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Motorola's circuit doesn't work.

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Have a look.

I was trying to use it with my 8051 microprocessor.

If I replace 100K with a short, the circuit 1/2 works (receives correct data) and 1/2 the time, it shuts off, turns back on, and then resets.

Would capacitors help?

I already have everything on a PCB and do not wish to change the layout.


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Hi Mik3ca,
The circuit should work fine with +12V, -12V RS-232 signals if they aren't too fast.

With the input at +12V then 11.3V will be across the 100k resistor resulting in a base current of 113uA. The datasheet of the 2N3904 shows that it saturates very well with its collector current 10 times its base current. Its collector current in the 4.7k resistor is only 1.1mA max so it will saturate well.

When the input is -12V then the transistor is turned off and the diode clamps its reversed emitter-base voltage to less than its breakdown voltage of 6.0V.

It will probably also work well if the RS-232 signal is from only +5V to only -5V.

Since you replaced the 100k current-limiting resistor with a short, you might have blown the RS-232 port or the base-emitter junction of the transistor.

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It seems that I only get ANY result if the 100K is replaced with a lower value resistor (330 ohms and under).

As for blowing the transistor, I don't think I broke it.

For the processor, I don't think I broke it either.

No matter what I do, the best I get is that 1/2 the data is received correctly, and sometimes the system resets itself and displays screwed-up results.

Maybe I'll try 1K, but I don't know if it will work.

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Processor? Didn't you connect the Motorola transistor circuit to an RS-232 signal??
The RS-232 signal doesn't come from the processor. It comes from the pc or from a modem at a dual-polarity fairly high voltage and fairly high current signal.

The microprocessor circuit you showed in your other thread uses a MAX-232 IC to make a +8V, -8V RS-232 signal that would feed Motorola's transistor circuit perfectly.

You shorted the RS-232 signal by feeding it directly to the base of the transistor and to the diode. So the RS-232 output IC and the transistor might both be broken.
Maybe the transistor doesn't work because its collector and emitter pins are backwards. Check its datasheet.

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Your circuit is a standard level converter, but the resistor values are high for connection to micro. Try 10K instead of 100K and 3K3 instead of 4K7. Also, the PC should be connected to the 10K and the micro should be connected to the pin marked TTL. Note that this is a one way communication from the PC to the micro.


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