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Full Discussion: Serial Lines Explained
Top Forums UNIX for Dummies Questions & Answers Serial Lines Explained Post 302912179 by edfair on Wednesday 6th of August 2014 11:32:31 AM
Old 08-06-2014
Since most serial communications is via a UART this might help with the data transfer part:
Universal asynchronous receiver/transmitter - Wikipedia, the free encyclopedia

The UART voltage swings are between 0 and 5 volts. This signal is inverted and expanded to higher voltages by line drivers for transmission and inverted again and reduced to 0/5 swings by line receivers on the receiving end.

The transmitted signal starts degrading immediately and at higher transmission speeds it can be expected to be useless beyond 50 feet due to RC effects. (with a scope you would see rounding of what started as sharp shapes of voltage transitions)
 

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URL(4)							   BSD Kernel Interfaces Manual 						    URL(4)

NAME
url -- Realtek RTL8150L USB Ethernet driver SYNOPSIS
url* at uhub? urlphy* at mii? DESCRIPTION
The url driver provides support for USB Ethernet adapters based on the Realtek RTL8150L USB-ether bridge chip. The url driver supports the following adapters: CompUSA USBKR100 GreenHouse GH-USB100B Melco Inc. LUA-KTX Sitecom LN013 For more information on configuring this device, see ifconfig(8). DIAGNOSTICS
url%d: watchdog timeout A packet was queued for transmission and a transmit command was issued, however the device failed to acknowledge the transmission before a timeout expired. url%d: no memory for rx list The driver failed to allocate an mbuf for the receiver ring. SEE ALSO
arp(4), mii(4), netintro(4), usb(4), ifconfig(8) HISTORY
The url device driver first appeared in NetBSD 1.6. AUTHORS
The url driver was written by Shingo WATANABE <nabe@nabechan.org>. BSD
September 18, 2006 BSD
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