RS232 Isolated Interface Circuit Assistance

[turbo22]

I have designed an isolated RS232 interface for a PIC16F877 circuit. Isolation is necessary since my circuit is powered from a direct mains regulator (no power transformer). I'm using a MAX232 converter and a couple of 4N35 optoisolators for the TX/RX paths. At 19200 baud, I'm starting to get garbage characters at the PC end. Has anyone successfully build an isolated interface that will handle this (or higher) baud rates? What would be the fastest optocouplers to use in this situation? I would like to get 115k baud but that's probably just wishful thinking. Thanks.

Bill Stottlemyer

stottlemyerb@decorp.com

 

[cbarn24050]

Hi, any optocoupler will work at these speeds, recheck your circuit.

 

[Lewish]

Hello turbo, I have done 2 megabaud with HP optos.

 

[stefandk]

Place resistor E-B on optotransistor.

 

[ooge]

Stefandk's suggestion of the E-B resistor has worked for me, but a better option would be to use an optocoupled logic gate (H11L1 or similar).
These work at much higher speeds than opto transistors. 115200 baud should be no problem.

 

[yesbee]

Try with HP2630 optos speed 10 Mbaud) it will work.

 

[OptoMark]

A single transistor coupler will work at those speeds if the circuit is tweeked for that coupler. If margin is added for temp and age, then the single transistor couplers drop to about 10-15kB.

You can switch to a high speed analog (PIN diode) type coupler like the NEC PS8101, that will get you up to 1Mb. The full digital couplers are overkill for what you want.