Step 2: Learn about the Protocol
A protocol is one or a few sets of
hardware and software rules agreed to by all communication parties
for exchanging data correctly and efficiently.
Synchronous and
Asynchronous Communications
Synchronous
Communication requires the sender and receiver to share the same
clock. The sender provides a timing signal to the receiver so that the
receiver knows when to "read" the data. Synchronous Communication
generally has higher data rates and greater error-checking
capability. A printer is a form of Synchronous Communication.
Asynchronous
Communication has no timing signal or clock. Instead, it inserts
Start/Stop bits into each byte of data to "synchronize" the
communication. As it uses less wires for communication (no clock
signals), Asynchronous Communication is simpler and more cost-effective.
RS-232/RS-485/RS-422/TTL are the forms of Asynchronous Communications.
Drilling Down: Bits and Bytes
Internal computer
communications consists of digital electronics, represented by
only two conditions: ON or OFF. We represent these with two numbers: 0 and
1, which in the binary system is termed a Bit.
A Byte consists
of 8 bits, which represents decimal number 0 to 255, or
Hexadecimal number 0 to FF. As described above, a byte is the
basic unit of Asynchronous communications.
Baud rate, Data
bits, Parity, and Stop bit
RS-232 Logic Waveform (8N1)
The baud rate is
the communication speed that measures the number of bit transfers
per second. For example, 19200 baud is 19200 bits per second.
Data bits are a
measurement of the actual data bits in a communication packet. For
example, the above graphic shows eight (8) data bits in a communication
packet. A communication packet refers to a single byte transfer,
including Start/Stop bits, Data bits and Parity. If you are
transferring a standard ASCII code (0 to 127), 7 data bits are
enough. If it is an extended ASCII code (128 to 255), then 8 data
bits are required.
Parity is a
simple way to error-check. There are Even, Odd, Mark and
Space indicators. You can also use no parity. For Even and Odd parity, the
serial port sets the parity bit (the last bit after the data bit)
to a value to ensure that the data packet has an Even or Odd
number of logic-high bits. For example, if the data is 10010010,
for Even parity, the serial port sets the parity bit as 1 to keep
the number of logic-high bits Even. For Odd parity, the parity bit
is 0 so that the number of logic-high bits is Odd. Mark parity
simply sets the parity bit to logic-high and Space sets the
parity bit to logic-low, so that the receiving party can determine if
the data is corrupted.
Stop bits are
used to signal the end of a communication packet. This also helps
to synchronize different clocks on the serial devices.
Handshaking
(Flow Control)
Handshaking is also
called "Flow Control". The main purpose of Handshaking is to
prevent receiver overloading. By using Handshaking signals,
receivers will be able to tell the sending device to pause data
transmission if the receiver is overloaded. There are three types of
handshaking: Software handshaking, Hardware handshaking and Both.
Software
handshaking uses two control characters: XON and XOFF. The
receiver sends these control characters to pause transmitter
during communication. XON is decimal 17 and XOFF is decimal 19 in
the ASCII chart. The drawback of Software handshaking is that these
two control characters can not be used in data. This is quite
important when you are transmitting Binary data as you might need
to use these two codes in your data.
Hardware
handshaking makes use of actual hardware lines, such as RTS/CTS,
DTR/DSR, and DCD/RI (for modem).
In DTE/DCE
communication, RTS (Request to Send) is an output on the DTE and
input on the DCE. CTS (Clear to Send) is the answering signal
coming from the DCE. Before sending a data, the DTE asks
permission by setting its RTS output to high. No data will be sent until
the DCE grants permission by using the CTS line.
The DTE uses the
DTR (Data Terminal Ready) signal to indicate it is ready to accept
information, whereas the DCE uses the DSR signal for the same
purpose. DTR/DSR are
normally ON or OFF for the whole connection session (e.g.
Off-hook), while RTS/CTS are ON or OFF for each data transmission.
DCD (Data Carrier Ready) is used by the modem
when a connection has been established with remote equipment, while RI (Ring Indicator)
is used by the modem to
indicate a ring signal from telephone line
Data formats
(Binary, Hex, Dec, Oct, and ASCII)
Serial devices
use Binary for communication, which consists of just two unique
numbers: 0 and 1. Binary is the Base-2 numbering system. One byte
of data consists of 8 binary digits, from 0000 0000 to 1111 1111.
Hexadecimal is
the base-16 system, which consists of 16 numbers: 0 to 9 and the
letters A to F (decimal number 15). The Hexadecimal numbering
system is useful because it can represent every byte as two
consecutive hexadecimal digits, and it is easier for humans to
read Hexadecimal numbers than Binary numbers. Most of the
manufacturers use Hexadecimal in their protocol documentation. It
is simple to
convert a value from Hexadecimal to Binary. Just translate
each Hexadecimal digit into its 4-bit binary equivalent. E.g.
Hexadecimal number F3 equals Binary number 1111 0011.
Octal refers to
the base-8 numbering system, which uses just eight unique symbols
(0 to 7). Programmers often use Octal format because it is
relatively easy for people to read and can easily be translated
into binary format: each Octal digit represents 3 binary digits. E.g. Octal number 73 equals to Binary number 111 011.
Decimal refers to
numbers in base 10, which is the numbering system we use most in
everyday life. It's not as easy as Hexadecimal and Octal to
converter Decimal to Binary number, but it is easier for us to
understand Decimal.
ASCII (American
Standard Code for Information Interchange) is a character encoding
based on the English alphabet. ASCII codes (both readable and
unreadable) are widely used in communications, such as Modem
communications. Letters A to Z and numbers 0 to 9 are readable
ASCII codes. Some ASCII codes are unreadable, such as the control
codes: XON and XOFF, which are used in Software flow control.
Checksum
Many serial
protocols use checksum (additional bytes added at the end of the
data string) to check the data integrity, as errors might occur
during data transmission.
There are many
types of checksum, from the simplest uses of it in Modula or BCC to
sophisticated CRC calculation. Using Modula as an example, we
learn that before
data transmission, the sender adds all command bytes together then
mod it by 255 (decimal) to get an additional byte. This is to be
added at the end of the command string. When the receiver receives
the command string, it will first check the added byte to see whether
data remain unchanged or not. If that is the case, it will accept the data, and
if not, it will ask the sender to resend the data.
Examples of
protocol commands
A protocol command is a data
string sent from one serial device (e.g. a Computer) to another
(i.e. a Modem). Here are some examples:
ASCII command example: ATI1<CR><LF>
to query Modem manufacturer's information. (Note: <CR><LF> are the
control codes: Carriage Return and Line Feed)
Convert the command string above to
Hexadecimal and it becomes:
41 54 49 31 0D 0A
Convert the command string above to
Decimal and it becomes:
065 084 073 049 013 010
Convert the command string above to
Octal and it becomes:
101 124 111 061 015 012
Convert the command string above to
Binary and it becomes:
01000001 01010100 01001001
00110001 00001101 00001010
Step 1: Understand RS-232
Connections & Signals
Step 3: Start controlling your RS-232 devices
by using 232Analyzer
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