L-1011 Trainer: Digital and Analog Bus Integration

Digital and Analog Integration Planning

L-1011 Digital I/O Bus

To make the simulator work there are a number of hardware integration methods that we need to put in place to allow the actual flight hardware to interact with the simulator software and vice versa. We need to be able to do the following:

Digital Input and Output (I/O)

Digital I/O is used to read switch states (on/off) as well as setting of annunciator lights. A large amount of digital I/O channels are needed to drive the L-1011 cockpit. All in all, there will be a need for 530 Output Channels as well as 582 Input Channels. The digital I/O bus also needs to be highly distributed across the sim (FE Station, MIP, Overhead, etc) and it needs to be resistant to electrical noise and power transients. Since everything in the simulator operates on either 24VDC or 115V AC ... the Digital I/O system needs to be able to directly interface with 24VDC without much need to do signal conditioning prior to entering the I/O interface (e.g. TTL level conversion). Digital Output will also need to be done mostly on 24VDC and will need to be able to be sourced as well as sunk.

Analog Input and Output (AIO)

The simulator instruments will required a number of analog signals to be generated. These generated analog signals will simulate the aircraft sensors. For example, engine tachometer signals need to be generated the same way they would be sent from the engine transducers to the instruments. Also, many of the flight controls will, of course, produce analog input signals to the simulator software which need to be digitized. For example, movement of the yoke or the flight control trim. So there is a need for a great amount of Digital to Analog (DAC) as well as Analog to Digital (ADC) conversion.

ARINC 429 Protocol 

Some of the primary navigation as well as attitude control instruments use the ARINC 429 bus protocol. The correct protocol needs to be generated and sent to the simulator hardware to display the right navigation and attitude data.

Overall

Since the simulator will be used in a museum and public setting where safety and reliability is of most  importance, it is critical that we use industrial strength, serviceable and commercial off-the-shelf (COTS) hardware. While building a lot of custom hardware might be more fun and perhaps a lot less expensive, it also makes it hard to maintain the sim in the future. When custom built components break down ... they can be very difficult or impossible to replace. So, at the current we are looking at using the following COTS technologies:

• Digital I/O -- Phoenix Contact Interbus (old, but very very reliable and trusted)
• Analog I/O -- National Instruments DAQ Equipment and Phoenix Contact Interbus 
• ARINC 429 -- V1 Avionics Simulator Interface 

Again, the key is to use as much off-the-self and easy to replace hardware as possible.