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Mission Statement 

The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

I am not a professional journalist.  Writing for a cross section of readers from differing cultures and languages with varying degrees of technical ability, can at times be challenging. I hope there are not too many spelling and grammatical mistakes.


Note:   I have NO affiliation with ANY manufacturer or reseller.  All reviews and content are 'frank and fearless' - I tell it as I see it.  Do not complain if you do not like what you read.

I use the words 'modules & panels' and 'CDU & FMC' interchangeably.  The definition of the acronym 'OEM' is Original Equipment Manufacturer (aka real aicraft part).


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If you see any errors or omissions, please contact me to correct the information. 

Journal Archive (Newest First)

Entries by FLAPS 2 APPROACH (207)


Altitude and Speed Intervention Explained 

The flight deck can be an extreme work environment, especially during the high-task descent and approach phase of the flight. 

Altitude and Speed Intervention were designed to allow pilots to easily and quickly change either the altitude or speed of their aircraft without re-programming the FMC, disengaging VNAV, or spending excessive time 'heads down'.

LEFT:  Altitude Intervention (ALT INTV) button on MCP.

The intervention buttons are strategically located on the MCP.  When the buttons are selected, the aircraft's altitude or speed can be altered quickly on ‘the fly’

In this article, I will examine the use of Altitude and Speed Intervention and demonstrate the use of these modes.  In a follow-on article, I will discuss alternate methods that can be used to change altitude whilst maintaining Vertical Navigation.  The reason for separating the two articles, is to avoid confusion that can develop between the different modes.

I use the terms Flight Level (FL) and Cruise Altitude (CRZ ALT) interchangeably.  I also use the terms CDU and FMC.  The Control Display Unit (CDU) is the keypad used to interface with the Flight Mode Computer (FMC).  The FMC forms part of the Flight Management System (FMS).

I recommend reading the appropriate section in the Flight Crew Operations Manual (FCOM) , Flight Crew Training Manual (FCTM) and the Cockpit Companion for a more thorough understanding. 

Furthermore, whether intervention modes function in the simulator will depend upon which avionics suite and FMC software version is used.  This article will deal only with ProSim-AR (ProSim737 avionics suite) which uses U10.8 A.  In this article I use the words Cruise Altitude (CRZ ALT) and Flight Level (FL) interchangeably.

Important Points:

  • Altitude and Speed Intervention are company options that may or may not be ordered at the time of airframe purchase.
  • Altitude and Speed Intervention will only operate when an a route has been programmed, and is active, in the CDU.  VNAV must be selected for either intervention mode to function..
  • Altitude and Speed Intervention is more often used when a temporary change in altitude and/or speed is required with a return to the original altitude/speed imminent.  

MCP, VNAV & FMA Nomenclature and Displays

Prior to examining Altitude and Speed Intervention, it may be fruitful to quickly discuss common words that are used when describing the operation of VNAV and the MCP.

(i)       CONDITION means that a mode will become active only when a condition(s) occurs;
(ii)      ARM means that a mode is armed pending engagement;
(iii)     ACTIVE means the mode is engaged/selected;
(iv)     SELECT means to select or engage the mode (turn on); and,
(v)      DESELECT means to deselect or disengage (turn off) the mode.

An often misunderstood facet of the MCP is that the annunciators illuminate to indicate a particular mode is active.  This is not entirely correct, as the presence of an illuminated annunciator light does not always indicate whether a mode is active or not.

Table 1:  FMA displays observed when Altitude and Speed Intervention is engaged (click to enlarge).  To download as .pdf click FMA displays.

For example, the VNAV annunciator on the MCP will remain illuminated when VNAV is either active or armed.  Furthermore, active modes that are not able to be deselected, do not display an illuminated annunciator.

To determine whether a mode is active or not, the Flight Mode Annunciator (FMA) should be consulted.  The FMA is located above the Primary Flight Display (PFD) and displays various alerts and status messages.  

Refer to Table 1 for a synopsis regarding the various displays that the FMA will generate when intervention is used.



Important Points:

  • A mode change highlight symbol (green rectangle) is displayed around the command name, in the Flight Mode Annunciator (FMA), whenever a mode has been armed and is about to become active.  The green rectangle will remain displayed for a period of 10 seconds.
  • It’s prudent to cross reference between the FMA, MCP and CDU to determine what mode is armed or active at a given time.
  • Altitude and Speed Intervention, when active, will take precedence over VNAV, although VNAV will remain in the armed mode.


The aircraft is flying at FL150 (15,000 feet) at 275 kias.  The FMC has an active route (Company Route) that includes altitude and speed constraints (in the LEGS page of the CDU). 

In level flight, with autopilot, LNAV and VNAV selected, the following will be observed:

(i)     LNAV and VNAV will be active;
(ii)    The FMA will display MCP SPD / LNAV / VNAV PTH;  
(iii)   The annunciators on the MCP - LNAV, VNAV & CMD A/B will be illuminated;
(iv)   The speed window located on the MCP will be blank (no speed displayed); and,
(v)    LNAV/VNAV will be displayed in white text on the PFD.

LNAV will be controlling the lateral navigation of the aircraft while VNAV will be controlling the speed and vertical altitude of the aircraft.

ATC request a decrease in speed from 275 kias to 240 kias.

Speed Intervention (SPD INTV)

Select (press) the SPD INTV button on the MCP.  The MCP speed window becomes active and displays the current speed of 275 kias.  Dial into the speed window on the MCP the new speed requirement of 240 kias. 

LEFT:  Speed Intervention (SPD INTV) button.

Notice the speed indicator display above the speed tape on the PFD has changed from 275 kias to the new speed of 240 kias.  Also note that the VNAV annunciator light on the MCP remains illuminated - in this case VNAV is active.  The speed of the aircraft will be reduced to 240 kias.

If you cross check with the Cruise Altitude in the CDU (CRZ ALT key/TGT SPD), the CDU will still indicate the original cruise speed of 275 kias.  This is because the speed is an intervention speed and, as such, will not have been updated in the FMC.

If you wish to stay at this speed (240 kias), you will need to manually change the cruise speed to 240 kias in the CDU.  However, in this case the reduction in speed is momentary, and ATC advise you to return to your original speed.  

Returning to Original Speed

Press the SPD INTV button (or unselect and reselect VNAV on the MCP).  Doing this, will return the speed to the original speed (275 kias).  It will also change the speed indication on the PFD from 240 kias back to 275 kias.  The MCP speed window will also become blank (no speed displayed). 

Important Point:

  • When SPD INTV is active, the FMA will display MCP SPD.  When SPD INTV is not active (deselected) the FMA will revert to FMC SPD.

Altitude Intervention (ALT INTV)

Altitude Intervention is slightly more sophisticated in comparison to Speed Intervention.  This is because, amongst other factors, the relationship changes when the aircraft is ascending or descending.

LEFT:  Altitude Intervention (ALT INTV) button on MCP.

In level flight, with autopilot, LNAV and VNAV engaged, the following will be observed:

(i)     LNAV and VNAV will be active;
(ii)    The FMA will display FMC SPD / LNAV / VNAV PTH;  
(iii)   The annunciators on the MCP - LNAV, VNAV & CMD A/B will be illuminated;
(iv)   The speed window located on the MCP will be blank (no speed displayed); and,
(v)    LNAV/VNAV will be displayed in white text on the PFD.

ATC request a descent from FL150 to FL120.

DESCENT Using ALT INTV (descent from FL150 to FL120)

Dial into the altitude window on the MCP the new altitude (FL120). 

Notice the altitude display above the altitude tape on the PFD has changed from FL150 to the new altitude of FL120.   Also note that the VNAV annunciator light on the MCP remains illuminated - in this case VNAV is armed.  ALT INTV takes precedence over VNAV.  

LEFT:  CDU cruise page showing 12000 in scratch pad.  Selecting line select 1 left (LS1L) will update the CDU to the new Flight Level.

Select (press) ALT INTV button on the MCP and the FMA will annunciate FMC SPD / LNAV / VNAV PTH.   The aircraft will descend at 1000 fpm (default descent speed) until FL120 is reached.  

If you cross-check the Cruise Altitude in the CDU (INIT PERF/PERF/CRZ ALT or CRZ key/CRZ ALT), it will display the original Cruise Altitude of FL150.  The FMC has NOT automatically updated the Flight Level to the lower altitude – this is normal and not a fault.  

If you want to remain at FL120, you will need to manually update the Cruise Altitude in the CDU (INIT PERF/PERF/CRZ ALT), or (CRZ key/CRZ ALT) and press the EXEC key.  

Important Points:

  • When the CDU page is open on CRZ (CRZ key), it will display in the scratch pad any change to the altitude in the MCP.  This provides a ‘shortcut’ to insert the new flight level should it be desired to make it permanent.  All that is needed is to press the CRZ/CRZ ALT (in the CRZ page) and the FMC cruise altitude will be updated.  The altitude in the LEGS page will also be updated.
  • By default, Altitude Intervention will always maintain a vertical descent at 1000 fpm.

Returning to Original Flight Level

To return to the original Flight Level (FL150), dial into the MCP the previous Flight Level (FL150) and press ALT INTV.  The aircraft will ascend to FL150.  

Important Points:

  • The FMC will NOT automatically update the Flight Level to the lower altitude.  If desired, this will need to be done manually.
  • When returning to the original Flight Level, VNAV will not engage unless the original Flight Level (FL150) is dialled into the altitude window of the MCP.  For VNAV to be active, the Cruise Altitude in the CDU and the altitude set in the MCP must be identical.
  • ALT INTV takes precedence over VNAV.  The VNAV annunciator on the MCP will remain illuminated and  VNAV will be in armed mode (when ALT INTV is selected).
  • To determine if VNAV is the active mode (or not) the FMA display must be consulted – not the annunciator light on the MCP.

ASCENT Using ALT INTV (ascent from FL120 to FL150)

The ALT INTV button operates a little differently when you ascend.   For a start, it automatically replaces (updates) the Flight Level (CRZ ALT) in the CDU.  It will also update the altitude in the LEGS page in the CDU. 

The FMA will annunciate  N1 / LNAV / VNAV SPD during the climb phase of the flight, changing to FMC SPD / LNAV / VNAV PTH when the new flight level is reached.  When climbing using ALT INTV, the thrust mode uses N1.

Important Point:

  • When a Flight Level of a higher altitude is dialled into the altitude window, and ALT INTV selected, the CDU will automatically update the data to the new Flight Level.

Considerations When Using ALT INTV

When using ALT INTV, several variables that relate to the altitude constraint (s) will change, depending upon whether you are in VNAV climb, cruise or descent.  Rather than rephrase what already has been written, I have scanned the appropriate page (below) from the Cockpit Companion written by Bill Bulfer.

Using ALT INTV and SPD INTV During a VNAV Approach Phase

ALT INTV is a very handy tool, if during an VNAV approach, the flight crew fail to change the altitude in the MCP to the next lowest altitude constraint.  

To demonstrate, the aircraft is flying a published STAR that will join an VNAV approach.  VNAV and LNAV are active and the flight plan has several altitude and speed constraints.  To meet these constraints, the crew must update the MCP altitude to the next lowest altitude (displayed in the LEGS page of the CDU) prior to the aircraft crossing the constraint.

If the crew fail to update the MCP to the next lowest altitude constraint, then the aircraft will transition from descending flight (VNAV PTH) to level flight (VNAV ALT).   In this situation a crew could engage LVL CHG or V/S,  however, doing so would deselect VNAV.  

A simpler solution is to change the altitude in the MCP window to the next lowest altitude constraint (or MDA) and press ALT INTV.  This will command VNAV to descend the aircraft, at a variable descent rate, to meet the required constraint.   By using ALT INTV, the aircraft will remain in VNAV.

Additionally, SPD INTV is a straightforward way to control the speed of the aircraft during the approach while maintaining VNAV.  Company policy at some airlines insist that Speed Intervention be used approximately 2 nautical miles from of the Final Approach Fix (FAF).

Reliability of ALT INTV in Descent Mode - ProSim-AR

ProSim-AR (Version 1.49) exhibits difficulty in holding a lower altitude level when ALT INTV is used.

The Boeing system is designed that once the V-Path is intercepted, the Flight Director (FD) cross hairs maintain the new altitude by pitch.  In ProSim-AR this pitch is often difficult to hold and a resultant pitching of the aircraft (up and down) occurs as the system attempts to hold the lower altitude.  When using LVL CHG or V/S this does not occur.  Note that this behaviour does not occur when using INTV ALT to ascend.

It is not certain if this behaviour is common only to my system or is more widespread; but a way to solve the issue is to either:

(i)   Use an alternate descent mode; or,
(ii)  Manually change the altitude values in the CDU (INDEX/PERF/CRZ ALT), or (CRZ key/CRZ ALT) and press EXEC.

Proceedure (ii) manually changes the Cruise Altitude (CRZ ALT) to the lower altitude in the CDU.  This causes the command logic to switch from the logic that commands Altitude Intervention to the logic that commands altitude in thr FMC.  The aircraft will not pitch and will be stable.

The developers at ProSim-AR are continually tweaking these variables.  In future software releases (post version 221.b12) this issue may well be rectified.

Final Call

There are many of reasons an aircraft will need to alter altitude and/or speed; be it to divert around a localized weather event, or to abide by an Air Traffic Control directive.  Whatever the reason, often the changes are short-lived and a return to the original altitude/speed constraint imminent.

In these situations Altitude and Speed Intervention enable the aircraft to easily and quickly transition between Flight Level changes whilst VNAV is active.   Furthermore, the use if this functionality can minimise the time spent in the ‘heads down’ position during the high-task descent and approach phase of a flight.

In this article, I have explained the Altitude and Speed Intervention functionality of the Boeing 737.  I also have documented "work-arounds" should VNAV not function as anticipated. 

Acronyms and Glossary

Annunciator - A push button to engage a particular mode – often has a light that illuminates
ALT INTV - Altitude Intervention

CDU – Control Display Unit (display screen and keyboard to input data into the FMC)
Flight Level – Altitude that the aircraft will fly at (set in FMC)

FMA – Flight Mode Annunciator

FMC – Flight Management Computer  (part of the Flight Management System)
FMS – Flight Management System
LNAV – Lateral Navigation

MCP – Mode Control Panel 

PFD – Primary Flight Display

SPD INTV - Speed Intervention

VNAV – Vertical Navigation

This article has been updated and amended as at 12 SEP 2019


Alternate Use for OEM Rudder Pedal Circuit Breakers

The picture at left is of an OEM circuit breaker that has been removed from an OEM rudder pedal control mechanism.  The front plate of the control mechanism has several circuit breakers on the Captain and First Officer-side of the flight deck.

LEFT:  OEM circuit breaker switch.  The two connectors on the rear of the switch are very easy to connect to an interface card for push/pull functionality (click to enlarge).

Although connection of the circuit breakers, to the original functionality that was assigned to the switch in the aircraft, is not necessary (unless wanted), there is no reason why the circuit breakers cannot be used for additional functionality outside of the simulator environment.   Many enthusiasts have specially made panels that reside in the center pedestal to address such a need. 

The circuit breakers are basically an on/off push/pull switch.  Each switch can be easily wired to a standard interface card, such as a Pokeys or Leo Bodnar card, and then configured in ProSim-AR to a particular function.  If using FSUPIC, the functionality of the switch can be assigned to any on/off function.

For example, using FSUPIC (buttons) it is possible to assign each circuit breaker to a simulator function such as: pause, sim acceleration, jetway extension, etc.  The list is almost endless.

In my simulator, I have the Captain-side circuit breaker switches configured to simulator pause and simulator time acceleration.  These commands are readily accessible within the FSUPIC framework.

The circuit breaker switches are aesthetics, therefore, configuring the switches to regularly used commands is a way to minimize keyboard usage, and declutter the flight deck.


OEM Rudder Pedal Mechanism and Handles

The OEM rudder handle mechanisms have been sitting in storage for considerable time, and I thought it was time to add them to the simulator and replace the very poorly made and ‘cheesy’ reproductions that I was using.

LEFT:  OEM rudder handles and mechanism installed to Captain-side kickstand.  The stick shaker can be seen in the foreground (click to enlarge).

The rudder mechanism is not a small item that you can easily screw to the kickstand.  Each handle attaches to a 8-inch-long box, that houses the various circuitry, cabling and a dozen or so aircraft circuit breakers. 

Connection to the aircraft’s system is via two Canon plugs at the rear of the unit, while movement of the pedals forward or aft is facilitated by a long metal cable that connects to the rear of the handle.

The mechanism is not light-weight and weighs in at just over 1 kilogram.

The rudder handles do nothing other than add to the aesthetics of the simulator.  However, if wanted the various circuit breakers can be connected to an interface card (something I will not be doing).

LEFT:  Rudder handle mechanism (prior to cleaning).  The long metal cable that connects to the rear of the handle (enabling the forward and aft adjustment of the pedals) has been removed.  The white handle hangs loose and needs to be attached to the box using plastic fasteners (empty holes).  The black circular pull on/off circuit breakers can be seen below the white handle  (click to enlarge).

Installation to MIP

There are several methods that can be used to install the mechanism to the Main Instrument Panel (MIP).

If you are using an OEM MIP, then connection of the mechanism to the kick-stand is a matter of using the existing bolts and placement holes.  Fitment to a reproduction MIP is accomplished differently and depends upon how the MIP is constructed. 

I fabricated an aluminium cradle (saddle) that is attached by two nuts and bolts to the lower portion of the kickstand (under the kickstand out of sight).  The rudder mechanism slides into the cradle and a small screw holds the mechanism in the correct place.  A similar assembly could easily be made from wood and painted Boeing grey.

Note that the method of attachment differs to the way the mechanism is attached in the real aircraft  (classic or NG).  I have abutted the upper section of the mechanism against the lower kickstand.  In the real aircraft the mechanism is attached by a metal mounting bracket and screws.

The rudder mechanism I have installed is from a classic 500 series.  The difference between the classic and the NG is minimal, however the method that the mechanism is attached to the lower kickstand differs considerably. 

The classic is as shown in the abiove two images while the NG, shown at left, connects directly to the kickstand via a a series of brackets that form part of the kickstand structure. 

LEFT:  NG style rudder mechanism (click to enlarge) courtesy @Karl

Read about an alternate use of the circuit breakers.


OEM B737 CDU Conversion - Introduction

One of the slower projects is the conversion of two B737 CDU units.  The two units were purchased from an aircraft scrap-yard in the US and were formally used in a Boeing 737 operated by United Airlines.  

LEFT:  Straight from United Airlines to me.  Two OEM CDU units.  The rear unit has already had its CRT display removed and is partially  'gutted' (click to enlarge).

The two CDUs came from an airframe of a B737-500, which in 2008 was retired along with other Boeing classics, due to United Airlines decision to adopt the Airbus A-320.

The rear of each unit has a chronometer showing the hours of use - one unit has 5130 hours while the other has 1630 hours.

The CDU presently used in the simulator is manufactured by Flight Deck Solutions (FDS) and although I have been pleased with its operation and reliability, there is little resemblance, other than appearance, to the OEM unit.

LEFT:  Detail of the keyboard and DIM knob.  Interestingly the DIM knob dims the actual screen and not the backlighting (click to enlarge).

The prominent difference is external build quality and the tactile feeling when depressing the keys on the keyboard; the keys don't wobble in their sockets, but are firm to press. 

There is also a strong audible click when a key is depressed.  Furthermore, the backlighting is evenly spread with each key evenly lit.

The OEM CDU is large and VERY heavy.  I was surprised at the weight - a good 6 kilograms.  Most of the weight is made up by the thick glass CRT display screen and other components that reside within the sturdy aluminium case.

LEFT:  The casing removed to show the electronic boards that are secured by lever clips.  Like anything OEM, the unit is made very well from solid components (click to enlarge).

Like the casing, the internal structure is also made from aluminium and has four rails to enable the electronic boards to be installed and secured into place. 

Whenever I look at anything OEM, I am amazed at the workmanship that has gone into producing the item; the CDU does not fall short in this area.

A myriad number of small screws hold together the aluminum casing that protects the internal components.  Not only screws are used, but also special miniature DZUS fasteners than enable the side of the casing to removed easily for maintenance.


When discussing the CDU there are three similar terms that are often used interchangeably: CDU, FMC and FMS.  In this website, I use the terms CDU and FMC interchangeable which is not quite correct - let me explain.

LEFT:  Protective cover removed to show the main pin-out board, rear of the CRT display, power supply, and electronics.  These parts cause the CDU to be quite heavy.  The two Canon plugs  are just visible at the right of the picture enable connection to the aircraft. (click to enlarge to see detail).

The Control Display Unit (CDU) is the interface that the flight crew use to interrogate the data from the Flight Management Computer (FMC); it's basically a screen and keyboard.  The FMC in turn is but one part of a complex system called the Flight Management System (FMS).  The FMS is capable of four dimensional area navigation.  It is the FMS that contains the navigational database.

CDU and MCDU - Nomenclature

The acronyms CDU and MCDU are often used interchangeably.  The older units used in the classic airframes (up to 500 series) are referred to as a CDU, while the Next Generation airframe (600-900 series) units are called a MCDU.  M stands for multipurpose or multi-function.  Basically, the MCDU has a different key called a menu key.  This key, when pressed, accesses another layer of information that is not available in the earlier CDUs.

For those more military-minded, the CDU in military parlance is called a mission computer.

Aesthetic Differences

The CDU dates from 2008, therefore; it is not exactly identical to the CDU used in the Next Generation airframe, however, it is very close.

Main Differences Between the 500 Series and Next Generation CDUs

(i)    The dim knob is a slightly different shape;
(ii)   The display screen is rounded at the edges (the NG is more straight-edged);
(iii)   The absence of the horizontal white lines located on the inside edge of the display frame bezel;
(iv)   The display screen is different - cathode ray tube (CRT) verses liquid crystal display (LCD);
(v)   Two of the keys are different.  The NG has a menu and space key whilst the older CDUs have a DIR INTC and a blank key (no lettering on key); and,
(vi)  The fonts and colors between units differ.  Earlier units a monochromatic or green in colour while later units are in multicolour.

Fonts and colours are not important in the simulator environment as the avionic suite controls the displasy output.

To a purist, these differences are probably important, and if so, you will have to contend with a reproduction MCDU or pay an exorbitant amount for an NG unit. 


The software used in the OEM CDU is not used in the simulator.  The CDU functionality is dictated by the avionics software (ProSim-AR) in use.  This is also true for the font type and colour.

LEFT:  Completely gutted.  All unnecessary and unusable electronic components have been removed.  These two CDU units will soon operate flawlessly with ProSim-AR and flight simulator (click to enlarge).

Converting the CDU

I am liaising with an Australian company that specialises in converting avionics components used in commercial flight simulators.  This company has had considerable experience converting B747 avionics and is keen to see if their expertise will similarly work with the B737.

In a second article, I will explain in more detail how the conversion was done, and examine some of the problems that needed to be resolved.  I also will discuss the mounting of the unit into the CDU bay. 

More photographs of the CDU are located in the image gallery.  Additional images will be added to the gallery in due course.


OEM - Original Equipment Manufacture (aka reral aircraft part).

CDU - Control Display Unit.

MCDU - Multipurpose/multifunction Control Display Unit.


Troubleshooting Power Management Settings and Solving USB Disconnects 

Remember when all that was required to run flight simulator was one display monitor, joystick and a keyboard – those days are long gone.   

LEFT:  High-speed 5 volt powered USB hub.  This hub resides in the Throttle Interface Module (TIM).  Note ferrite choke. (click to enlarge).

Depending upon the level of system complexity, a flight simulator may require a dozen or more ports to connect peripheral items to a server or client computer (s).  Historically, connection of peripherals has been via USB.  

USB is an acronym for Universal Serial Bus and, generally speaking, if only a few peripherals are attached to a computer, there usually is not a problem with communication between the computer and the attached device.  However, as interface cards and peripherals become more complicated and numerous, there is a propensity for disconnects to occur more frequently.  A USB disconnect usually announces itself by the sound card playing the ‘ding-dong’ sound as the peripheral disconnects itself from the computer.

Guidelines (golden rules)

There are several ‘golden rules’ to remember when using USB.

(i)      Try and keep all USB cables as short as possible;

(ii)     Do not join USB cables together;

(iii)    Always use quality USB cables with quality connectors;

(iv)    Do not ‘kink’ the USB cable or wrap the cable so tightly that the wires are at a 90-degree angle;

(v)     Do not lie USB cables beside one another so they are touching, but maintain some space between them;

(vi)     Use a USB cable fitted with noise limiting nodes (NLN);

(vii)    Use a USB cable/port that is rated at the highest output (USB 3 or above);

(viii)   For multi USB connections use a quality powered USB hub; and,

(ix)     Try to maintain space between USB cables and power cables.

Noise Limiting Node (NLN)

A noise limiting node (NLN), also known as a 'ferrite choke' is a small cylindrical node that sits at each end of a USB cable.  Briefly explained the nodes are made from a solid ball of ferrite which is magnetic and therefore quite heavy.

LEFT:  Ferrite choke on USB cable.

The purpose of the NLN is to stop electromagnetic interference (EMI) transferring from the peripheral to the computer.  EMI can be produced from any number of peripheral items and a USB cable running between the peripheral and the computer acts as an antenna, picking up and transmitting EMI current.  The current can, but not necessarily always, cause havoc with either the operation of the peripheral or the computer itself.  

Adding USB Ports

As the number of add-on peripherals increase, the number of available ports falls short and additional USB ports need to be added to the computer.  Additional ports can easily be added to a computer via a PCE card which enables (on average) an additional 4 USB ports to be added to your computer.  A PCI card is attached to your motherboard.

Power Requirements

One of the main reasons that USB disconnects occur, relates to the power that is available to the computer’s USB port.  Often the power requirements of the device will be greater than that provided to the USB port; this causes a disconnect.  Additionally, depending upon your computer, it is not uncommon for power to fluctuate between USB ports as the computer’s motherboard directs power to various processes.

Depending upon how your system is set-up, when several devices 'come on line' a minor spike can be generated.  Often, this spike can momentarily exceed the amperage rating of the USB port.  This can cause a disconnect to occur.

It’s important to understand that not all USB ports are made identical.  In general, the ports on the rear of the computer are part of the computer’s motherboard; these ports are rated as high power ports.  However, USB ports that are not part of the motherboard, and usually located on the front of the computer may not receive the same power rating.  

Often a supply company will provide a computer will a dozen or so USB ports, however, to save money will choose to use what is called a ‘front panel USB header’ which has a small piece of circuitry that acts as a hub.  In this case, the power to the front panel USB is reduced.  Furthermore, it is probable that these ports may not be USB 3 and if used for a high-demand peripheral will cause a disconnects to occur.

USB Hubs

Another strong recommendation is to use a high quality powered USB hub rather than connecting several USB cables directly to a computer.  A powered hub should be used rather than an unpowered hub as the former provides its own direct power source which is usually rated at a higher amperage than the computer’s USB port.  

The interface modules that form the core of my simulation system have one or two powered hubs installed to the module.  The interface cards are then connected by very short USB cables to the hub.  A high quality USB cable (with a NLN) then connects the interface module directly to the computer.

Windows Power Management Settings (PMS)

Not all USB peripherals will be required at all times.  Often a device will not need to communicate with the computer until something is required – such as a change to a radio frequency, an input from the control column or a key press to the MCP or CDU.

LEFT:  Screen grab of Windows 7 PMS.  Windows 10 is similar (click to enlarge).

Windows has a nasty habit of ‘putting to sleep’ a USB connection that is not being used.  It does this to save power.  It is very imperative that you ensure that all power saving modes are turned off with regard to USB.  

To do this open your control panel and search for device manager.  Scroll down until you find Universal Serial Bus.  Under this tab you will find all the USB ports that you have attached to your computer.  Open each in turn and check the power management settings and ensure they are turned off.

Troubleshooting USB Disconnects

It is paramount to try and discover which peripheral is causing the disconnect.  The easiest way to troubleshoot a disconnect issue is to remove ALL the USB cables from the computer, and then one by one re-connect the cables to the allocated port and test.  Make sure you switch your computer off and on as you add each of the cables in turn.  Hopefully, you will eventually discover which cable/device is causing the issue.  The problem device will generate ‘ding dong’ if a secure connection is not possible.

If USB disconnects continue, try swapping the cables between different USB ports on the computer.  The disconnect issue maybe caused by the USB port/cable combination you are using.  As mentioned, not all USB ports have the same amount of power/amps available to them. 

Try to place peripherals that require minimal power, such as a mouse or keyboard, on lower-powered USB ports, and place more energy-requiring peripherals on powered hubs; perhaps only a few devices on the one hub.  Doing this will ensure that the hub will always have enough power (amps) to power the devices attached (cancelling out possible spikes as discussed above).  

Final Call

Hopefully, if you apply the above-mentioned suggestions USB disconnects will cease.  However, you will eventually reach the limit of USB capability, and at this point the use of Ethernet should be investigated to augment, or to replace the reliance on USB.

This article is but a primer.  I am not an IT expert and welcome any comments.

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