PiPhin
Posts: 9
Joined: Wed Apr 09, 2014 2:48 am

RS232 with 3.3V?

Wed Apr 09, 2014 3:04 am

Hi there,

I need to connect my Raspberry Pi to a Viessmann heater, it uses an Optolink-interface (using 2 LEDs for sending and receiving IR-light for data-transmission) which is RS232 as far as I understand it. Is there any prob to drive the LEDs directly through the GPIO pins? I use this circuit:

http://openv.wikispaces.com/Bauanleitung+LAN-Ethernet

I don't use the 5V regulator part though, obviously. :)

Can I use the generic (documented) way to open the interface in Raspian?

klricks
Posts: 6503
Joined: Sat Jan 12, 2013 3:01 am
Location: Grants Pass, OR, USA
Contact: Website

Re: RS232 with 3.3V?

Wed Apr 09, 2014 1:25 pm

If the IN and OUT in the circuit were designed to connect to true RS232 signals then you must use a MAX3232 chip between the RPi GPIO and the circuit. The power pin on the MAX3232 would need to be connected to 3V3.

If the circuit was designed for TTL level serial then it would probably work direct connected. Note that the RPi uses 3V3 LVTTL signals so some adjustments may need to be done if the circuit is expecting 5V TTL serial.

Maybe someone better at electronics could look at the circuit and know what the intended voltage levels are?

The circuit does not have a 5V regulator it shows a 3V3 regulator which you would connect to the 5V GPIO pin. If the circuit, and everything else connected to GPIO, draws less than 50mA then you could connect the power for the circuit to the 3V3 GPIO and not need the regulator at all. Note that the IR LED draws 30mA by itself.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

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