RS-232 to TTL Logic Question

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danner123

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Hi everyone,

I am using the MAX3232 (RS-232 Transceiver). I need to test this board. From my tester I have a digital output card which provides +5V (for HIGH) and 0V (for LOW).

Will this chip work if I send in from the RS-232 side +5V for a HIGH and 0V for a low. My VCC=3.3V. I do recall that for a low in the RS-232 protocol it needs to be a negative voltage but maybe this still will work.

Thanks so much for your help!
 

Probably won't work. RS-232 needs +/- 3V minimum. Why don't you just loopback the TXD to RXD?
 
Thanks Barry. That should work!
 

Checking a random datasheet, a voltage below about +0.8 Volts counts as a low, while above 2.4 Volts will be a high (actual switch at about 1.5 Volts).

So while 0 volts and +5 Volts are outside RS232 spec, your MAX3232 may well accept these levels at RS232 input.

Negative voltages would normally be expected at RS232 receiver, but many modern chips will accept 0V as a low.

Check the datasheet for the chip you are using to be sure.

Not a wonderfully good test, but may be good enough for whatever your purpose is.
 

You're looking at the spec for the TTL side, not the RS232 side!!!!

I take it back. You're right. That's a weird, unexpected specification. It kind of defeats the noise immunity of RS-232 with only a 0.3volt deadband.
 
I didn't understand the question, actually.

Take into account that RS-232 takes from -5V to -15V as "1" and from +5V to +15V as "0". Noise immunity is about 10V, at least in the serial port DB-9.

If you put 5V in the TTL input the RS-232 output should be above -5V.
 


Yes it will work. All RS-232 signals although bipolar for noise immunity, the input translator uses 2 PN drops or +1.3V to invert the output, same as all TTL.
 
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