I have a cx CAS and I would like to know how I can do I/O through the USB port i.e. what commands do I use?

I thought there was only one usb port?

EDIT: Sorry, I see it's for, not 4. For some reason I read it as USB through pin 4 (presumably in the serial port.

TI has always been concerned more about locking the calculator down and making it less useful to users. They're only worsening over time.

Depending on what you want to control, embedded ARM-based boards such as the highly popular Raspberry Pi, the Cubieboard, the BeagleBone Black, and others, are all of:
* much more powerful;
* much more open;
* much easier;
* much cheaper;
than any calculator on the market. Even the newly released HP Prime.

Well you say they spend more time locking than fixing. This simple equation actually works : TI = APP£€ * Micro$oft.

I was replying to the following sentence of yours:

I just wanted a simple way to use it to control things for my 10 year-old son

I'm not sure what that encompasses, but I can confirm you that the Nspire is a very poor platform for programming, as you've found.
Even though it has a built-in screen, keyboard and battery, it's overly expensive and limited, compared to other recent platforms whose programmability is a design goal of (instead of being considered an anti-feature by) the manufacturer.

The problem is that its their hardware and patents etc so ideally we could spend a lot of time decrypting their algorithms or as we say in EW countermeasures and they will countermeasure our countermeasures etc ad infinitim until one of  goes bankrupt or...

True, although to clarify, once you buy a TI-Nspire CX, TI no longer owns it nor its hardware. They still hold the patents and copyrights, but after the calc gets out of the store, they can't dictate what you can do with it anymore, so it's obvious that the TI programming community will attempt to fight for that right. TI might have reasons to block the hardware, but we live in free countries for the most part (or countries where doing what we want with purchased hardware is allowed).

Also, the Internet can get pretty dangerous if people feel that a company goes too far (in some case it can cause damages to both the company and users). Look what happened when Sony tried to sue George Hotz for the PS3 jailbreaks: Anonymous retaliated by launching numerous attacks on Sony's servers, causing PSN to collapse and compromising millions of user accounts.

True, although to clarify, once you buy a TI-Nspire CX, TI no longer owns it nor its hardware. They still hold the patents and copyrights, but after the calc gets out of the store, they can't dictate what you can do with it anymore, so it's obvious that the TI programming community will attempt to fight for that right. TI might have reasons to block the hardware, but we live in free countries for the most part (or countries where doing what we want with purchased hardware is allowed).

Lets suppose I write some code to bypass all the test mode security but make it look like it is in the test mode and even have access to other additional capabilities the teacher is unaware of and I post it out on the internet. What do you think would happen? It would be banned from all tests and less people would buy it and TI would lose money and go after me even if it was difficult to find me. That's their greatest fear. If we keep beating their security at every OS version they will eventually give up and stop producing the calc or calcs in general. Another concern is intellectual property in that they invested a great deal of money in developing some of the software and they don't want others to have access to it. What algorithm do they use for computing the ln(matrix)  and e^(matrix) for example? Not easy to do it well. Why didn't they include matrix root(matrix) since they already have exp,ln and matrix^-1? There are a billion questions like these but it all comes down to money. If we zero or reduce their profit enough then they have to invest the money elsewhere.

Also, the Internet can get pretty dangerous if people feel that a company goes too far (in some case it can cause damages to both the company and users). Look what happened when Sony tried to sue George Hotz for the PS3 jailbreaks: Anonymous retaliated by launching numerous attacks on Sony's servers, causing PSN to collapse and compromising millions of user accounts.

Sony did sue him and he did settle and He thought Anonymous was wrong for doing what they did. I don't think that it should be legal to sue him for what he did or others do!

Regarding programs to bypass the test mode, if you post such file on Omnimaga or Cemetech forums, I doubt it would remain online for long, as moderators would delete it in a heartbeat. I am pretty sure on TI-Planet it would be the same.

Multiple such programs were already posted in 2012, right after (and clearly in retaliation to) OS 3.2. The end result of moderation was that such software ended up being posted on cncalc. The software would be far harder to remove from so-called "takedown-resistant hosting", should such hosting be used.

Of course you can cheat with nLaunch but that's not the purpose of this software, which is why forum people are relunctant about helping anyone using it and why the documentation isn't very clear.

TI-Planet's tutorials are believed to be clearer.

Also, if Ndless could cause the Nspire to be banned from tests, then why hasn't the TI-83 Plus, 84 Plus and 84 Plus C Silver Edition (which all have native ASM support) been banned already?

There's no proper CAS for any member of the TI-Z80 series; the Nspire series does have a proper CAS. nLaunch / nLaunch CX / nLaunchy would be more likely to get the Nspire banned than Ndless is.