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I accidentally swapped the TX and RX pins on a UART bus when hooking up a to a 3.3V bus with this 3.3V cable: https://www.sparkfun.com/products/12977.

Did I cause any permanent damage to this board as I debug it?

Can we all agree to use TXO, RXI?

tarabyte
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    Happens all the time. Normally no permanent damage occurs. In fact I have never heard of TX or RX lines getting damaged due to cross-connection. I think for the schematic symbol, TXO and RXI are good names. But the nets in the schematic should be called HOST_TO_DEVICE and DEVICE_TO_HOST or some such (you can substitute some other identifier for DEVICE and HOST). – user57037 Oct 11 '16 at 06:06
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    @tarabyte: You've also swapped the 's' and 'x' on a UART bus. See your title. – Transistor Oct 11 '16 at 23:54

2 Answers2

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It's quite unlikely that there'll have been any permanent damage. Typical LVCMOS output stages can survive contention with each other - they will source/sink 20 mA or so, which is usually below their damage limit. But the details do depend on the ICs involved, you could check the datasheets if you're thinking of building these particular parts into a high reliability system and are concerned about latent failures.

pericynthion
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  • Also it's only a signle pin and both sides idle at the same level. This means that the heat geneated will be far less then if say a pair of large busses was in contention. – Peter Green Oct 13 '16 at 00:38
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You cannot damage them, in fact the #1 test if you don't have data is to switch the TX/RX.

Din and Dout are also a standard but TX means transmit and RX means Receive so it's pretty clear what those mean. The problem is no matter what you call them it will always be easy to get confused due to which device you are looking at since the opposite lines always go together.

EDIT: My apologies for not being more thorough in my explanation. The only situation where there will be contention is TX to TX as RX to RX would be two inputs. First consider that the output will never be a constant HIGH signal as the output buffer is transmitting a structured packet. The duty cycle is reduced even further when connecting two TX pins VS one TX pin to Vcc or Gnd. Swapping the pins on any serial bus has been a problem from the beginning and you will find all serial protocols provide some level of fault tolerance. Take a look at the specifications for RS-485 as an example.

The exact electrical specifications for the drive current of a TX pin varies between device manufacturers. Take a look at the datasheet for the TI TL16C752D. The maximum output current per pin is .5mA HIGH and 1mA LOW. The PL2303 has a maximum drive current of 4-8mA. The FT232 says 24mA but I suspect that is for the GPIO pins only. Some TX pins are open collector outputs relying on an internal pull up resistor and most provide clamping diodes to protect the pins that can handle well over 10mA.

Neelix
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