Serial Programming - upload.wikimedia....

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in 4B5B coding, 8b/10b encoding; etc. Other systems, such as 9-bit protocols, 891011121314
transmit 9 bit symbols. Typically the first 9-bit symbol of a packet has its high bit set to 1,
waking up all nodes; then each node checks the destination address of the packet, and all
nodes other than the addressed node go back to sleep. The rest of the data in the packet
(and the ACK response) is transmitted as 9 bit symbols with the high bit cleared to 0,
effectively 8 bit values, which is ignored by the sleeping nodes. (This is similar to the way
that all data bytes in a MIDI message are effectively 7 bit values; the high bit is set only on
the first byte in a MIDI message). Alas, some UARTs make it awkward, 1516 difficult, or
impossible to send and receive such 9-bit characters.
Some communication protocols break "unique start" -- i.e., they allow the no-longer-unique
start-of-packet symbol to occur elsewhere -- most often because we are sending a file that
includes that byte, and "simple copy" puts that byte in the data payload. When a receiver is
first turned on, or when cables are unplugged and later reconnected, or when noise corrupts
what was intended to be the real start-of-packet symbol, the receiver will incorrectly interpret
that data as the start-of-packet. Even though the receiver usually recognizes that something
is wrong (checksum failure), a single such noise glitch may lead to a cascade of many lost
packets, as the receiver goes back and forth between (incorrectly) interpreting that data byte
in the payload as a start-of-packet, and then (incorrectly) interpreting a real start-of-packet
symbol as payload data.
In order to keep the "unique start" feature, many communication protocols break "simple
copy". This requires a little extra software and a little more time per packet than simply
copying the data -- which is usually insignificant with modern processors. The awkwardness
comes from (a) making sure that the entire process -- the transmitter encoding/escaping
a chunk of raw data into a packet payload that must not include the start-of-packet byte,
and the receiver decoding/unescaping the packet payload into a chunk of raw data -- is
completely transparent to any possible sequence of raw data bytes, even if those bytes
include one or more start-of-packet bytes, and (b) since the encoded/escaped payload data
inevitably requires more bytes than the raw data, we must make sure we don't overflow
any buffers even with the worst possible expansion, and (c) unlike "simple copy" where a
constant bitrate of payload data bits results in the same constant goodput of raw data bits,
we must make sure that the system is designed to handle the variations in payload data
8 uLan ˆ{http://ulan.sourceforge.net/} : 9-bit message oriented communication protocol, which is
transferred over RS-485 link.
9 Pavel Pisa. "uLan RS-485 Communication Driver" ˆ{http://cmp.felk.cvut.cz/~pisa/ulan/ul_drv.
html} "9-bit message oriented communication protocol, which is transferred over RS-485 link."
10 Peter Gasparik. "9-bit data transfer format" ˆ{http://www.rtjcom.com/6811/jackpot/
rs485-commspec.html#3}
11 Stephen Byron Cooper. "9-Bit Serial Protocol" ˆ{http://www.ehow.com/facts_7735117_
9bit-serial-protocol.html} .
12 "Use The PC's UART With 9-Bit Protocols" ˆ{http://electronicdesign.com/article/embedded/
use-the-pc-s-uart-with-9-bit-protocols6245.aspx} . 1998.
13 Wikipedia: multidrop bus ˆ{http://en.wikipedia.org/wiki/%20multidrop%20bus} (MDB) is a 9-bit
protocol used in many vending machines.
14 ParitySwitch_9BitProtocols ˆ{http://www.docklight.de/examples_en.htm} : manipulate parity to
emulate a 9 bit protocol
15 "Use The PC's UART With 9-Bit Protocols" ˆ{http://electronicdesign.com/article/embedded/
use-the-pc-s-uart-with-9-bit-protocols6245.aspx} . Electronic Design. 1998-December.
16 Thomas Lochmatter. "Linux and MARK/SPACE Parity" ˆ{http://www.lothosoft.ch/thomas/
libmip/markspaceparity.php} . 2010.
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