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By dngsparkfun
#196348
Will the SparkFun RS232 Shifter (PRT-00449) drive negative voltage at the RS232 output?
(I am talking about the TTL/RS232 device.)

I had assumed so, but I do not understand the circuit, and I just realized I have not noticed anything explicitly saying it will.

It would appear to receive and convert RS232 negative voltage to TTL High, but does send TTL High as RS232 negative voltage?

Also, if the unit is powered at say 5V, what voltage swing can one expect at the RS232 output?

Thanks.
By jremington
#196358
Yes, that circuit will produce a negative voltage on the RS232 output line.

The circuit uses a trick -- the TX output of RS232 port in the resting state is negative, which puts a negative voltage on C4 (which is why it is "upside down", and why the diode is necessary). The actual voltage depends on the characteristics of the RS232 serial port, so measure the voltage on TX during idle.

This trick won't necessarily work for full duplex transmissions. In that case, use a MAX232 converter to provide true RS232 +/- driving voltages. See https://www.sparkfun.com/tutorials/215