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I have a device, namely a Synology DiskStation. It has a 3.3V RS232 interface on board. All I have is a 5V RS232 to USB adapter. Can I attach it to my DiskStation without making any smoke?

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up vote 5 down vote accepted

I'm not an electronics expert but everything I've ever read about attaching serial ports to boards that have 3.3v UART pads says to use a TTL converter, or convert the voltage somehow. So I believe you really need something to convert to the proper voltage.

You might get away with it if you only use it to receive serial data from the UART, but I wouldn't chance it.

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The RS232 standard specifies +/- 3-15V

If any device is advertised as RS232 and emits smoke when presented with a valid 15V RS232 signal, I'd say it is suffering from a manufacturing defect which the manufacturer should rectify at no cost to you.

"RS-232 drivers and receivers must be able to withstand indefinite short circuit to ground or to any voltage level up to ±25 volts." - Wikipedia

You could always ask the manufacturer?


The serial port on a Synology NAS is not RS232.

"The serial port on Synology NAS boxes is using 3.3V TTL levels, which have to be converted into regular RS232 levels by a level shifter circuit." - NetBSD

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RS232 is specified +- 3-15V, but actually this is only true for computers. Things like embedded devices often have levels of +- 3.3V and +-5V. – Max Ried Jun 14 '11 at 14:23
If it doesn't produce at least +/-5V it isn't RS232. If it doesn't accept +/-25V it isn't RS232. This applies to embedded devices, not just to computers. – RedGrittyBrick Jun 14 '11 at 14:33
Paper doesn't blush. This might be the standard, but in real world, most current devices that have a "RS232" interface, speak "RS232" logically, but downscaled +-3.3V or +-5V. So it might no be entirely technically correct to speak of RS232, but it's common speak. – Max Ried Jun 14 '11 at 14:48
@bot47: Yes I accept what you say. However if a consumer device presents a DB-25, DE-9 or even modular-socket and it's manual says you can connect RS232, it is reasonable to expect that this can be connected to a +/-15V RS232 device. If (as may be the case here) it has a 6-pin header on a circuit board not normally accessible to end users, it is not a safe assumption. In the latter case the question arguably belongs on – RedGrittyBrick Jun 14 '11 at 16:10

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