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that we understand the hardware part of the picture, let's dive right
into the software part. We'll take a look at each part of the puzzle
by defining a few of the common terms. Ever wondered what phrases
like 9600-8-N-1 meant?
Do you use software-handshaking or hardware-handshaking at formal parties
for a greeting? If you're not sure, read on!
is a human-readable to computer-readable translation code. (i.e.
each letter/number is translated to 1's and 0's) It's a 7-bit (a
bit is a 1 or a 0) code, so we can translate 128 characters. (2^7
is 128) Character sets that use the 8th bit do exist but they are
not true ASCII. Below is an ASCII chart showing its "human-readable"
representation. We typically refer to the characters by using hexadecimal
terminology. "0" is 30h, "5" is 35h, "E"
is 45h, etc. (the "h" simple means hexadecimal)
||most significant bits
bit- In RS-232 the first thing we send is called a start bit.
This start bit ("invented" during WW1 by Kleinschmidt)
is a synchronizing bit added just before each character we are sending.
This is considered a SPACE or negative voltage or a 0.
bit- The last thing we send is called a stop bit. This stop
bit tells us that the last character was just sent. Think of it
as an end-of -character bit. This is considered a MARK or positive
voltage or a 1. The start and stop bits are commonly called framing
bits because they surround the character we are sending.
bit- Since most plcs/external equipment are byte-oriented (8
bits=1byte) it seems natural to handle data as a byte. Although
ASCII is a 7-bit code it is rarely transmitted that way. Typically,
the 8th bit is used as a parity bit for error checking. This method
of error checking gets its name from the math idea of parity. (remember
the odd-even property of integers? I didn't think so.) In
simple terms, parity means that all characters will either have
an odd number of 1's or an even number of 1's.
Common forms of parity are None,
Even, and Odd. (Mark and Space aren't very common so I won't
discuss them). Consider these examples:
send "E" (45h or 1000101b(inary))
In parity of None, the parity bit is
always 0 so we send 10001010.
In parity of Even we must have
an Even number of 1's in our total character so the original character
currently has 3 1's (1000101) therefore our
parity bit we will add must be a 1. (10001011) Now we have
an even number of 1's.
In Odd parity we need an odd number
of 1's. Since our original character already has an odd number of
1's (3 is an odd number, right?) our parity bit will be a 0. (10001010)
During transmission, the sender calculates the parity bit and sends
it. The receiver calculates parity for the 7-bit character and compares
the result to the parity bit received. If the calculated and real
parity bits don't match, an error occurred an we act appropriately.
It's strange that this parity method is so popular. The reason is
because it's only effective half the time. That is, parity checking
can only find errors that effect an odd number of bits. If the error
affected 2 or 4 or 6 bits the method is useless. Typically, errors
are caused by noise which comes in bursts and rarely effects 1 bit.
Block redundancy checks are used in other
communication methods to prevent this.
rate- I'll perpetuate the incorrect
meaning since it's most commonly used incorrectly. Think of baud
rate as referring to the number of bits per second that are being
transmitted. So 1200 means 1200 bits per second are being sent and
9600 means 9600 bits are being transmitted every second. Common
values (speeds) are 1200, 2400, 4800, 9600, 19200, and 38400.
data format- (baud rate-data bits-parity-stop bits) This is
the way the data format is typically specified. For example, 9600-8-N-1
means a baud rate of 9600, 8 data bits, parity of None, and 1 stop
picture below shows how data leaves the serial port for the character
"E" (45h 100 0101b) and Even parity.
important thing that is sometimes used is called software handshaking
Like the hardware handshaking we saw in the previous chapter, software
handshaking is used to make sure both devices are ready to send/receive
data. The most popular "character flow control" is called
XON/XOFF. It's very simple to understand. Simply put, the receiver sends
the XOFF character when it wants the transmitter to pause sending data.
When it's ready to receive data again, it sends the transmitter the
XON character. XOFF is sometimes referred to as the holdoff character
and XON as the release character.
last thing we should know about is delimiters. A
delimiter is simply added to the end of a message to tell the receiver
to process the data it has received. The most common is the CR
or the CR and LF pair. The CR (carriage return) is like the old typewriters.
(remember them??) When you reached the end of a line while typing, a
bell would sound. You would then grab the handle and move the carriage
back to the start. In other words, you returned the carriage to the
beginning. (This is the same as what a CR delimiter will do if you view
it on a computer screen.) The plc/external device receives this and
knows to take the data from its buffer. (where the data is stored temporarily
before being processed) An LF (line feed) is also sometimes sent with
the CR character. If viewed on a computer screen this would look like
what happens on the typewriter when the carriage is returned and the
page moves down a line so you don't type over what you just typed.
an STX and ETX pair is used for transmission/reception as well. STX
is "start of text" and ETX is "end of text". The
STX is sent before the data and tells the external device that data
is coming. After all the data has been sent, an ETX character is sent.
we might also come across an ACK/NAK pair. This is rarely used but it
should be noted as well. Essentially, the transmitter sends its data.
If the receiver gets it without error, it sends back an ACK character.
If there was an error, the receiver sends back a NAK character and the
transmitter resends the data.
has a lot of information to absorb, so feel free to reread it.
It's not so difficult if you take your time and think about each step.
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