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Design issue Serial Comm. Isolator with Level shifting

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geobabu

Member
Hi,
I am trying to implement a isolation in between a serial communication system. One side its have a RS232-TTL(MAX232 circuit) level terminal & other side contains a 3.3V based microcontroller whose power is directly driven from a transformer less capacitor based power supply. I tried a setup a circuit that mentioned in the attached file ,but it doesn't helped helped me. Please help me to rectify the error in this circuit.
Thanks&Regards,
Geo
 

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creakndale

New Member
It looks to me like it should work.
You do know on the 16-pin MAX232 IC that, even though it's not shown on the schematic, you need to:
1) Connect Pin 16 to VCC (+5v)
2) Connect Pin 15 to GND

Also, the striped part of the capacitor symbols are the minus leads of the 10uF electrolytics.

creakndale
 

geobabu

Member
Yes the The VCC & GND are connected in the actual circuit. Here in some schematic editors it wont show, but the net will connect internally.

Whether this BC547 is a good choice for the 3.3V logic levels and baud rates like 57600bps

Thanks & Regards,
Geo
 

ericgibbs

Well-Known Member
Most Helpful Member
Yes the The VCC & GND are connected in the actual circuit. Here in some schematic editors it wont show, but the net will connect internally.

Whether this BC547 is a good choice for the 3.3V logic levels and baud rates like 57600bps

Thanks & Regards,
Geo
hi,
On simulation, its the 4N35 thats the problem at 57Kbaud, not the BC547.

If you look at the 4N35 spec, the 'tr' time is given for 10V is 3us typ, 10us max.[your Vs is only 3.3V]
At 57000bps you have a 17us, high/low period.

I would suggest you do a bench test on a 4N35/BC547 using a square wave signal generator and a scope. [remove the 1K in the BC547 base lead]
 

geobabu

Member
Hi ..thanks for the reply...

One thing is , when I placed a CRO(for testing I used an isolated 3.3V supply) prob across the C-E pin of 4N35 in the Rx part (Also in RXD line), I can see some Sq.waves according to the Key press in the other terminal. This made me more confused.
So we need to replace the 4N35.? Whether MCT2E is suggested to do the job. Its tr is [email protected]

thanks & Regards,
Geo
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi ..thanks for the reply...

One thing is , when I placed a CRO(for testing I used an isolated 3.3V supply) prob across the C-E pin of 4N35 in the Rx part (Also in RXD line), I can see some Sq.waves according to the Key press in the other terminal. This made me more confused.
So we need to replace the 4N35.? Whether MCT2E is suggested to do the job. Its tr is [email protected]

thanks & Regards,
Geo
hi,
I see that Chris has posted some circuits, what baud rates were these running at Chris.?

I see that Geo is running at 57000Baud.
 

chris54

New Member
If My memory is correct It worked well up to 57k baud but the radio I had would only talk to it at 4800 if I rember right. I did use it on another radio for the interface and it worked. I will verify this and let you know I need to look at the radio settings.

I did build an interface using the 4n35 and had problems with them. For some reason as I rember I just could not get them to talk. That is when I used this circuit.
 

ericgibbs

Well-Known Member
Most Helpful Member
If My memory is correct It worked well up to 57k baud but the radio I had would only talk to it at 4800 if I rember right. I did use it on another radio for the interface and it worked. I will verify this and let you know I need to look at the radio settings.

I did build an interface using the 4n35 and had problems with them. For some reason as I rember I just could not get them to talk. That is when I used this circuit.
hi.
If Geo has more problems I'll modify one of my project boards and give it try out.

Using the LTSpice with the circuit he posted gave a very poor performance for the 4N35 at 57000.
It would be wise to check it in the real world.:)

EDIT: the sim worked OK at 9600baud.
 
Last edited:

geobabu

Member
hi,

Thank you Chris for the schematic, I can refer this circuit.

Thank you. ..ericgibbs. :) ...it will be very helpful.. ..

I have a doubt, why the CRO showing the sq. wave according to the key press, even in this 57.6K baud.

So I think its better to change the 4N35 itself. Ericgibbs, how about the MCT2e.
Thanks & Regards,
Geo
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,

Thank you Chris for the schematic, I can refer this circuit.

Thank you. ..ericgibbs. :) ...it will be very helpful.. ..

I have a doubt, why the CRO showing the sq. wave according to the key press, even in this 57.6K baud.

So I think its better to change the 4N35 itself. Ericgibbs, how about the MCT2e.
Thanks & Regards,
Geo
hi,
I use the 6N137 as Ubergeek as posted.:)
 

ericgibbs

Well-Known Member
Most Helpful Member
Can some one tell me the fomula to convert us to baud rate. Thanks
In a binary channel the baud rate is equal to the bit rate, therefore 1 baud = 1 bit a second. [bps]
 
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