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Protocol
Specifics
The protocol has been designed for flexibility and efficiency. This byte-wise
protocol communicates information by transferring bytes of data back and
forth between master and slave. A group of bytes for each communication
is called a packet.
The master and slave(s) transfer information via data packets.
These packets will always contain bytes to start communication, an
address, a packet length, a command, and a checksum.
Additional optional variable-length data bytes or optional parameters
can also be sent or received. This section is a brief overview of the
protocol specifics.
Data and Packets
A byte is a piece of data. It is a way to indicate information
and is composed of eight bits of information. A bit is
the smallest possible piece of information. It can only
be two possible values, 1 or 0 (True or False). A byte can
have a decimal value from 0 to 255. In hexadecimal
representation, a byte's value can range from 0x00 to 0xFF.
Hexadecimal representation will be used for the remainder of
this manual. The number may be interpreted in a manner
other than a number, for example, as a letter.
Using ASCII characters, where 0x41 is the letter A, 0x42 is
the letter B, 0x43 is C, and so on, a message can be written
as these numbers. Each number is a byte. For example, the
word "BAD" is 0x42, 0x41, 0x44.
The collection of the three bytes in our message (0x42, 0x41,
0x44) may be referred to as a packet, since these bytes are
always associated with each other.
Just as all words are not the same length (number of letters),
packets may also vary in length. One way to denote the size
of the word is to put the number of letters you have in the
word as the first number in your packet. Your data packet
containing the word (or command) "BAD" will then become
0x03, 0x42, 0x41, 0x44.
5
CM4 Protocol Manual
Rev. 2.4 (3/05)
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