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USB Termination RC Placement

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wuchy143

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Hi All,

I'm working on a USB hub design. It's a 5 port hub. 1 upstream port and the others are all downstream ports.

I'm grappling with where to place the termination resistors for both the upstream and downstream ports. All the literature I have read says to: "Place both the Ct and Rt resistors as close to the driver chip as practicable." I assume they mean the hub in my case. Though, they don't give too much theory as why you would do that. Also, does that apply to both the upsteam and downstream ports? If anyone has some background with this I'd really appreciate it. Thanks
 
It's a transmission line terminated at each end with the characteristic Z. The beginning of the line is the xmitter chip. The end is the receiver. Minimizes reflections.
 
It's a transmission line terminated at each end with the characteristic Z. The beginning of the line is the xmitter chip. The end is the receiver. Minimizes reflections.

thanks for the reply. So just to be clear. In my case where it's a USB hub chip would it be safe to say that for the upstream and downstream USB ports it's ideal to have the RC near the hub chip? I ask because USB is bi-directional and in my head you would want to put the RC on both ends(transmitter and receiver) becuase it depends on the direction of the data.

What am I missing?
 
Rs485 is somewhat similar to USB as in there are two wires. Look here: **broken link removed**

Think of the two wires, the twisted pair transmission line gets terminated at each end. You worry about your end and the device it's connected to worries about that end.

Your terminating the medium of transmission at the ends of the line, not the chip per se. Xmit and receive use the same medium, thus one set of two terminators total. One at each end.

Missing concept: Either the transmitter or receiver is enabled, not both at the same time.
 
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