gpioradiors232serial
Is it okay to connect a 3.3V GPIO pin to the RXD and TXD pins of the RS232 port and supply it with 12VDC from an external source for serial radio? Or would I need to step up the voltage to 12VDC on the RXD pin to the RS232 and step down the voltage respectively for the TXD pin?
I'm using n920 – OEM 900 MHz Spread Spectrum Wireless Modem serial radio.
It will depend of the software that you're using.
A) In my case, as I develop my own PC software, I've never used other pins, only RxD, TxD and GND. So, in this case you can left other pins open. This is called as null modem without handshaking
B) Yes (assuming GND is being considered)
C) Again, it will depend of the software on PC side. Some softwares you can configure to not take care of hardware control. Others will require RTS/CTS handshaking, so you can do a null modem with loop back handshaking
Source of the pictures: RS232 serial null modem cable wiring
It's possible that the device is powered through the clamping diodes on the TxD input, since that will be higher than Vcc if you haven't that connected.
Doing so is very naughty, the diodes are meant as overvoltage protection, not for powering the device. It's not because something works that it's wise to use it that way.
edit re your comment
You hadn't said anything about the USB connection! In that case the 3.3 V is not unconnected: it's the output of an on-board LDO voltage regulator, which also powers the UART converter. Chances are that it's an FTDI converter, probably an FT232R. In that case the 3.3 V regulator is on the FT232R IC.
Up to 50mA can be drawn from this pin to power external logic if required. (p. 8)
Which is OK for a small microcontroller circuit, but not for the Rpi, which needs up to 700 mA.
Best Answer
The specs on the N920 radio say that the serial interface is TTL, it is NOT RS232. There is no purpose in giving either the microprocessor or the radio a transmit/receive RS232 interface, unless they are to be separated by a large distance. The serial I/O terminals on the N920 radio can be directly connected to GPIO pins.
Unless other hardware needs it, there is also no requirement for 12V.