Conversion of OEM CDU - Part Two — Flaps 2 Approach

In this second article, I will explain how the OEM Control Device Unit (CDU) was converted to enable a SimStack Foundation Board to be installed inside the unit and connected to ProSim737.

SimStacks are manufactured by Simulator Solutions, which is a Sydney based company in Australia and their foundation boards can be used with ProSim737 and ProSim320 avionics suites.

This is but one method to convert an OEM item to be used with flight simulator.

This article will mainly address the mechanical conversion of the CDU. A future article, after flight testing, will provide a review of SimStacks interface cards.

Conversion of OEM CDU Part 1.
Various photographs of the CDU and conversion process can be viewed in the image gallery.

Conversion

Many of the OEM parts used in the simulator have been converted using Phidget cards, and to a lesser extent Leo Bodnar and PoKeys interface cards. Phidgets provide a stable platform, despite the disadvantage that they, at time of writing, can only connect via USB to the server computer, and don’t enable every OEM function to be used in ProSim-AR. The primary advantage of using Phidgets is that they have been used in a wide variety of applications, are inherently stable, and their configuration is well documented.

I decided that, rather than use Phidgets, a different system would be trailed to interface the CDU with ProSim737.

he SimStack Foundation Board mounted on an angular bracket inside the CDU. Fortunately there is ample room to mount the board inside the CDU

SimStacks by Simulator Solutions

The conversion of the CDU was done in collaboration with Sydney-based company Simulator Solutions Pty Ltd. Simulator Solutions use their propriety interface boards called SimStacks to convert OEM parts for use in commercial-grade simulators.

SimStacks is a modular, stackable, and scalable hardware interface that is designed to integrate OEM parts into your simulator with little or no modification. One of the many advantages in using a SimStack board is that the interface can connect with either the server or client computer via Ethernet (as opposed to Phidgets).

To date, Simulator Solution’s experience has been predominately with the conversion of B747 parts and Rodney and John (owners) were excited to have the opportunity to evaluate their software on the 737 platform using ProSim737.

Converting the CDU - Choose Your Poison

There are two main camps when discussing how to convert an OEM part. The first is to use as much of the original wiring and parts as possible. The second is to completely ‘gut’ the part and convert it cleanly using an interface that connects seamlessly with the avionics software in use (ProSim-AR). A third option, although expensive and in many respects ‘experimental’, is to use ARINC 429.

ARINC 429 is a protocol used in real aircraft to enable panels etc to be connected with the aircraft’s systems, and although it can be used in a simulated environment, it’s not without its shortfalls, in particular, the use of AC power (in contrast to DC power).

To use SimStacks the internal components of the CDU had to be removed, with the exception of the internal shelf divider and keypad. In hindsight, the pin-outs of the Canon plugs could have been used, but in doing so a female Canon plug would have been required, and for the use of a couple of pins, the price of a Canon female plug was expensive.

Keypad and Screen

The keypad and screen are the two most important parts of the CDU.

The keypad forms part of the lightplate. The backlighting for the keypad is powered by 21 5 Volt incandescent bulbs, strategically located to ensure even backlighting of the keys.

table 1: provides an overview of bulb location, part number and quantity

Like anything, bulbs have a limited left and, although OEM bulbs are renown for their longevity, there is always a chance that some bulbs are broken. In this case, there were 3 bulbs that needed replacement.

Disassembling and removing the keypad from the main body of the CDU is straightforward; several small Philips head screws hold the keypad in place. Once the keypad has been removed, any ‘blown’ bulbs can be replaced.

The most important area is the keypad is what is called the terminus (bus). Several wires from the keypad travel to the bus and then to the various (now removed) parts in the CDU. The Simstack Foundation Board is wired to the bus, therefore, care must be taken to not damage these wires between the bus and the keypad.

737 CL CDU showing older green coloured text displayed on CRT screen

I found that the wires were quite short and needed to be lengthened; this can be done by splicing longer wire to the existing wire. Although it's possible to replace the wire to the keypad, this would entail re soldering the wires to the various keypad points - a process that requires very exact soldering.

CRT and LCD Screen

The Classic CDU from airframes up to the Boeing 737-500 is fitted with a solid glass cathode ray tube (CRT) screen.

The CRT screen is approximately 2 cm thick, curved in design, and fits snugly within the display frame of the CDU. Although it’s possible to make this screen operational, the display will be mono-colour (green) and the screen resolution poor. Therefore, the CRT was replaced with a custom-sized high resolution colour LCD screen.

To replace the CRT screen is not without its challenges. The first being that the LCD screen is not 2 cm in thickness and will not fit snugly within the curved display recess of the CDU frame. To rectify this shortfall, a piece of clear glass must be ground to correctly fit within the frame. This piece of glass replaces the 2 cm thick, curved CRT glass.

Photo showing how the thin LCD screen was secured with tape the glass screen. Although the process appears rudimentary, it's functional

The thin LCD screen is installed directly behind the clear glass using high density tape. Commercial grade double-sided sticky tape is the easiest method, but it is rudimentary. The reason that tape is used, is that should the screen fail, it’s easy to remove the tape, install a replacement screen, and then tape the screen in place.

During the design phase, it was thought that the thick piece of glass would cause a refraction problem. However, although the theory suggests refraction will occur, the practical application has been such that any refraction is not readily noticeable.

Installing the SimStacks Foundation Board and Screen Controller Card

To enable the CDU to operate, four items need to be mounted inside the CDU.

(i) The generic Interface card that controls the LCD screen;

(ii) The LCD screen controller (buttons that control brightness, contrast, etc);

(iii) The SimStack Foundation Board; and,

(iv) The wiring to connect the keyboard to the Foundation Board.

Fortunately, there is ample room in the cavernous interior of the CDU to fit these items.

The SimStack Foundation Board is mounted on an angular metal bracket that is attached directly to the bottom of the CDU, while the LCD interface card has been installed on the upper shelf along with the screen controller. A ribbon cable connects the LCD screen to the interface card while a standard VGA cable connects the LCD screen to the client computer and Ethernet switch.

The SimStack Foundation Board is Ethernet ready and requires a standard Ethernet cable (CAT 6) to connect from the card to an Ethernet switch (located behind the MIP). In addition to the Ethernet and VGA cable, six power wires leave the CDU via the rear of the casing; four from the SimStack Foundation Board (5 and 12 volts +-) and two from the keypad (5 volts +-) to control the backlighting.

The specialist switch and wiring (Ethernet, power and VGA cables) extruding from the rear of the CDU

Specialist Switch and Power Supply

A standard two-way toggle switch is mounted to the rear of the CDU casing.

This switch is used to control whether the LCD screen, used in the CDU, is always on, or is only turned on when ProSim-AR is activated.

To operate the CDU requires a 5 and 12 volt power supply. The backlighting of the keypad is powered by 5 volts while the SimStack Foundation Board and CDU operation require 12 volts.

Backlight Dimming (keypad)

To enable the CDU keypad to be dimmed, the 5 volt wires are connected to a dedicated 5 volt Busbar located in the center pedestal. This Busbar is used to connect the backlighting from all OEM panels. The Busbar is then connected to the panel knob on the center pedestal. The ability to turn the backlighting on and off is controlled by opening or closing a 12 volt relay (attached in line between the panel knob and Busbar). Dimming is controlled by a dimmer circuit (see earlier article).

Installing the OEM CDU to Flight Deck Solutions MIP

It can be challenging attempting to install OEM panels, gauges and other items to a reproduction Main Instrument Panel (MIP). Unfortunately, no matter what the manufacturer states, many MIPS do not comply with real world measurements.

Before and after photograph of the FDS CDU bay showing the small flange from the shelf that needed to be trimmed to enable the CDU to slide into the bay recess. A small notch was made at the corner to facilitate the safe routing of the wires used to enable the Lights Test

The MIP skeleton is manufactured by Flight Deck Solutions (FDS) and the CDU bay, although fitted with OEM DZUS rails, is designed to fit FDS’s propriety CDU unit (MX Pro) and not an OEM unit.

The casing for the OEM CDU is much longer than the FDS CDU and measures 20 cm in length.

The FDS MIP design is such that the aluminum shelf (used by FDS to mount various interface cards) protrudes slightly into the rear of the CDU bay. This protrusion stops the OEM casing from sliding neatly into the bay to its fullest extent. To enable the CDU to slide into the CDU bay, the shelf must be ‘trimmed’.

To trim the metal away from the shelf, a small metal saw was used, and although an easy task, care must be taken not to ‘saw away’ too much metal. Once the piece of offending aluminum is removed, the CDU slides perfectly into the bay, to be secured by DZUS fasteners to the DZUS rail.

Functionality and Operation

The CDU is not intelligent; it’s basically a glorified keyboard that must be interfaced with ProSim-AR to enable the CDU to function correctly. The fonts and colour of the fonts is generated by the avionics suite (in this case ProSim-AR, but arguably it could also be Sim Avionics or Project Magenta).

To enable communication between the avionics suite and the SimStack Foundation Board, proprietary software must be installed. This software has been developed by Simulator Solutions.

SimStack Software (simswitch)

Screen grab showing SimSwitch software User Interface. SimSwitch is standalone once the initial configuration has been completed. The software can be configured to open in minimised mode via a batch file

To enable communication between the Foundation Board and ProSim737, propriety software, called SimSwitch must be installed to the computer that has the CDU connected.

SimSwitch is a JAR executable file, that when configured with the correct static IP address and port numbers, provides communication between ProSim-AR (on the server computer) and the network (clients). The switch must be opened for communication to occur between the Foundation Board, SimSwitch and ProSim737. The jar file can easily be included into a batch file (with timer command) for automatic loading when flight simulator is used.

When opened, SimSwitch displays the User Interface. The User Interface displays all OEM panels that have been connected using a SimStacks, can be used to monitor connected panels, and can display debugging information (if required).

Independent Operation

The Captain and First Officer CDUs are not cloned (although this is easy to do), but operate as separate units. This is identical to the operation in the real aircraft, whereby the Captain and First Officer are responsible for specific tasks when inputting the information into the CDU.

First Officer CDU

The First Officer CDU will be converted using a similar technique, with the exception that this unit will be converted more ‘cleanly’. Rather than use an angled plate on which to attach the SimStacks Foundation Board, a solid aluminum plate will be used. The LCD screen controller card will also be attached to the rear of the LCD screen. Finally, to enable fast and easy removal of the CDU, the connection of the Ethernet cable will be outside of the unit.

Additional Information

SoarByWire (another enthusiast) has written an excellent article dealing with interfacing SimStacks.

Below is a short video demonstrating the operation of the OEM CDU using ProSim737.

Main points to note in the video are:

Heavy duty tactile keys.

The definite click that is heard when depressing a key.

The solid keypad (the keys do not wobble about in their sockets).

Although subjective, the appearance of the OEM CDU looks more aesthetically pleasing that a reproduction unit.

Final Call

The conversion has been successful and, when connected with ProSim737 via SimSwitch, all the functions available in the CDU work correctly.

Glossary

ARINC 429 – A standard used to address data communications between avionics components. The most widely used standard is an avionics data bus. ARINC 429 enables a single transmitter to communicate data to up to 20 receivers over a single bus.
Standalone – Two meanings. Operation does not require an interface card to be mounted outside of the panel/part; and, In relation to software, the executable file (.exe) does not need to be installed to C Drive, but can be executed from any folder or the desktop.