Quick and Obvious 404 Trick

Posted by Justin A. Parr on November 22, 2022
Posted in: Other Stuff. Leave a Comment

Quick and Obvious 404 Trick

 

I’m sure someone has stumbled upon this before, but I thought I would share a revelation I had while working on a technical issue.

To get to the 404 page for any website, go to:

[website]/404

Unless the website in question specifically has content published at the path “/404”, you’ll get the 404 (not found) page.

Some examples:

For some reason facebook.com/404 redirects to some person’s page – not sure if that’s intentional, or maybe a sly back door left by a clever developer.

Rant: SMA vs. RP-SMA

Posted by Justin A. Parr on August 15, 2022
Posted in: Good Design - Bad Design, Rants. 4 comments

Rant: SMA vs. RP-SMA

A few years ago, I put up a wireless camera system, and I ended up needing an antenna extension cable.

I vaguely recall buying a SMA, and it was the wrong one, which of course meant that I had to buy a RP-SMA cable, which worked.

I have also occasionally had to buy RP-SMA cables to extend WiFi antennas from time to time – mainly due to line-of-sight issues.

And I never gave it much thought after that.

Having recently installed a new gate opener, I found that I needed to raise the antenna (again, due to line-of-sight issues), and I bought a RP-SMA cable, which I assumed was fairly standard.

Imagine my surprise when it didn’t work – ONE end connected just fine, but the other end wouldn’t connect at all.

“What idiot used the wrong standard?”, I thought to myself.

So I bought a SMA cable, and that worked out just fine.

However, I started thinking about it.  Why have two cables that are otherwise identical, but not interoperable?

 

Other than the pin configuration, SMA and RP-SMA are identical. In regular SMA, the male pin goes on the female-threaded connector. In “Reverse Polarity” SMA, the male pin goes on the male-threaded connector.

 

 

What is SMA?

The SubMiniature A (SMA) connector is a smaller, lighter-weight, and more precision version of the “F” connector.

If you’ve ever had a VCR or cable box in the 1990’s, you have at least one of these in the bottom of a drawer somewhere:

The venerable “F” connector at the end of a coaxial cable was once the standard way to connect devices that share a NTSC video signal.  Over time, this was replaced by newer standards, but even as late as the early 2000’s, you could go purchase a VCR, and it probably came with one of these cables.

The pin on the male end is typically the unshielded cable’s core, which protrudes from the insulation.  The male end has a female thread that freely spins, while the female end has fixed male threads.  To connect the two, you insert the pin in to the female end, and tighten the free-spinning “nut” on the male end.  Here is an RG59 elbow that has both male and female F connectors:

RG59 (“F” Connector) Elbow

 

The SMA connector is slightly smaller, lighter, and machined to more specific tolerances.

SMA Elbow

If you were unaware of the scale, SMA connectors look almost identical to F connectors.  However, where the female F connector is about 1/3 inch in diameter, the female SMA connector is about 1/4 inch – about 32% smaller.

Today, SMA connectors are found almost universally in any consumer application that uses an external antenna, including hand-held radios, FRS (walkie-talkies), garage door openers, and WiFi routers.

Well, not WiFi routers.

This is where RP-SMA comes in.

 

What is RP-SMA?

I always assumed that one or the other supported better attenuation, or more bandwidth, or maybe one was for outdoor applications.  But, that’s not the case.

“Reverse Polarity” SMA, or RP-SMA, was created as a way to appease the FCC’s requirement to keep people from being able to boost their WiFi signal by attaching a larger antenna.

I wish I was kidding.

You see, when WiFi was first standardized in 1997, the FCC required “unlicensed transmitters” to use an antenna connector that was either proprietary, or required professional installation.

In order to address the FCC requirement, some genius decided to move the pin on the then-standardized SMA connector from the female-threaded end to the male-threaded end.  So RP-SMA should really be called “Reverse Gender” SMA – polarity refers to the signal, and gender refers to the connector.

As an easy fix, the larger manufacturers adopted RP-SMA as an alternative to SMA, because inventing an entirely new connector would be unavoidably complicated and unreliable.

Because all of the larger WiFi manufacturers used RP-SMA, it became fairly standard, and because it became fairly standard, parts and cables eventually made their way in to the consumer market.  And once the parts and cables hit the consumer market, it was no longer considered proprietary.

So, just 3 years later in 2000, the FCC dropped the “unique and proprietary” requirement, because RP-SMA was no longer unique, and no longer proprietary.  And by that time, WLAN was fairly standardized, and the government had other things to worry about.

Unfortunately, the legacy of those bad decisions is that we have two incompatible but semi-interoperable standards.

 

Why RP-SMA is Bad

In summary, there is too much complexity, which leads to too many possibilities for problems.

In my case, I didn’t even know that I had the wrong cable until I went to connect the two male ends together.

SMA is on the top, RP-SMA is on the bottom.  In addition to the two gender changers for SMA and the two for RP-SMA, there are four additional converters between the two.  Also, RP-F is mechanically-compatible with SMA-F, but won’t pass a signal because there is no center pin on either connector.

Because the two standards use the same mechanical connection, it’s way too easy to mistake one for the other.  In addition to two gender changers for each, there are four additional converters, which means that there are eight possible combinations instead of the usual three (M-M, F-F, M-F) that you would find with any other connector.

Worse, you can connect a female RP-SMA to a female SMA, but it won’t pass a signal because there is no pin.

For example, if I didn’t know better, and simply bought a RP-F to SMA-F coupler, I could have spent weeks troubleshooting, and I bet others have done this.

It’s a bad standard, and now that the FCC no longer requires it, RP-SMA should be completely deprecated in favor of SMA, which was the original standard.

(And besides, “Reverse Polarity” SMA is not even named correctly)

 

How Do We Get Rid of RP-SMA?

  1. Stop making devices with RP-SMA connectors.  Eventually all of the old devices will die off.
  2. Stop making RP-SMA cables.  Make ONLY SMA cables, and continue to produce converters for situations where a cable needs to connect to an older RP-SMA device.

As with most pin-based connectors, devices that use either SMA or RP tend to have female connectors, because pins bend, and replacing a bent pin on a device is a lot more expensive than simply replacing a cable.  Further, most antenna cables tend to be male-to-female, and antennas tend to be male.  Therefore, for most applications, converting RP to SMA would only require a RP-male to SMA-female connector, and then everything after that, including the antenna, would be SMA.

 

Gate Opener Replacement

Posted by Justin A. Parr on August 7, 2022
Posted in: Other Stuff. Leave a Comment

We moved in 2019, and the new house has a gate opener.  After three years of loyal service to us, and who knows how many years of service to the previous residents, it finally died.

After some troubleshooting, I called Mighty Mule tech support (A+ by the way!) to discuss my options.

Faced with a cost of $350 for a new controller board for my now-obsolete MM560, or buy a new MM571W for about $100 more, buying a whole new unit made much more sense.

Here’s the full details of my troubleshooting and installation efforts:

Mighty Mule MM571W

Gauge and Gauge (Bore and Sheet Metal)

Posted by Justin A. Parr on June 29, 2022
Posted in: Good Design - Bad Design, Math and Science, Other Stuff. Leave a Comment

Unfortunately, these two things have absolutely no relationship to each other.

Recently, while working on a sheet metal project, I was annoyed that I had to go find a sheet metal gauge chart, because sheet metal is pretty much only sold in “gauge” thickness.  I was even more annoyed to find that different metals have different gauges.  So 14 gauge stainless is not the same as 14 gauge copper, and neither of these are the same as 14 gauge aluminum

 

Bore Gauge

Although this is an outdated system, its only modern vestige applies to shotguns.

And it goes like this:

The bore gauge number (n) is the diameter of the bore, such that n lead balls of equal diameter weigh 1 pound.

Therefore, the greater the bore gauge, the smaller the diameter.

So when is 12 > 20?  When you’re talking about bore gauge.

Yep.  That’s bad.  But, it’s at least manageably bad.  There is at least a formula.

Lead weighs about 11.8 ounces of weight per ounce of volume.

A pound of lead (16 weight ounces) would therefore occupy a mere 16 / 11.8 = 1.35 fluid ounces.

To relate volume and diameter, we have to convert fluid ounces to inches, and 1 ounce of volume = 1.8 cubic inches.

Therefore, 1.35 fluid ounces of lead = 1.35 * 1.8 = 2.43 cubic inches.

At this point we have 2.43 cubic inches of lead, which we know is enough for a “1 gauge” lead ball, but we need to find the diameter.

Volume of a sphere (V) = 4 / 3 * Pi * r^3

r = Radius

r^3 = Radius to the 3rd power (cubed)

Since we have the volume and we need the diameter, we need to rearrange this to solve for radius, then multiply by 2 (diameter = 2 * radius)

d  = 2 * ( (3 * V) / (4 * Pi)) ^ 0.3333

By inverting ^3 (cubed) as ^ 0.3333 we get the cubed root

If we compute this for our 2.43 cubic inches of lead, we get:

d = 2 * ( (3 * 2.43) / (4 * Pi)) ^ 0.3333

d = 2 * ( 7.29 / 12.57 ) ^ 0.3333

d = 2 * 0.58 ^ 0.3333

d = 2 * 0.83

d = 1.667 inches

A 1 gauge bore diameter is 1.667 inches.

For the general case, we can start by computing the amount of lead.  Since volume and weight are proportional, and the volume of a pound of lead is constant:

V = 2.43 / g

2.43 = 1 pound of lead in cubic inches

g = gauge number

We then take the same formula, substitute for V, and pre-compute some constants:

d = 2 * ( ( 3 * 2.43 / g ) / (4 * Pi )) ^ 0.3333

d = 2 * ( 0.58 / g ) ^ 0.3333

We can cube both sides and further simplify:

d ^ 3 = 8 * 0.58 / g

d ^ 3 = 4.64 / g

d = 1.668 / g ^ 0.3333 

And, we can use this for any gauge number (g):

Gauge Diameter
10 d = 1.668 / 10 ^ 0.3333

d = 1.668 / 2.154

d = 0.77 inches
12 d = 1.668 / 12 ^ 0.3333

d = 1.668 / 2.289

d = 0.73 inches
20 d = 1.668 / 20 ^ 0.3333

d = 1.668 / 2.714

d = 0.61 inches

And, you can plug any number in to this formula, so if you wanted a 14.5 gauge bore size (for whatever reason), you would use the same formula above, and that bore size is 0.68 inches.

You can even reverse the process to find the bore gauge corresponding to a given diameter measured in inches, by solving for gauge:

d = 1.668 / g ^ 0.3333

d ^ 3 = 4.64 / g

g = 4.64 / d ^ 3

A .357 magnum has a nominal bore diameter of 0.354 inches.  If we plug in 0.354, we get:

g = 4.64 / 0.354 ^ 3

g = 4.64 / 0.0444

g = 104.6

So the next time you threaten someone, you can say “Don’t make me pull out my 104 gauge!”, and at least this has an obscure but discernible meaning.

How did we get stuck with this standard?  Nobody really knows!  However, most texts state that gauge was used even as late as 200 years ago, when there weren’t any good, universal measuring standards.  If you ordered ammunition in gauge size, you were assured that it would match your gun’s bore diameter.  As the rifle (gun with a rifled barrel) is a relatively new invention, measurements were much more standardized by that time, and it makes sense that rifle bore sizes (caliber) are always measured in inches or millimeters.

However, to this day, most shotguns are still smooth-bore, because shotgun cartridges have multiple, small projectiles (called pellets) that spray out in a disc formation when the cartridge is fired.  Because the pellets are much smaller than the shotgun’s bore, they don’t come in to contact with the barrel enough for rifling to be effective.  Although there are rifled shotguns (called slug guns) that are designed for shooting a single, solid lead shot (called a slug), these are uncommon.  Therefore, it makes sense that we’ve kept the tradition of measuring a shotgun’s smooth bore size in gauge, rather than using caliber.

Fortunately, today, bore gauge is only used for shotguns, and rifle bores have become so standardized that I can go anywhere in the world and purchase ammunition for a gun of the same caliber manufactured anywhere else in the world.

Now, let’s talk about sheet metal gauge.

 

Sheet Metal Gauge

Here is the formula for sheet metal gauge:

There isn’t one.  Consult a gauge chart.

But, it gets even better.

Sheet metal gauge is based on density, so not only do you need a gauge chart, you need a different gauge chart for each type of sheet metal.

How did we end up with this?

Sheet metal gauge size is based on wire gauge size, but not AWG, which is the American standard for wire gauge size.

Back in the early days, each vendor had their own wire gauge size, based on the number of machining steps required in order to get down to that gauge size.  E.g. 10 gauge wire requires 10 steps.  Eventually, all of that got standardized as the American Wire Gauge (AWG) standard.

Wire has been mass-produced much longer than sheet metal, so when sheet metal started to be mass-produced, the sheet metal manufacturers adopted the wire gauge standard (but not AWG), and each manufacturer had their own.

Over time, like AWG, sheet metal gauge sizes became standardized.

Unlike AWG which specifies a standard wire diameter independent of any other factor (e.g. 20 gauge wire is the same diameter in copper or aluminum, or steel, or whatever), sheet metal gauge is based on density, so the gauge size changes based on the specified material.

Nominally, every reference to sheet metal states that gauge size is based on “a density of 41.82 pounds per square foot”, but that’s not the case.  If it was the case, you would be able to work backwards to find the gauge number.  For example, going back to the way bore gauge works, if 41.82 pounds = 1 (sheet metal) gauge, then dividing that by 2 = 2 gauge, etc.  But that’s not the case.  10 gauge (steel) = 0.1345 inches, and if we multiply that by 10, we get 1.35 inches.  20 gauge = 0.0359 inches, and if we multiply that by 20, we only get 0.72 inches.  So there is clearly no proportionality based on weight or thickness.

If you google for a sheet metal gauge formula, you can find plenty of references that all begin with:

Sheet metal gauge size is based on 41.82 pounds per square foot.  Consult your sheet metal gauge chart, and…

As it turns out, this is a formula for calculating the weight based on gauge size, not for calculating gauge size.

Why do we still use this archaic standard?  Sheet metal gauge is codified by U.S. law, for tax purposes:  15 USC 206: Standard gauge for sheet and plate iron and steel.

So, because the government regulates the thickness of sheet metal for tax purposes, you have to consult a gauge chart whenever you order sheet metal.

 

C – Access Other Data Types As Byte Array

Posted by Justin A. Parr on June 19, 2022
Posted in: Tech Support. Leave a Comment

C – Access Other Data Types As Byte Array

 

I’m working on an Arduino project.  Yes, I know I complained about them before, but that’s before I found out about the Espressif ESP32 and it’s smaller cousin, the ESP12 / ESP8266.  These have built-in WiFi and Bluetooth, and have all sorts of cool features.

So, if you’ve visited this site in the last couple of years, you may have noticed that in the upper-right corner of the page, there is a widget called “Temperature at Casa de Parr”.  This runs on a Raspberry Pi Zero W (like the Gate Controller).  There is a DS18B20 temperature probe attached to the Pi, which is in a weather-proof case, and a second one connected via a wire, which sits just below the water level inside the pool.

At the time, the Pi was WAY overkill for this type of application, but for about $30 and some coding, I had a WiFi-accessible thermometer – very cool.

Well, I’ve been working on re-creating this using an ESP12, which is about $4, and I can use the same DS18B20 temperature probes.

Although the DS18B20 temperature probes are very consistent (precise), they are not very accurate, and therefore there is a need to store a calibration factor – basically an offset that you can add or subtract to the measured temperature, which may be two or three or even four degrees off, in order to calibrate it to the “real” temperature.

Of course, this assumes that the error is a constant at all temperature ranges – we could talk about two-point (linear-gradient) or three-point (curved) calibration, or even use an n-point bezier curve, which would be much more accurate, but for my needs, a constant is probably sufficient.

Unlike the Pi, which uses an SD card as its file system, the ESP can’t read or write any files unless you attach an SD card reader and write some code to read and write the files.  Instead, they are designed to have a single, static program that gets stored in flash memory, which loads and runs automatically at boot.  In addition, it has 512 bytes of EEPROM that’s accessible to the user, to store things like state or settings – 1/2 KB isn’t very much storage.

So to store my calibration factor, I need to write a float to EEPROM, and the method for writing to EEPROM is to write one byte at a time.  A float is 4 bytes, and therefore I have to perform 4 write operations to 4 separate EEPROM addresses.

In order to accomplish this, I need to access my 4-byte float as a byte array.  The easiest way to do this is to create a casted pointer, and I thought the concept behind this was interesting enough that you, dear reader, might be interested in it as well.

float f;
char *c=(char *)&f;
for(int i=0;i<4;i++)
  EEPROM.write(iBaseAddress+i,c[i]);

 

How This Works

The “char” (or byte) data type is one byte – normally, you would use an array of char as a string:

char myString[]="Hi There!";

At this point, you can access the individual bytes of myString as elements in the array:

myString[3]=='T';   //true

In addition, I can create a char pointer to my string, and access the pointer as if it was my original string:

char *c=&myString;  //& means "Address of"
char *c=myString;   //The compiler knows what you're trying to accomplish
c[3]=='T';          //true

Using the address operator (&) we can assign the pointer c to the address of our string &myString.  However, if you just assign a pointer to a string, most compilers know what you’re trying to accomplish, and just directly assigning it results in the same thing – the pointer c holds the address of myString.

If we try to do this with our float, we get a type conversion error:

float f=3.14;
char *c=f;          //Error:  Can't convert float to char
char *c=&f;         //Error:  Can't convert float to char

However, if we cast our float as a char, we are telling the compiler to treat the variable f (a 4-byte float) as if it was a char.  Using casting with the address operator returns a pointer to a char, which really points to our float:

char *c=(char *)&f;

Now, c[0] through c[3] are the four bytes of our float.

 

Conclusion

I wrote this out within my code, and I got to thinking about it – I don’t do very much c coding these days, and I should probably double check.  I found NO GOOD answers, and the top answer seemed to be to use a union:

union {
  float f;
  char  c[4];
} fl;
fl.f=3.14;
for(int i=0;i<4;i++)
  EEPROM.write(iBaseAddress+i,fl.c[i]);

This works, and I could certainly create a union type:

union fltype {
  float f;
  char  c[4];
}
union fltype fl;

This is certainly more readable, but the only time I need to read a float as a byte array is when reading or writing to EEPROM.  Using type casting just seems like a cleaner solution.

Did You Know – Most Browsers Have a Built-in Color Picker?

Posted by Justin A. Parr on March 12, 2022
Posted in: Other Stuff. Leave a Comment

Did You Know – Most Browsers Have a Built-in Color Picker?

Well, I didn’t (If you said ‘no’ then response.write “either”) !

Here is an example that uses JavaScript to keep a color picker and a text input in sync with each other:

<script>
  function sync(s,t) {
    var a=document.getElementById(s);
    var b=document.getElementById(t);
    if (a.value!=b.value) b.value=a.value;
  }
</script>
<input id="PickColor" type="COLOR" value="#207cff" onchange="sync('PickColor','HexColor');" />
<input id="HexColor" type="TEXT" value="#207cff" onchange="sync('HexColor','PickColor');" />
Here is the Color picker without the JavaScript:

<input name="WebColor" type="COLOR" value="#207cff" />

If you add this to a form, when the user submits the form, one of the form fields will be “WebColor”.

 

The Log4J Problem Was Both Insidious And Unnecessary

Posted by Justin A. Parr on January 13, 2022
Posted in: Good Design - Bad Design, Rants. 1 comment

If you have anything to do with technology, the chances are pretty good that you wasted at least part of the month of December patching, upgrading, or reconfiguring software that uses Log4J.

Now that the dust has settled, we can look at the vulnerabilities that were announced in December, what factors led to the problem, and why it should never have occurred.

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Thoughts on the ‘Rust’ Shooting, AKA the ‘Alec Baldwin Incident’

Posted by Justin A. Parr on January 1, 2022
Posted in: Analyses and Responses. Leave a Comment

It’s easy to play armchair quarterback by offering early criticism, so I felt that it was important to withhold any commentary until a majority of the facts came to light.

On October 21, 2021, in New Mexico, on a set for the movie “Rust”, a live round was discharged from a firearm, resulted in the fatal shooting of the cinematographer and injury to the director.

As a result of the incident, key people involved in handling the gun have been scrutinized by the media:

  • Alec Baldwin, who was handling the gun at the time of the discharge.
  • Hannah Gutierrez-Reed, who was the Weapons Master, also known as the armorer, who has ultimate responsibility for all firearms used as props, as well as to ensure the safety of the cast and crew.
  • Dave Halls, who was the Assistant Director, had handled the gun.  Allegedly, Gutierrez-Reed handed the gun to Halls, who handed it to Baldwin.

In addition, the shooting incident has sparked a debate about the general use of firearms as props in TV and movies.

I want to state unequivocally that I have no background in the film and theater industry, but I do have a level of knowledge and experience with many different kinds of firearms that exceeds at least 99% of the general public.

The purpose of this post is to review the facts of the incident (current, as of this writing), provide my analysis and thoughts about what may have led up to the incident, as well as what could and should have been done to prevent it.

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