Flight Simulation

INTRODUCTION

I created this section to explain how a flight simulator works. This is purely based on my experience working on CAE's flight simulators as a software developer. I hope this provides a good high level explanation to a layman on how these simulators work.

I hope I've provided this with a fair amount of accuracy based on my notes from many years ago and from memory. I hope you enjoy the read.

A FULL MOTION FLIGHT SIMULATOR

We'll refer to the diagram below as a starting point to describe how a flight simulator works. Basically it's a combination of powerful computing, mechanical systems and a lot of complex software algorithms.

Figure 1 - High Level Flight Simulator Conceptual Model

A full-motion flight simulator is a highly sophisticated training device used to replicate the experience of flying an aircraft in a controlled and safe environment. It provides pilots with a realistic flight experience, allowing them to practice various flight scenarios without the risks associated with actual flight.

Flight simulators have also been used to re-create aircraft accidents to understand what went wrong and what could have been done to avoid it. Recall the US Airways flight 1549 incident on January 15, 2009. An A320 crash landed in the Hudson River, due to bird ingestion by both engines. With no operating engines, the plane became one large glider. The same conditions were then duplicated on an A320 simulator to understand if any other course of action could have been taken. The simulation indicated that the ditching could have been avoided by returning the LaGuardia airport. While simulations showed that the plane might have just barely made it back to LaGuardia, those scenarios assumed an instant decision to do so, with no time allowed for assessing the situation. Only seven of the thirteen simulated returns to La Guardia succeeded! The ditching strategy provided the highest probability of survival.

The eight key components of a flight simulator are:

1 - Cockpit

The cockpit is the most critical component of the simulator as it replicates the aircraft's flight deck, controls, instruments, and displays. It resembles the real aircraft cockpit in terms of layout, switches, buttons, and overall appearance. The cockpit is mounted on a motion platform that allows it to move in response to the simulated flight inputs, creating a sense of motion and immersion for the pilot. Somme simulators are purely cockpit procedure training devices (FTDs). These would not have a motion platform as the training is mostly focused on instrument familiarity and cockpit procedures which don't require motion. FTDs usually don't require a visual system either. In my day at CAE we built several Lockheed C5 Galaxy FTDs, but only one full flight C5 simulator (FFS).

The same instruments used on a real aircraft are also on the simulator. The Flight Management System (FMS) is the same unit as used on an actual aircraft. On the A320 the Flight Augmentation Computers (FACs) is the actual aircraft hardware. The overhead breakers a pilot sees on an aircraft are the same on the simulator and behave exactly the same way. Cockpit instruments communicate with the simulation computer over the ARINC-429 serial instrument bus as they do on a real aircraft.

2 - Motion System

The motion system is responsible for providing physical sensations of movement, acceleration, and deceleration to the pilot. It allows the simulator to replicate the aircraft's movements during flight, such as pitching, rolling, and yawing. The motion system may use hydraulic actuators, electric actuators, or a combination of both to achieve the desired motion effects.

In Figure 1, you can see the motion system is hydraulic. This was the dominant motion system in the 80s and early 90s while I still worked at CAE. Subsequently, electric actuators were used which is much simpler than having a fluid based hydraulic system with a hydraulic power pack and accumulator tanks. The image below is that of a newer simulator using electric motion actuators. Note the lack of hydraulic accumulator tanks as compared to that of Figure 1. This eliminated the need for the hydraulic power pack and all the issues associated with maintaining hydraulic hoses and managing leaks. It also reduces the simulator installation time at the client site.

The motion effects are realistically duplicated. From the takeoff, to landing, you have the same sensation of motion as you would on a real aircraft. On landing, a pilot will experience the same thud of the aircraft when it hits the runway. I've seen cases where the aircraft lands hard and the motion system issues a violent jolt to duplicate the hard landing.

When the aircraft taxies to the runway, you will feel every bump on the taxiway as the pilot guides the aircraft along. It really feels as if the aircraft is moving along on a real runway.

The system simulates a variety of other motions due to:

• Rough air and turbulence
• Stalling and high speed buffeting
• Landing gear deployment
• Engine vibrations
• Turbulence
• Tire failures; main gear or the failure of anyone of the gear trucks
• Hard landings and go-around events

One day we had a glitch with the motion system while the CAE test pilot was in the simulator. The data sent caused the motion system to actually vibrate. It didn't move much either vertically or horizontally but physically caused the simulator cabin to vibrate so much it sounded like a buzz. It didn't last long, we cut the interface quickly. After stopping it, the pilot (an elderly fellow; retired airline pilot) stepped out. He was furious; red in the face furious! It must have felt like sitting on a jack hammer. It never happened again.

3 - Visual System

The visual system is a crucial aspect of a flight simulator as it provides the pilot with a realistic and immersive view of the external environment. This is usually achieved through a high-quality projection system or a set of high-definition screens that display computer-generated images of the terrain, airports, and other objects. The visual system renders realistic graphics and updates them in real-time to match the pilot's movements and inputs.

In my days at CAE I recall interfacing General Electric (GE) visual systems over Ethernet. There were other manufacturers as well. Today, CAE manufactures their own simulator visual system. The visual rendition of the landscape is so good, that when I moved to Toronto from Montreal I recognized the buildings around Pearson airport from the time spend in simulator cockpits. Also depending, on what airport the simulation is at, you can also see the runway tire marks from previous aircraft landings.

By the way, when you crash land the aircraft, the whole display area goes red!

4 - Control Loading System

The control loading system is responsible for providing the appropriate force feedback to the pilot's control inputs. It ensures that the controls feel like those of a real aircraft, with appropriate resistance and response based on the simulated flight conditions.

5 - Avionics and Systems Simulation

A full-motion flight simulator replicates the avionics systems and flight dynamics of the specific aircraft it is designed for. This includes the aircraft's flight management system (FMS), autopilot, communication systems, navigation equipment, engine controls, and other essential systems. Accurate simulation of these systems allows pilots to practice realistic procedures and emergencies.

6 - Sound System

The sound system plays a vital role in providing an immersive experience to the pilot. It recreates the various sounds that a pilot would hear during a real flight, including engine noise, airframe vibrations, and communication with air traffic control.

CAE's flight simulators typically have eighteen speakers in various places in and around the cockpit to simulate the sounds from various sources. This is driven by an eight channel digital sound system. All sounds are generated digitally and are accurate to the degree of direction, frequency and amplitude. A sample of the sounds that are faithfully replicated by the digital processors are:

Rumble Sounds:

• These reflect the noises experienced while the aircraft taxies on the runway. This is coordinated with the motion system which provides the bumps to simulate an actual runway environment.

Engine Sounds:

• Effects of altitude and airspeed on engine sounds
• Ground effects
• Acceleration
• Engine compressor stall
• Engine surges
• Thrust reverser

Aerodynamic noises as a function of flight actions:

• Landing gear doors opening
• Landing gear extension and retraction
• Spoilers
• Slats
• Flaps

Audible cabin sounds:

• Auxiliary Power Unit (APU) sounds while staring, running and shutting down
• Nose gear noise
• Windshield wipers
• Air conditioning air flow
• Clicking sounds from relays and other external generated equipment sounds
• Precipitation noises on the aircraft's skin and windshield
• Touchdown sounds
• Gear collapse and tire bursts
• Decompression

7 - Instructor Station

The instructor station is where the instructor or flight supervisor controls and monitors the simulator's operation. From the instructor station, they can set up flight scenarios, control weather conditions, introduce malfunctions, and evaluate the pilot's performance. The instructor sits within the simulator cabin, just behind the pilots, so they can initiate various scenarios and observe the pilot's responses. Some of the scenarios an instructor may initiate are:

• Engine fires and other engine failures
• Cockpit fire/smoke conditions (yes they really generate smoke conditions)
• Air to air collisions
• Inclement weather (wind gusts, wind shear, down drafts, rain, fog, icing conditions, etc.)
• Turbulence
• Landing gear failures
• Flight control malfunctions

8 - Computer System and Software

A powerful computer system drives the entire simulator, processing vast amounts of data, calculating flight dynamics, and managing all the interconnected components.

Figure 2 - Flight Simulation Software

I consider the computer, the software running on it and the interfaces to the cockpit to be the heart of the simulator. The computer and simulation software tie all the other components together to form the technological marvel that a flight simulator is.

Referring back to Figure 1, the simulation computer complex is one or several computer systems interconnected to run the many programs that simulate the aerodynamic behaviour of the aircraft under different conditions. The engine behaviour is simulated, the way the aircraft feels and behaves on the ground and in flight are all controlled by hundreds of program models running on the computer. Interfaces such as the instruction facilities and the various micro controllers provide inputs and outputs to the various simulation programs. The motion system's data, where and when to move, are , generated by programs which take cockpit and flight model input data (e.g. pilot's column, weather conditions, etc.), process it and move the resulting command data to the motion system.

A flick of a switch in the cockpit comes back as data into the computer's memory which is then analyzed by a program and performs an equivalent output to the visual display or to an action on a cockpit instrument. For example, turning on the autopilot, will set a data variable in the computer's memory that the autopilot is ON; this will change the logic in the flight programs, resulting in how the aircraft flies. As well, switching on the nose wheel landing lights, sets a variable in the computer memory to indicate to the visual system to light up the path ahead of the aircraft.

In summary, pilot actions and instructor settings combine to form inputs to the computer models which depending on many factors (simulated malfunctions, weather conditions, aircraft weight, etc.) results in feedback to the cockpit by way of motion changes, visual acknowledgments, audible warnings (pull up, pull up!), other sounds (aerodynamic noise variations) visual changes and/or motion system changes. Different mathematical models (engine, airframe, ground effect, oleo, etc.) interact to produce a realistic simulated cockpit environment for the pilot.

The various models are written in the FORTRAN language and then compiled on VMS or MPX operating systems. FORTRAN (FORmula TRANslation) is ideal for scientific computing and hence it's well suited for composing simulation models which are mathematically complex algorithms.

Each program has access to a COMMON block of memory. That is every program can effectively see the variables common to all programs. For a simulator, the COMMON memory block is a very large memory area containing thousands of variables used by the simulator programs. When programs are compiled, they include the COMMON block variables that the program needs to read and write into. Data produced by programs are used by other programs which drive subsystems, such as motion, cockpit indicators, visual, sound, etc.

A simulator application referred to as the "Simulation Executive" (SIMEX) is responsible for loading the simulation programs and the COMMON block. SIMEX can inspect every variable in the memory block. There are literally thousands of variables in COMMON supporting the various simulation models. This is an extremely useful feature for debugging and monitoring the simulator's model behaviour. As well, SIMEX can write into the variables to change certain conditions on the simulator, mostly under test conditions (e.g. turn off the auto-pilot). SIMEX would not normally change model variables on a running simulator.

To understand how this works, let's look at an example. Referring to Figure 1; note in the lower left of the illustration there is a switch in the cockpit panel that is labeled ANTISKID. This is similar to the ABS system on your car, it prevents the aircraft from skidding off a center line when landing on on wet or icy runways. When the pilot depresses the ANTISKID button, a micro controller (DMC) samples the various input cards in its chassis. It detects on the DIP-64 card that the ANTISKID switch is depressed. The DMC packages this data along with other inputs and sends the data over its Ethernet interface back to the simulator computer. A program on the computer maps the data back into COMMON variables. A variable in memory called ANTISKID (as an example) is now set to TRUE. Appendix A has a description of the various DMC I/O cards.

When the pilot attempts to land the aircraft (assuming landing gear is successfully deployed) one of the variables the flight model inspects is if ANTISKID is set to TRUE. If the runway is wet, then the flight model takes the ANTISKID into account and simulates a landing whereby the aircraft does not skid. If the switch is FALSE, then the flight model may cause the aircraft to skid off the runway, depending on the amount of water on the runway, aircraft weight, wind factors, runway surface properties and likely other variables. Likewise, an instructor setting a landing gear fail scenario is really a set of variables put into COMMON memory to indicate which trucks failed (one or all three) and the flight model responding appropriately to those variables. ANTISKID status would be ignored if the landing gear is deployed.

In summary, the setting of one variable, or even a change of a small number of variables, can impact the actions of many models.

Figure 2.1 is a logical representation of the various simulation programs that read and write into the COMMON data space. It illustrates the block of COMMON memory that all programs have access to and the I/O controllers access to that same block of COMMON.

The I/O controllers read and write out of this space through Direct Memory Access (DMA); that is the controllers access the memory without CPU intervention. Of course, the controllers must be told where to get the data within the COMMON memory, but this is accomplished with channel programs that when called will have the logical address of data converted to physical memory addresses and how much data to move. Most often a logical block of data in COMMON, for example an INTGER array with 1000 elements is logically sequential, but in the computer's physical memory, those 32 bit Integers may be partitioned physically as three separate blocks of data. A controller which moves data in our out of memory in DMA mode, needs to know the physical locations of data in the computer's memory. This is noted as logical to physical memory mapping.

The simulation cycle time is 33ms, that means all programs must run within a 33ms frame time. Actually, in most cases they must run within 28ms as contractually the company must leave at least 5ms of spare computer time per cycle. The spare time is used to accommodate future enhancements, but also to allow time for users to have access to background utilities. For example, if a user is inspecting COMMON variables through SIMEX, then there must be sufficient background time to run SIMEX with a running simulator. Simulation programs (foreground) have a higher process priority than background (terminal based) programs. If the foreground tasks consumes all available CPU capacity, then there is no time available for anyone to even log into a terminal and run background tasks such as doing file editing. File editing or other non-simulation activities are generally not performed while the simulator runs for this reason.

So how do hundreds of simulation programs all run within 28ms? A very powerful CPU with very fast memory would be required to do that. Instead, another approach is used; allocate the programs to multiple CPUs with a shared memory configuration. All programs running on the various CPUs would see the same COMMON shared memory. Therefore each CPU runs a set of simulation models; a divide and conquer strategy.

There is one more nuance to executing programs (models) and that is that not all programs need to run within 28ms. Only some models are required to run every cycle. Other programs can be run only every two cycles or every three cycles. For example, the instructor facilities program doesn't have to run every cycle; the human interaction with the program is slower than several simulation cycles.

Figure 2.2 illustrates this concept. In the figure, programs P1 and P2 run every 33ms cycle. Programs P3 runs every two cycles (every 66ms) and P4 runs every three cycles (99ms).

Therefore the combination of multiple CPUs, distributed processing and variable task scheduling enables all tasks to execute within their scheduled cycle time.

Computer System

The systems I worked and am most familiar were the Encore 32-bit Super minis of that time. CAE used 32-bit VAX/VMS systems from Digital Equipment Corporation (DEC) before the Encore systems arrived. The VMS operating system was a much better development environment. For example the code debugger is excellent, allowing the user to step through the source code one line at a time and examine the variables/data at each step. By contrast, MPX has no such facility. You would have to supplement your program with debug code to understand what's going on.

CAE duplicated, with limited success, VMS user facilities with their GED-1 software package; giving Encore's MPX OS an improved software development environment. Despite lacking a great user environment, it was a computer well suited for scientific number crunching in a real-time environment. It lacked VMS's paging and swapping, which is great for time sharing, but for real-time work this would have been a hindrance (imagine swapping out programs to disk on a 28ms cycle time).

Rumor has it that when Air Canada was looking for a simulator, the Encore sales person approached them about using their computer instead of DEC's VAX system. It was positioned that if the bidding process was truly going to be open, then Air Canada should be looking at a variety of computer suppliers that provided the best value. Rumor or not, CAE simulators for Air Canada were powered by Encore systems thereafter.

Referring to Figure 3, this represents a typical Encore simulation computer configuration. Multiple CPUs are used along with shared memory to allow for a distributed processing system working off the COMMON data block. I hope that this section adequately explains why the Encore systems were so well suited for simulation.

Figure 3 - Flight Simulator Computer Configuration, Encore CONCEPT 32/67 Series

The configuration consists of a Master and Slave computer with shared memory. The Master computer consists of a CPU and IPU (Internal Processing Unit) combination with floating point accelerators on each (FPA). The CPU and IPU reside on the SELBus along with the Ethernet, HSD and Multi Function Controller (MFP). The CPU and IPU are single board processors implemented in VLSI CMOS gate arrays. Programs run on both the CPU and IPU, however only the CPU can perform I/O. Therefore all I/O commands are initiated by the CPU.

The MFP provides the main console access, connectivity for up to seven asynchronous terminals, a printer port and SCSI disk control. Originally, the simulators used Control Data Storage Module Disk Drives (SMD) from Control Data Corporation (CDC) but as lower costs SCSI disks became available CAE switched to these. I ran a benchmark (MPX re-build) to assess if the SCSI disks provided the same performance as the SMD disks; the SCSI disks were only 15% slower. It was good enough. We started using SCSI disks henceforth. In simulation, the disks are primarily used for loading the programs but get very little I/O use while the simulator runs. There are no real-time tasks writing to disk.

On the Master computer the two Ethernet controllers exchange data with the Datapath-C Micro Controllers (DMC). The DMCs drive the six type of interface cards to the cockpit instruments as in Figure 1. Refer to Appendix A for a description of the cockpit interface cards.

The Slave computer is configured similarly. It features a CPU with an FPA, but no IPU. An IPU could be added to this configuration if needed for more computing capacity. It also features an MFP for console access only, but it does not have it's own disks. The purpose of the Slave is to provide computing capacity and offload I/O through the Ethernet controllers to the Motion and Visual systems. Programs are loaded from the Master to the Slave, so it does not require its own disks.

By splitting the I/O controllers between Master and Slave ensures that no SELBus is overwhelmed with I/O activity. If more computing or I/O capacity is required, additional slaves could be added to this configuration, thereby providing a very scalable and powerful architecture.

There is one key advantage the Encore computer has over its VAX counterparts; its the reflective memory system. Note that the 8MByte memories in this example are dual ported (DPIMM). Each memory features a read sense controller (RSC) and a write sense controller (WSC). Dual ported memory means that two systems can share the same memory unit.

In shared memory systems one of the challenges is that when a CPU needs access to memory for reading instructions or reading and writing data, it temporarily blocks other CPU access to memory. In this configuration, if the CPU accesses memory, then the IPU has to wait until the CPU finishes. Now imagine a system with four CPUs, all vying for access to memory, each one waiting for the other to complete. This is why adding more CPUs to a shared memory system has diminishing value as more processors are added. The memory contention each CPU introduces negates the value of adding additional processors. For example, a shared memory system with four CPUs is really only equivalent to 3.5 CPUs.

Most computer applications generally spend 80% of their time reading from memory and 20% writing data into memory. Think of a running program; every instruction must be read from memory. When there are multiple CPUs, each running a program in memory, means each one has to get its instructions from a common memory; this is where contention happens. This is somewhat alleviated by the CPU's cache memory but contention remains an issue. Most modern CPUs feature large primary cache (L1) and larger secondary (L2) cache memories. Generally, when programs write into memory it's to update variables in the program.

Understanding this constraint, Encore devised a memory system whereby when a change is made in the memory of the Master or the Slave, the same change is propagated between the memories of each system. For example, if the Master computer receives a block of data from the Ethernet controller into memory, the same block of data is copied to the memory of the Slave computer. In this way both computers always have the same data. Each memory system "reflects" the changes to the next computer; hence the moniker "Reflective Memory". This method allows a portion of the memory to be set as "reflected". This mechanism uses a high speed fiber optic link between both memory systems to keep Master and Slave memories consistent.

Why this is important is because 80% of memory access is reads. With each system having a copy of the shared memory, means that the contention for memory is diminished as each system has a copy of memory. Therefore the Master CPU and IPU can access memory without having to contend for the same access with the Slave CPU or its controllers. This is a key differentiator for Encore computers. This improves their real-time performance capabilities. These systems also have other improvements such as "Shadow Memory": sections of the memory system that can be replaced (or shadowed) with very fast static memory. Similar to having defined sections of main memory as fast as the CPU's cache memory.

From a conversation with an Encore representative, I understood that the "Reflective" memory concept was first pioneered by Honeywell. The patent description for reflective memory and prior works can be read in more detail *click here*.

Oleo-Pneumatic Landing Gear Model

A flight simulator is a combination of several inter-dependent mathematical models that simulate flight dynamics, motion and sound. The most demanding compute time is when the aircraft is rolling down the runway to take flight. In such a scenario the simulation has to model both the ground effects (air compression under the airframe as the aircraft picks up speed) and the flight dynamics as the aircraft begins to ascend. Once aloft, the flight models take over.

One example, to illustrate what models do, is the oleo model. In flight simulation and aircraft dynamics, oleo model equations are mathematical equations used to model the behavior of oleo-pneumatic landing gear systems. Oleo-pneumatic landing gear systems are commonly used in aircraft to provide shock absorption during landings and takeoffs, reducing the impact forces on the aircraft structure and passengers.

The oleo model equations describe the dynamics of the landing gear strut, which consists of an inner cylinder (piston) and an outer cylinder (housing) separated by a column of compressed gas (usually nitrogen) and hydraulic fluid. When the landing gear experiences vertical loads, the gas and fluid compress, allowing the landing gear to absorb the impact energy.

The general form of the oleo model equations can be represented as follows:

Gas Law Equation: The gas law equation describes the relationship between the pressure, volume, and temperature of the gas within the landing gear strut. It is typically based on the ideal gas law, which states:

P * V = n * R * T
where:
P = Pressure of the gas
V = Volume of the gas
n = Number of moles of gas
R = Ideal gas constant
T = Temperature of the gas

Fluid Flow Equations: The oleo model equations also consider the flow of hydraulic fluid in and out of the landing gear strut as it compresses and extends. These equations take into account the flow rate of the fluid and the hydraulic pressure within the strut.

Force-Displacement Relationships: The oleo model equations establish the relationship between the forces applied to the landing gear and the resulting displacements or deflections of the landing gear strut. These relationships are crucial for understanding the shock-absorbing capabilities of the landing gear system.

Damping and Stiffness Coefficients: The oleo model equations also include damping and stiffness coefficients that represent the energy dissipation and stiffness properties of the landing gear system.

Overall, the oleo model equations are essential for analyzing and simulating the landing gear behavior under various landing and takeoff conditions in flight simulation.

OTHER SUPPORT SYSTEMS

There are many other mechanical components to a flight simulator installation. Compressed air is needed to simulate the pilot's oxygen supply. An external air conditioning unit is required to cool the simulator cabin. Water is needed to cool the hydraulic heat exchangers. Of course, an adequate electrical power source is needed (3 phase) to power the whole facility.

SIMULATOR CERTIFICATION

Aircraft simulators must meet a level of certification. For example, a client may demand that their flight simulator meet a Phase III level of certification. The certification phases for aircraft simulators are outlined in the Federal Aviation Administration (FAA) Advisory Circular (AC) 120-40A, "Airplane Flight Simulation Training Device Qualification." This document provides guidance on the qualification and evaluation of various flight simulation training devices (FSTDs), including full flight simulators (FFS) and flight training devices (FTD). The different certification phases outlined in AC 120-40A include:

Phase I - Initial Qualification: During this phase, the simulator manufacturer or operator applies for initial qualification of the FSTD. The FAA evaluates the simulator to ensure it meets the specified requirements for its intended use and training tasks. This phase involves the examination of hardware, software, flight model, and visual system capabilities, among other factors.

Phase II - Continuation Training and Qualification: After the initial qualification, the FSTD enters Phase II. This phase focuses on the evaluation of the simulator's ongoing performance and capability to deliver accurate and reliable training. Regular evaluations are conducted to ensure that the simulator maintains its qualification standards over time.

Phase III - Re-qualification: The Phase III evaluation is a more extensive assessment that occurs periodically (typically every two years) to re-qualify the FSTD. This phase is similar to the initial qualification but involves a more comprehensive evaluation to verify that the simulator continues to meet the latest standards and requirements.

Phase IV - Advanced Qualification: For certain types of simulators, such as Level D Full Flight Simulators (FFS), an additional Phase IV evaluation may be required. This phase involves specific tests and checks to assess the simulator's capability to conduct advanced training tasks, including those associated with upset prevention and recovery training (UPRT).

Each of these phases plays a crucial role in ensuring the accuracy, reliability, and quality of flight simulators for pilot training. The simulator must meet the respective requirements of each phase to obtain or maintain its qualification status. This certification process allows flight simulators to serve as effective training tools, providing pilots with realistic and immersive training experiences that enhance their skills and safety in actual flight operations.

SUMMARY

The integration of all components allows a full-motion flight simulator to provide a highly realistic and effective training experience for pilots, enhancing their skills and situational awareness in a safe and controlled environment. This has been a brief explanation on how a simulator works. A more detailed explanation would require a book! I am familiar with the computing systems but by no means am I qualified to describe the various simulation models.

APPENDIX A

Card ID	Description
ARINC429	Aeronautical Radio INC (ARINC) is a global organization made up of airplane manufacturers and major airlines. It objective is the promotion of aircraft equipment standardization within the industry. The ARINC 429 is today the most widely used data bus for transport and commercial aircraft. The messages get transmitted at either a bit rate of 100 kilobits or 12.5 per second to other elements of the system monitoring the messages coming through the bus. The transmission and reception is done through separate ports making it necessary to have lots of wires on an aircraft, which typically use avionics systems in large numbers. The CAE ARINC 429 I/O card drives eight output channels and four input channels. This connects to real avionics hardware used in the simulator such as the FMS.
SOP/ACI	This card produces two outputs with an output defined by a 12-bit data word. The card also contains four AC inputs that can accept up to 12Vrms.
AOP-32	Analog Output - this is an analog card with 32 analog outputs ranging from +/- 10VDC. A 12-bit data word for each channels drives the digital to analog converters.
AIRO-16	Analog I/O Output - this card handles 16 analog inputs in the range of +/- 10V. The card uses a 12 bit analog to digital converter that scans each input ans stores the data in memory for reading by the DMC.
DIP-64	Discrete Inputs - each card can sense the status of 64 discrete inputs. For example an on/off switch from the cockpit would be monitored by such a card. When the DMC queries the card, the DMC receives four 16-bit words that relay the status of the 64 inputs.
DOR-32	Discrete Output Relay - this card drives 32 contact relays. Two 16-bit words are sent by the DMC and latched on the card to drive the status of each relay.

Compiled on 08-28-2023 07:35:35