Like the AC/DC Metering panel in the forward overhead there is no ready solution for the IRS display (Honewell and Litton), both in terms of hardware and software.  To modern standards this display is very primitive, twelve 7-segment displays, of which two are extended with a few segments so they can display a W and N character.  Looking for a solution I decided to use the Phidgets LCD display, which can display two lines of 20 characters each.  Initially I was hoping by using customer characters to recreate the original 7-segment display, but unfortunately only 8 customer characters are supported.  Well, let's move up in the world and use both lines.  In the top line the IRS position is always displayed, while the bottom line shows the information as commanded by the DSPL SEL selector.  Because of the luxurious space the information can be presented in a clearer way.

Because the IRS Panel has a number of switches I needed to interface these as well.  In the AC/DC panel I used the digital inputs that are switched (as in a matrix) by the digital outputs of the integrated interface board.  Because the interface board also has 8 analog inputs I thought I use these.  By interconnecting the switch positions with a resistor a discrete potentiometer is created.  For example the DISP SEL has five positions.  By interconnecting with 4 resistors and applying a fixed voltage (5 volts), each step will return a different voltage: 0, 1.25, 2.5, 3.75 and 5 V respectively.  The Phidgets board reads these analog values and with some simple software these values are converted to a position value (1, 2, 3, 4 and 5) which then can be fed to the proper FSUIPC value.

The IRS panel is part of the Aft overhead panel.  It features a number of switches that also need to be interfaced.  For this I am using six of the eight digital inputs of the Phidgets interface card.

I have extended the panel software so it can drive the IRS panel as well.  In order to do so I modified the structure of the accompanying XML file. It now shows the configuration of the various inputs for the Phidgets interface board that drives the IRS display.  It allows up to 8 selector switches via its analog inputs and 8 on/off switches via the digital inputs.

 

PPOS - Currently prompt only	True wind direction and speed
True Heading and status		The image on left the shows the wiring of a 4-position switch.  The 1.2 kΩ for the resistors is not critical, I happened to have these lying around.  Obviously they all must have the same value.

I made the following changes/additions:

5720 1 IRSDispSel   (sysvar.txt)          
5721 1 IRSSysDisp (sysvar.txt)          

IRSDispSel 1 (pm737.lgc)

element roundknobsmall1,-121,206,IRSSysDisp (pm737.txt)
element roundknobsmall2,-121,224,IRSDispSel (pm737.txt)

For more information about the software see the panel software page.  If you have any questions or remarks please send me an e-mail at: pvrijald@planet.nl.

True track and groundspeed