Hello all,
Sorry for this negative first post. I've been a big fan of arduino for a while had a great time using the Uno, but I'm trying to run a project about the Internet Of Things in my office and we have 5 or so Arduino Yuns and I'm finding it really hard to get them working. I've been careful to remove focus from the Arduino programming window every time I headbutt my keyboard in frustration, thus far I'm fairly sure that the code I'm writing is okay.
Everything I try something on the Uno it works perfectly, but trying it on the Yun systematically fails.
The reason for this is that many of the things I'm trying to do involve a serial connection. From what i've understood Yun doesn't have a working Tx and Rx (thus rendering most sheilds useless! hooray!) so I've been looking at software serial. I somehow managed to get an RFID reader working but only if I use pins 10 and 11 (can not figure a reason for this, defies logic, saw someone else use these pins with a yun and an rfid and it worked for them). I'm now trying to get TextStar LCD to work with it : Nope, no chance, not on any pin. (Uno : fine, obv).
I'm trying to help introduce beginners to this tech, and I fully accept the problems may lie with me but this is incredibly frustrating. So far the best option someone I work with has found appears to be to connect an Uno to the Yun and wire transfer stuff across, so the solution appears to be don't use the yun?
Please somebody tell me I'm missing something here. Any pointers really helpful, including how to clear a toilet that is mysteriously blocked by having 5 PCBs flushed down it.
From the MCU point of view, a Yun is a Leonardo. Not every shield works with the leonardo. Some/most (I don't have a number) work fine by overriding some pin numbers in the related library
To give you an example, see this tutorial, the leonardo paragraph in particular
[quote author=Federico Fissore link=topic=237171.msg1704755#msg1704755 date=1398964855]
From the MCU point of view, a Yun is a Leonardo. Not every shield works with the leonardo. Some/most (I don't have a number) work fine by overriding some pin numbers in the related library
To give you an example, see this tutorial, the leonardo paragraph in particular
[/quote]I think there are just too many people that just don't read any documentation, regardless how many times you push their nose onto it... 
Ralf
Sorry all, the above post was the result of much frustration. In reply to peoples comments I realise the yun is more like a Leonardo, when I mentioned the uno just worked I wasn't referring to shields, just the software serial. Shields I realise can be wired up on a breadboard and made to work, have done this but still minorly annoying. Saying RTFM is never helpful, could anyone maybe help me by letting me know why an rfid with software serial only works on pins 10 an 11 for instance? I'm all for reading the manual, but can't find a reason for this... Thanks in advance for any help and sorry for venting.
...also, need to update the yun image too I see now in the main part of the forum and peoples signatures, this is helpful, thanks 
I would check the datasheet for descriptions of internal pull up resistors in the avr chip. see if those are identical on uno and Leonardo chips.
From the product description page:
It is not possible to access the I/O pins of the Atheros AR9331. All I/O lines are tied to the 32U4.
Each of the 20 digital i/o pins on the Yún can be used as an input or output, using pinMode(), digitalWrite(), and digitalRead() functions. They operate at 5 volts. Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor**(disconnected by default)** of 20-50 kOhms.
MarkTheCaz:
Sorry all, the above post was the result of much frustration. In reply to peoples comments I realise the yun is more like a Leonardo
Well, the links to the documentation were made as it clearly states that the AVR part is in fact a Leonardo (using the same chip), so all the differences between Uno (and preceding boards, as well as the Mega(2560)) and a Leonardo do apply.
Before complaining that this and that doesn't work on a Yun, people need first to check that what they are trying to do works on a Leonardo.
On top of that, the basic serial port of the Leonardo part of the Yun is used for the bridge, the communication between the AVR and the MIPS/Linino part. This restriction is also clearly stated in the documentation for the Yun.
As for your problems that the RFID shield only works on certain pins of the Yun, not knowing the exact requirements of that RFID shield, it might be very well down to the differences in the pin layout/capabilities between the Uno and the Leonardo...
Ralf
PS: A nice blog about some of the practical differences (particular pertaining to bad/old habits of Uno/ users) between Uno and Leonardo can be found at Dr. Monk's DIY Electronics Blog: Arduino Leonardo vs. Arduino Uno
Thanks for the above replies, still banging my head against the wall a little.
I'm not sure the differences between the Uno and the Leonardo/Yun are the issue here. I've got the RFID working but it does so intermittently.
Yesterday it was fine, today it's not. The only thing I've noticed is that the 5v coming out of the yun is not 5v: when the rfid works it's about 4.5v, when it's not working it goes down to about 3.7v. Dont know why this might be (or even if it is worryingly low: my instinct says yes).
I'm guessing something is shorting out or drawing too much power(?) I've had a search on the forum but cant see anyone else having this issue, is it likely to be the problem? currently I get 3.8v, if I remove the power lines to my breadboard I'm getting 4.0v. Dont know if this is even the cause of the problem, but I'm not sure what to try now: I'd be happy if the board worked all the time, or have something to work with if it failed all the time but this sort of situation (with a tight deadline looming) is pretty stressful.
MarkTheCaz:
Yesterday it was fine, today it's not. The only thing I've noticed is that the 5v coming out of the yun is not 5v: when the rfid works it's about 4.5v, when it's not working it goes down to about 3.7v. Dont know why this might be (or even if it is worryingly low: my instinct says yes).
Yun 5V give ~4.5V is hardware bug. Someone at this board post hardware mod by short diode but it will be void the warranty. What is your RFID reader's model? for testing purpose you could separately power up reader, and only connect ground, Rx, Tx between reader and Yun.
We had an issue using this nfc shield with the yun. Fabio Varesano has a blog post about that
In essence, the shield is made to work with an Uno. Making it work with a Leonardo/Yun requires some hacking