 OUT pins are currently NOT IN USE.
1) Pin 5 = RS-232 OUT 4
2) Pin 6 = RS-232 OUT 3
3) Pin 7 = RS-232 OUT 2

I read in this thread GTN 650 Aviation out config software Bug? that the GTN650 has a limitation of GTN V2.00 software that only one serial port may provide Aviation Out.

I spoke with Garmin support and he confirmed they'll be coming out with SW v3.00 by then end of Dec/Jan that eliminates this limitation.

As explained to me by the tech at Stark Avionics:
Garmin did not design the GTN series any differently than the GNS series, when it comes to the RS-232 ports. You must continue to use the RS-232 out one for all your ?aviation out? needs. If putting that many lines on one output starts to become a problem, then you will need to buy a RS-232 splitter/amplifier.

So my questions is....

Doe anyone have running either a GTN650, GNS 430 or 530 with a single RS-232 OUT signal feeding 4 devices?

Is it working properly....or do you experience any issues with data integrity?

What is your hardware configuration?

The Garmin Tech explained the rule of thumb is a max of 3 devices being fed by one RS-232 signal. More than that...and the data signal goes from square wave to more sinusoidal....impacting possible the data quality and how its read by the receiving device.

I'd rather fix this now....while the wiring bundle is exposed, then dig it out later.

Thanks,
Howard
hplevyak@mac.com

[BobTurner]

Not what you asked, but...
Do you have any unused serial out ports on one of the GRTs? Why not use one of them? (I understand that makes the ELT dependent on both the EFIS and the GPS, that's a trade off.) Or does the transponder have an unused serial out?

Or, run two wires now, one from the EFIS and one from another serial port on the gps, and swap wires at the ELT if and when the Garmin bug is fixed.

[mosquito]

Looks like Vern Little's RS232 'Buffer in a Backshell' has been open-sourced and is available fully assembled or as a kit:

http://makerplane.myshopify.com/prod...uffer-trx-232a

Just stumbled across http://makerplane.org while looking for the Vx site... very interesting!

-jon

[Sparky]

My GNS-430W RS-232 Out 1, P4001 - Pin 56, is connected to 5 devices and works great...

1) AFS AF-4500 EFIS
2) AFS Pilot (DigiFlight II) Autopilot Controller
3) TruTrak ADI
4) GTX-327 Transponder
5) ACK E-04 ELT

If it doesn't work well you could always use an RS-232 Buffer/Amplifier as mentioned.

Most modern RS-232 transmitter devices will easily drive up to 5 loads, as long as the distance to the load isn't excessive. On our RV's, none of the distances would be considered excessive.

[Plevyakh]

Originally Posted by Sparky

My GNS-430W RS-232 Out 1, P4001 - Pin 56, is connected to 5 devices and works great...

Most modern RS-232 transmitter devices will easily drive up to 5 loads, as long as the distance to the load isn't excessive.

Paul,
What distance are you carrying the RS-232 to your ELT? Both my ELT and the Trig Avionics TT-22 Transponder are AFT in the tail cone...so about a 20ft long run considering snaking it down the panel and aft.

I'm thinking I'd breakout the TT-22 transponder and ELT and put on RS-232 Out 2 for reliability and do the GTN V3.00 upgrade. Those are the one's I'm worried about data drop out due to wire run lengths.

Thanks guys for the tip on the MakerPlane RS-232 booster device as well!

I'd prefer not to daisy chain the signal from GPS to EFIS and then to the device...makes debugging a nightmare if a problem comes up.

Howard

[Sparky]

Howard,

My ELT run is about 12 feet, to behind the baggage bulkhead, and is the longest run and when I tested it the signal was strong. 12 feet is definitely not considered very long for RS-232 signals. The other 4 runs are shorter as they are mounted to the instrument panel.

I didn't daisy chain any of the runs. I ran the output of the GNS-430 to a female DB9 connector located behind the subpanel and created a "RS-232 Bus" by connecting all the DB9 pins together on that connector. I then have each of the RS-232 loads connected to the other (male) side of the DB9 connector. This way I can add or remove a device by simply accessing this connector and adding or removing a pin.

Edit: See here on how to do this...
http://www.aeroelectric.com/articles...s/minibus.html

[Neal@F14]

The "only one RS232 output gets the GPS aviation data" limitation has been in Garmin units since the very first GNS-4xx units were released way back when.....

Lots of folks have successfully paralleled multiple RS232-receiving devices from one Garmin GNS-4xx/5xx output before... probably not much collective experience yet on the 6xx series yet since they're so new, but it will probably work fine. You might want to use 2-conductor shielded wire for the longest run though.

In a former lifetime at a former job many aeons ago, I had a bunch of serial dot-matrix printers (3 or 4) in a large warehouse connected to one computer serial output port (for "broadcast printing"), with no flow control, and one of them was a few hundred feet away. It was connected using a couple strands of a plain old CAT5 cable duct-taped across the concrete warehouse floor. The printers all seemed to work fine, except whenever someone would drive the electric-powered forklift across that longest cable, it would spit out a couple of garbage characters on that one printer (from the forklift's large motor inducing EM interference into the long wire)