Instrument_Interfaces

Matt's Multi-Mission Military Flight Simulator

SIM various

James & Ted visit 2003

Sim pilots 1999-2004

Basement tour 2004

Canopy & projector

Sound system enhancement

SIM TV studio

Instructor's station

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Instruments

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Childhood sims to present

Evolution of current sim

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Planned upgrades

EPIC interface

Instrument interfaces

HUD development

Other projects

Software

Parts sources

Instrument interfaces were all built by hand with whatever ICs I happened to have. Above is the simplest of interfaces, which is a DC instrument driver. A simple 54LS373 latch is accessed by the 32 point output module where a number is then placed into that latch. The digital number now held in the latch is sent to a digital to analog converter IC, such as the AD565, DAC-85 or DAC-0808, which provides a DC signal corresponding to the digital number. The output can unipolar or bipolar depending on the need. For instance, a deviation bar on an ADI needs a bipolar output where 128 sent to the latch would center the needle, zero would deflect the needle all the way left and 255 would deflect the needle all the way to the right.

An example of a unipolar output would be the turbine temp indicators where zero would output zero volts and 255 would translate into 10 volts out. The voltage would then be stepped down before reaching the temperature indicators since they are really looking for a tiny voltage supplied by a thermocouple mounted to the engine.

One of the most important interfaces is the digital to synchro interface. Since most aircraft instruments, such as ADIs and HSIs require multiple 3 phase 400Hz sychro transformer inputs, there has to be an interface to convert a binary digital number to a corresponsing 3 phase 400Hz angular output.

This is accomplished via the use of digital to synchro converters as pictured above. The EPIC 32 point output module accesses a pair of octal latches, as with the other interfaces, and places a 16 bit number in those latches. The output of the latches is sent directly to the digital to synchro converter.

The converters require a 26VAC 400Hz reference signal and +5V, +-15V inputs to provide a 3 phase 400Hz angular output that is sent directly to the indicator.

Back when I was using purely analog tachometers, I was faced with the dilemma of simulating a three phase tachometer generator. The easiest solution is pictured above.

The EPIC 32 point output module sends a number to the latch, as usual, and the D/A converter provides a 0-10VDC output. This output is then sent to a power operational amplifier that directly drives a DC motor. The motor stops at zero volts and is at full speed at 10 volts. The motors are shaft-coupled the synchro transformers that have a DC current supplied to their rotors. This turns them into miniature three phase generators. The output of the synchros is then amplified via power op amps to deliver the voltage level expected by the RPM indicator. As far as the indicator knows, it's being fed signals by true tachometer generator. This interfce was later abandoned when I converted to new indicators that ran on pulses alone.

The completed circuit boards are installed within these boxes in the nose of the sim. The cables leading from them connect to the multitude of terminal blocks all over the sim. Those, in turn, are connected to the control heads and instruments.

The servo loop assembly is calibrated via the adjustment of the shaft couplers when the module is separated from the box that houses it. At the head of the box is the servo loop circuitry (the board raised at the end) that drives the various modules.

The potentiometers on the servo loop module are used to set the motor speeds.

Everything in the boxes are easily extended for quick and easy troubleshooting and modification. In fact, the entire sim was designed to be maintainable via the modular nature of its construction. This allowed for the constant changes and improvements made to it since its inception. From removable boxes to hinged access panels, almost every area of the sim is accessible from the outside.