Avionics Bending: Inertial Navigation System (INS) - Part 3

Litton LTN-72RL CDU with Simulator Data

Took a few days off over the holidays. Hope everybody had a few nice days as well and is getting ready for 2013! I am now back to working on the Litton LTN-72RL implementation with the flight simulator using an ARINC 429 interface and the ARINC 575 characteristic. Before the CDU can talk to the simulator, however, the functions of the INS and IMU need to be simulated in software  as described in my previous blog posting.

The Litton INS is arranged into pages. Each page can have multiple lines and can also have sub-pages. The CDU, however, has room for 5 display lines with 16 characters each. In order to reach lines below the 5th line the up and down arrow keys are used. Litton referred to this as slewing the page up and down ... I guess today we would probably call it scrolling. The Litton CDU does not use ASCII code for the symbols but has a 6-bit character set defined by Litton. Therefore, in order to paint data on the screen the following needs to happen:

1. Translate each character from ASCII to the Litton Character set (this is done on the INS simulator side)
2. Package characters in groups of 4 into the 32bit ARINC 575 word. 
3. Transmit the ARINC 575 word to the LTN-72 CDU via the ARINC 429 interface. 

The same, of course, has to happen in reverse for every key entry. Again, for key entries, Litton encoded each key on the keyboard as a 6 bit character. 

In Litton's documentation, each screen and scroll sequence is sometimes represented by a stack of images (as seen below). So far I have worked on the database validation page as well as the position entry page. If you want to know more about the LTN-72RL document that I am using here for the blog, you can find it here: LTN-72RL Pilots Guide

Database Validation

From TP72RL-73-00 Litton Aero Products

Litton LTN-72RL CDU Simulator - Database Data Validation Entry

Once the database validates the INS will be ready for use. The LTN-72RL was originally not fully Y2K (there is blast from the past) compatible. However, it is now understood that "12" for the years does not stand for 1912 but for 2012. I also added the liberty of calling the version "72-73-CZ" instead of "72-73-00" as would have appears on the actual INS to assure that the screen I produce can not be mistaken for actual Litton screens. 

Initial Position Data Entry 

Prior to INS alignment, the initial aircraft position has to be entered as a set of longitude and latitude coordinates. The POSITION DATA page allows for entry of this latitude and longitude data as well as GMT. 

From TP72RL-73-00 Litton Aero Products

The GMT line is actually on line 6 which is initially hidden from view. To get to the GMT line the display has to be moved up one line.

LTN-72RL CDU - Position Data Page

LTN-72RL CDU - Position Data Page scrolled to see GMT