rs232 isolation

[dr pepper]

I need to have 2 micro's communicating with each other, but I need isolation, only around 50v or so.
I thought about using a couple of opto's but wondered if theres a practical way with a transformer, I thought about using a torroid with equal turns pri and sec, so long as the v/s doesnt saturarte the core this might work, and with 4 windings I could easily do half duplex from pic to pic, or by using another io pin maybe just 2 windings.
Anyone done this, or should I forget it and go get a couple of opto's

 

[Mr RB]

Opto's would be the way to go, they can easily be set up for good digital operation (ON and OFF setpoints). You need an opto good for the baudrate you like, from memory anything over 19200 baud can get problematic and starts to need "high speed" optos.

 

[ronsimpson]

There are 'digital' opto's that take TTL in/out. Very easy.

 

[be80be]

Here a link if you want it https://www.avagotech.com/pages/en/optocouplers_plastic/plastic_digital_optocoupler/

 

[dr pepper]

Thanks burt, sounds like the ACPL-021L is the way to go, a logic gate with an opto inbetween, too easy.

 

[bryan]

Have built this one before and it works.

**broken link removed**

 

[dr pepper]

Vnice and with standard opto's

 

[Jon Wilder]

Being someone who works with MIDI (baud rate of 31250), I recommend the 6N138 high speed opto. Rise time < 2uS. Just need a 1K-10K resistor from pin 7 to ground, ground pin 5, 470R pull up resistor on pin 6 and tie pin 8 directly to Vdd. Internal LED is on pins 2 and 3. Simple hook up and they work great.

**broken link removed**

 

[dr pepper]

Thanks I'll use that, I've made leads up for midi many times but never messed with opto's for it.

220r is either driving the led pretty hard, or vf for opto's is higher than theusual 1.8v.

 

[dr pepper]

Picture didnt work.

Ref the acpl021l I have to wait 2 weeks for one of those, 6n138's are in my parts box.

 

[Jon Wilder]

+V from TX is 5V. Typically you have a 2nd 220R pull up in the transmitter and a 220R in line with the data line from the transmitter, amounting to 660R total loop resistance.

Vf for 6N138 is 1.3V I believe. Total loop current ends up around 5.6mA -

3.7V / 660R = 5.6mA

I use that exact circuit for MIDI In all the time and it works just fine.

 

[dr pepper]

5.6ma does sound right, I didnt know about all the other resistances.

 

[Jon Wilder]

Yeah perhaps I should've explained a bit more in detail.

Basically you set up a current loop. On the TX device, you'll need a 220R pull up resistor to connect to the point that says "V+ FROM TX". Then, you connect a second 220R in series between TxD and the point that says "DATA IN".