Trans-Cal Altitude Encoder Model Ssd120-30N-Rs232

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The world's smallest and lightest altitude encoder.

There are two versions of this model:
  • Standard Version SSD120-30N-RS232
  • Version SSD120-30N-RS232-MOD1 for NARCO AT-5A, 6A, 50, 50A Transponders. The Narco AT-5A, AT-6A, AT-50* or AT-50A transponder will not accept data from the SSD120-(XX)N-RS232 Altitude Digitizer. A recent modification to remove the output decoupling capacitors is required and this version includes that modification.
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  • Featuring the best warranty in the business: 42-month manufacturer direct replacement. If your Trans-Cal encoder experiences a warranty-covered failure, it'll be replaced with a brand new unit with a new 42-month warranty.
  • Average MTBF: 9.7+ years. Thats right, Trans-Cal encoders are among the most reliable in the market, commonly providing reliable service for over 10 years.
  • Pin for pin replacement for most other manufacturers' encoders.
  • Optional: Available with adapter plates that will allow the installer to mount the unit in another manufacturer's mounting tray.


  • Operating temperature: -20c to +70c
  • Operating altitude: -1000' to +30,000'(available in altitudes from 30,000' up to 100,000')
  • Accuracy: 50' from -1000' to +30,000
  • Weight: 5.9oz.
  • Will operate on either 14 or 28 VCD at 0.270 Amps.
  • No warm-up time: +20c to +70c, -20c to +20c, 30 seconds
  • *The Narco AT-50 and earlier transponder models require a modification before thay will function correctly with any altitude encoder. This modification is outlined in Narco Service Bulletin AT-50A-5.


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Heinz S Verified Purchase


June 8, 2022

replaces the same p/n

Ron P Verified Purchase


August 21, 2021

Clifford S Verified Purchase


August 17, 2021

Compact, serial-interface, 10 resolution. This is my go-to encoder for all installations.

Lloyd A
March 29, 2021

This is the only Encoder we install along with ADS-B Transponders, thank you for keeping them in-stock!!! We are also so thankful for the Chicago Warehouse, we get our orders in 1-day!!

Diane W
December 19, 2019

An excellent plug and play replacement for the SSD120-120. This unit did not require any calibration after installation. The RS232 port wasn’t used but left the future upgrade option open.

June 8, 2018

Excellent quality and is very versatile. It did not require adjustment.

Gary H
November 5, 2017


Please note,'s personnel are not certified aircraft mechanics and can only provide general support and ideas, which should not be relied upon or implemented in lieu of consulting an A&P or other qualified technician. assumes no responsibility or liability for any issue or problem which may arise from any repair, modification or other work done from this knowledge base. Any product eligibility information provided here is based on general application guides and we recommend always referring to your specific aircraft parts manual, the parts manufacturer or consulting with a qualified mechanic.

Q: This unit mentions the Narco units up to the AT50A. Will the standard unit accept a Narco AT150A?

Yes, the Narco AT150A is compatible.

Q: Can this Trans-cal SSD120-30N-RS232 work as a drop in replacement for the AK-350?

Yes, they are both encoders so the Trans-cal unit can replace the Ak-350. However, since they are from different manufacturers, the wiring will be different and it will not be plug and play. It will require a new install.

Q: Does this Trans Cal SSD120-30N-RS232 encoder has a separate serial output connector from the connector that attaches to the transponder? I'm looking for a encoder that has a separate output connector for the serial to a gns430.

Per the manual, you will be able to give signal to both the transponder as well as the GNS 430.

Q: Is the Mounting Tray (P/N:103018) included in the package?

Per the manual, the mounting tray does not come included in the package.

Q: Will the Rs232 Data work with the Stratus ESGI?

Yes, part # 11-06983 will work with the Stratus ESGI for RS232 data.

Q: Just a comment for one of the answers. The encoder comes with its own mounting bracket for new installs, but if you are installing it in place of some other brands encoder, the adapter plate is an extra cost option.

Thank you for providing this information. We will post it for other users to see.

Q: Does the SSD120-30N-RS232 replace the SSD120-30A-RS232? Mine just went down and I need a replacement quickly?

Per the manufacturer, the new altitude encoder is backwards compatible in regard to wiring. There are also adapter plates available to accommodate the change in size and shape.