fischertechnik community forum

Dear Esther,

EstherM hat geschrieben:I am quite impressed, that this file, which has already been on the ftc website for more than three years now attracted so much interest, once it has found a place at the prototype of the new website. There is a clear warning message at the file download page.
I doubt that anybody who is not familiar with electronics will experiment with this circuit.

I share your bewilderment regarding the fuss this topic created. As indicated, I think Till misread vleeuven's original meaning.

However, I neither share your optimism regarding standard disclaimers and their actual impact (is your experience that people read them or take them by heart?), nor regarding possible users of this kind of information. Fact is, we both don't know. So why leave it up to chance? Why not just add a simple sentence of warning to the download description warning about using the pin as a 3,3V current source in other than this specific application, add a link to this thread, and be done with it?

Another part of the disclaimer reads "Falls Du einen Fehler findest, kontaktiere bitte Dirk Wölffel oder uns." In a sense, the OP did just that.

Jan

juh hat geschrieben:Let's call the example (using the TXT pin as 3,3V source for the level shifter) a "hack" and add a proper warning about its limits, and all is well.

Sounds fine to me. I always agreed that there are nicer and more elegant solutions. But there's simply nothing "dangerous" with this one. It's a hack, indeed. But a harmless and working hack. I already told Ether in PM that i might write an ftPedia article featuring a cleaner and less controversial solution.

Hallo...
A small interrupt
(The picture with the TXT and the I2C)

OK, i do not read all the I2C dokuments, but
-are at the +3.3V from the TXT the UART with 3.3V? (As an normal digital high Output to get the 3.3V?)
-are the GND all the same Ground? Or must put it through all peaces. Or i it internal connectet?

Ill try to get two of the Converter. If had never something to do with a mix of 3.3V and 5V at the I2C.
Best reagard
fishfriend
Holger Howey

Dear Jan,

juh hat geschrieben:Dear Esther,
Why not just add a simple sentence of warning to the download description warning about using the pin as a 3,3V current source in other than this specific application, add a link to this thread, and be done with it?

If you (or anybody else who is sufficiently familiar with the subject), provides me with a short description about possible isssues with this files, I will be happy to add it to the download page.
I will certainly not add a link to this thread, because the discussion here was (in my eyes) too unfriendly and too confusing to be remembered for any time longer.

Deutsch: Ich füge gerne eine kurze Beschreibung der möglichen Probleme mit diesem Schaltplan in die betreffende Seite ein, wenn mir ein Experte für das Thema eine liefert.
Auf den Thread möchte ich nicht verlinken, er war (aus meiner Sicht) im Ton unfreundlich und in der Sache verworren, so dass er hoffentlich irgendwann in Vergessenheit gerät.

Gruß / Cheers
Esther

fishfriend hat geschrieben: -are at the +3.3V from the TXT the UART with 3.3V? (As an normal digital high Output to get the 3.3V?)
-are the GND all the same Ground? Or must put it through all peaces. Or i it internal connectet?

Yes, the 3.3v come from the txt's uart transmit (tx) output. And yes, it is 3.3v since this is the digital high level which is preeent on the uart tx as long as no data is being sent there. This in turn does not happen since this is the output of the linux console which is waiting for a user to log in which never happens in this case as nothing is connected to the uarts input (rx) on the ext connector.

And yes, all GNDs are the same as they are internally connected inside the 9v regulator at the bottom as well as the level shifter itself.

If you plan to build this device i am sure it'll work exactly as depicted in the image. However, it would be a little cleaner design if you a) connect all GNDs as well and b) if you apply 5v to the supply of the 5v side of the level shifter. The latter will provide power to the pullup resistors on the 5v side which are currently left floating. This still works since the target device (the sensor) also comes with pullups which are properly powered by the sensor itself.

Funny side note: Did you know that three of the other pins on the ext port are a USB port? I never tried to use that as i am not even sure if it's a host or target port and if it's part of the linux controller or the motor controller of the txt. But i'll give it a try one day ... or maybe one of you ...

Hi Holger,

to avoid all these discussions regarding I2C voltage levels and level shifters we offered the plug play community project ft-Extender.
Easy to use with the build in level shifter and support of power supply for the external I2C devices.

Regards
Björn

Funny side note: Did you know that three of the other pins on the ext port are a USB port? I never tried to use that as i am not even sure if it's a host or target port and if it's part of the linux controller or the motor controller of the txt. But i'll give it a try one day ... or maybe one of you ...

Auf Rei Vilo´s Seite sind einige Ext.-Pins des TXT mit Rx und Tx bezeichnet. Wären ja Anschlüsse für einen seriellen Port mit getrennten
Sende- und Empfangs- Leitungen auf GND bezogen. Sind doch eigentlich asymmetrisch Signale.
USB Pins sind doch mit D+ und D- bezeichnet, eigentlich so mehr Richtung RS 485 - CAN usw. Dürften doch symmetrische Signale sein.
Mißverstehe ich etwas ?

Karl hat geschrieben: Auf Rei Vilo´s Seite sind einige Ext.-Pins des TXT mit Rx und Tx bezeichnet. Wären ja Anschlüsse für einen seriellen Port mit getrennten
Sende- und Empfangs- Leitungen auf GND bezogen. Sind doch eigentlich asymmetrisch Signale.
USB Pins sind doch mit D+ und D- bezeichnet, eigentlich so mehr Richtung RS 485 - CAN usw. Dürften doch symmetrische Signale sein.
Mißverstehe ich etwas ?

- RX und TX sind normale 3,3V-UART-Signale. Dort liegt die serielle Linux-Konsole des TXT. Das haben wir vor jahren mal benötigt, um den Bootloader des TXT in den Griff zu bekommen. Das sind keine USB-Signale. Will man USB nutzen muss man dort einen Adapter anschließen / RX and TX are regular 3.3V UART signals which we once used to access the TXTs bootloader. This needs an adaptor to be used with USB

- Die USB-Signale liegen auf anderen normalerweise nicht beschrifteten Pins. Elektrisch arbeitet USB mit 3,3V-Pegeln auf den Datenleitungen (was interessant ist, denn die USB-Versorgunsgspannung beträgt 5V). Auch USB-Signale sind relativ zu GND, allerdings sind es in der Tat differenzielle Signale und der Logikpegel auf D+ und D- ist normalerweise immer genau gegenteilig / the USB signals are on different usually unlabeled pins. The work electrically with 3.3V (despite the fact that USB power is 5V). The USB signals are also relative to GND but they are differential and usually carry the opposite logic levels

Guys,

please

a) stay to the topic (I smell thread piracy here....)
b) write in ENGLISH!
c) let's agree (that's also my opinion) that the use of the TX pin as a power supply is a rather dirty hack since the serial port, meant for firmware debugging, may engage anytime and thus suddently the 3.3V may become instable and I2C communication will collapse.
d) a permanent hardware damage is unlikely, as long as there is no external power source connected to any of the TXT extension pins, nevertheless a clean setup (common ground, but separate 3.3V supply for the I2C bus participants) is definitely the preferable solution. Of course you might use the radiator pipes in your home to also supply 230V, but ... I wouldn't recommend it (except for darwinistic reasons...)


Kind regards
Peter

Thanks Peter, time to close this subject