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I'm familiar with the idea that RX means receive, and TX means transmit. Therefore, if I'm wiring two IC's together, feeding the TX of the sending chip into the RX of the receiving chip makes sense. How does this labeling standard extend to other devices that aren't either the source or termination of the signal? For example, I have a USB connector that has certain pins labeled RX and TX. It seems like the RX pin could mean "receiving from the cable" or "receiving from the device" - so the meaning is not as clear. Obviously I can check the standards for USB to determine the answer to this particular question, but I am curious if there is a more general labeling system that applies in these non-obvious cases. And, is there ever a case where an IC would have a pin labeled RX that isn't a receive input (ie. trying to imply that it is driving a receiver or something)?

zplizzi
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  • Is it a USB 3.0 connector, or USB 2.0? USB 3.0 gains additional speed by having two extra cable pairs, one pair for RX and one for TX (along with the standard D+/D- pair). It would help to know what you "USB Connector" looks like. – Tom Carpenter Jul 01 '15 at 22:48
  • Yeah, should have mentioned that! I was thinking about USB 3.0 with the differential RX/TX pairs when I wrote that, although the question was intended to be more general - although maybe other standards just don't use the RX/TX naming. – zplizzi Jul 02 '15 at 00:41

2 Answers2

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Given that you are talking about USB 3.0, this does use two additional pairs. These pairs are high speed 5Gbps differential pairs. One of them carries data from the host to the device, and the other carries data from the device to the host. These actually get crossed over, much like with RS232/UART - so the RX from one goes to the TX of the other and vice versa. If it is a device itself then these labels would be specific enough to understand. But it can cause confusion when building things from breakout boards where connectors are labelled with no device connected to indicate their reference direction (whose RX is it? ours or theirs!) - in that case you are better of referring to the correct specification or any documentation that came with the board.

The same is used in PCIe devices which have one or more simplex pairs for RX and the same number again for TX - so for example an 8 lane PCIe device actually has 16 pairs, 8 in each direction. Again here the RX and TX are crossed over. These are not always named RX and TX, you can also find them called HSI and HSO ('H'ighspeed 'S'erial 'I'n and HS 'O'ut). You also sometimes see the pairs labelled as +/- or P/N or just one (the inverting one) labelled 'n'.

When it comes to the other signals for USB, the D+/D- pair, these are actually bidirectional, so RX and TX make no sense. Instead you have the positive and negative (inverted) 'D'ata pins. Again here the +/- can be named P/N as well.

Firewire (IEEE1394) is similar to USB 2.0 lines in naming, you have a data pair and a strobe pair.

Usually you wouldn't have a pin labelled RX that transmits. I say usually because some UART type devices unhelpfully label the RX pin as TX and vice versa in which case you would connect RX to RX. But I've only ever seen one device which did that and to be honest it is a stupid thing to do!

Tom Carpenter
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  • Thanks for the detailed response! I guess there isn't a good, clean answer that applies in all situations like I was hoping for :P – zplizzi Jul 03 '15 at 01:41
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USB doesn`t have RX/TX.

http://www.hobbytronics.co.uk/usb-connector-pinout

USB use a D+ and D- line of communication. The labeling depend of the protocol. Protocol aren`t made by the same company. I2C -> Phillips , SPI -> Motorolla, USB -> a regroupment of CO. Therefore, each protocol got their own labeling and logic behind them.

MathieuL
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