Siemens TC35 - Arduino - differing logic levels

Hi All,

Been getting some excellent advice in the programming section but another issue has arisen, it appears I have been 5v bit-banging the TC35 with the arduino and the TC35 states a max logic voltage of 3.3v. hopefully I have not caused any damage, it has only been used a couple of times briefly to test and has, and still is, working ok.

To drop the level from 5v to 3.3v on the arduino Tx line I am going to use voltage dividing with 10k/20k resistors and put a diode on the Rx line to stop and spikes with the arduino taking it high on startup etc.

My electronics knowledge is limited so any advice gratefully received? I do realise that this may not be the most protective method but I can live with that if you think it would work?

Thanks

Craig

1K - 2K2 might be a slightly better choice for one way communication 10/20K would work but might well be flakey at high data rates because of the higher resistance, drawing a little more current is better. and I could spend an hour "explaining" it... but I won't Just do it... It'll work fine.

Bob

Docedison:
1K - 2K2 might be a slightly better choice for one way communication 10/20K would work but might well be flakey at high data rates because of the higher resistance, drawing a little more current is better. and I could spend an hour “explaining” it… but I won’t Just do it… It’ll work fine.

Perfect Bob, I calculate that would give around 2.7v. Do you think a diode on the Rx is necessary?

Thanks

Craig

hey guys, I initially hooked up my TC35 developer board to my mega without much thinking and it worked fine (meaning 5V). Only after some reading on other issues I had I realized the voltage difference issue. I tried with my 3.3V Mini Pro and communcation resumed just fine.

Now, as the receive side on the mega with 5V was reading the signal state fine, I didn't see an issue there (actually, I am not sure if the hardware serial uses / activates the internal pull-up resistor or not ...?).

Anyway, I saw - as many suggested - that a voltage divider on the Arduino TX pin should kind of solve the issue. I stumbled across a 1.2 and a 2.2 k Ohm resistor in my supplies box and implementing the same and the communication was working fine!

Not sure about diodes in the send/receive circuit. They create (typically) a 0.7 voltage drop and I am not certain about the response at higher frequencies ... (ps.: I am an electrical engineer but have not dealt with the need for such circuitry ...).

Anyway, does anyone know if the mega hardware serial 2 RX pin (in my case) uses the pull-up resistor or not?

Cheers & thanks, Robert