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Sorry for the inexperience. This will be quick.

I have two Texas Instrument ICs, one is a device, the other is a hub.

I'm looking at a USB 3.0 device that has the TX and RX lines. When connecting directly to the hub, I should be switching those lines right?

TX (Device) <--> RX (Hub)

RX (Device) <--> TX (Hub)

This breaks down further...

TX+ (Device) <--> RX+ (Hub) TX- (Device) <--> RX- (Hub)

Is this correct? I'm looking at USB connectors to try and get an idea but it seems like the pinouts that are labeled in pictures match up the TX+ and the TX+ on each side and that doesn't make sense!

user2421188
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  • USB has D+ and D- lines, not TX/RX, maybe that is where you are confused? Do you have information on what the IC's are? We can probably come up with a schematic, but recognize that USB 3.0 speeds are very fast and require differential impedance traces and equal length routings. It isn't as simple as RS-232/422/485. – Ron Beyer Jun 22 '17 at 03:22
  • @RonBeyer USB 3 is different. – pericynthion Jun 22 '17 at 04:00

1 Answers1

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You are correct in that high speed transceivers are unidirectional, so in much the same way as UART you must connect transmit at one end to receive at the other.

Typically this crossover is done in the cable. What that means is that on the PCB you would wire the TX of a USB controller to the TX of a USB socket, and the RX of the USB controller to the RX of the socket. You would not cross them over on the board.

However, as you are not using a cable (missed that initially, sorry), they you will need to cross the signals over. The RX of one device must always connected to the TX of the other device and vice versa.

For the D+/D- lines, they shouldn't be swapped at all, simply connect with the corresponding D+/D- on the other end.

TL;DR Yes, you should connect the TX of your hub to the RX of your device.

Tom Carpenter
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  • So in the OP's situation (as I read it) where he has a device & a hub on the same PCB (no USb cable involved), he should wire TX(hub) to RX(device) and vice versa? – brhans Jun 22 '17 at 11:57
  • Thanks brhans, that's just what I was about to ask. @tom thanks for that heads up. I couldn't find this information anywhere! Now, if I have both the device and the hub on the same board - _no wires or cables at all_, then I should switch them accordingly? – user2421188 Jun 22 '17 at 11:59
  • Hi Tom. USB 3 supports automatic polarity inversion, so it is not necessary to connect Rx+ to Tx+. See https://electronics.stackexchange.com/questions/227892/rx-polarity-inversion-in-usb3-x – Peter Smith Jun 22 '17 at 12:36
  • @user2421188 sorry, missed that in your question. See my edit, yes you should connect TX to RX on your board. – Tom Carpenter Jun 22 '17 at 17:20