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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.

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    \$\begingroup\$ You are very likely to break your microcontroller if you do this. So, if you're trying to get the magic smoke out of your micro, it's perfectly okay, but if you want a working micro afterward, no, it isn't. \$\endgroup\$ – Hearth Apr 25 '17 at 2:43
  • \$\begingroup\$ @Felthry wouldn't that be MS232... \$\endgroup\$ – Trevor_G Apr 25 '17 at 2:49
  • \$\begingroup\$ The signal polarity is generally inverted, even the components were somehow not damaged the data would generally not be transferred correctly. \$\endgroup\$ – KalleMP Apr 25 '17 at 4:33
  • \$\begingroup\$ I know we're not supposed to answer the OP in comments, but not only no, but God no. It would never be the same. Anyway, there are great answers to this one. Upvote. And +1 for @Felthry for the magic smoke. \$\endgroup\$ – SDsolar Apr 25 '17 at 7:54
  • \$\begingroup\$ For the record, the signal is +/- 15 Volts for RS-232 data pins. You need a GND for reference. And a converter chip, lest bad things happen to TTL circuits of any normal voltage range. \$\endgroup\$ – SDsolar Apr 25 '17 at 7:56
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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.

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  • \$\begingroup\$ Basically sound thinking, with a caveat: a "TTL" interface usually implies 5V and can cause trouble with a 3.3V GPIO. \$\endgroup\$ – rackandboneman Apr 25 '17 at 11:12
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    \$\begingroup\$ The radio module is powered from 3.3V, so its "TTL" capability is unlikely to exceed that voltage level. \$\endgroup\$ – Whit3rd Apr 25 '17 at 11:17
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Can I directly connect GPIO to RS232 RXD and TXD? 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?

Simply, no.

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?

A transceiver IC (e.g. MAX232) is needed.

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I would suggest using a transceiver IC such is ADM3252. Below is a high level schematic of the IC

ADM3252E

ADM325x support +/- 30V on Rin. Below are the date sheet for you reference.

Reference:

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