Altitude encoder or not?

[Markmp]

In receiving quotes from a few avionics shops for a GPS+ADS-B upgrade, I've noticed some shops say I need a separate altitude encoder, specifically the GAE-12 to work with the proposed Garmin GTX345. Other shops say I should get the ACK A.30 encoder. Some shops say I don't need either with the GTX but I can install if I want to. 

Can someone shed some light on whether I need a separate encoder or not, and which one is appropriate? (and why?). Assume a GTX345+GTN650 setup.

Thanks!

[Bob - S50]

We did not need to buy a new encoder when we had our GTX345 installed.  The one we had worked just fine.  However, if I was going to buy one, I'd probably get the one that Garmin makes (GAE-12) just because it is small, light weight, and easy to install.

[Andy95W]

The GTX needs altitude encoder input.  Most modern avionics use RS-232 which is fairly simple to connect, but older avionics used a 10-wire system called Gilham or grey code.  Your old encoder may only output grey code, and the Avionics shop wants to install the more modern (and easier) RS-232.  

IMO, if your encoder really is that old, get a new one.  They really aren't that expensive and you could be avoiding problems down the road.

[Cruiser]

The GTX 345 requires external uncorrected pressure altitude input.

If multiple sources are available the GTX 345 will use the following priority for input

ARINC 429 data

RS-232 data

GAE module

Gray Code

 

[jetdriven]

I Put in the garmin encoder. It was only like 250$ and low installation cost.  It snaps into the gtx backplate. 

[nsrrlearjet]

I just put in a GTX 345 & GTN 650 a few months ago. So far so good.


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[bradp]

Garmin unit is tucked away and small.  Price is in line with a blind encoder   

Cruiser has it right - 345 has multiple sources in an order of priority for pressure altitude.