6 Rev. 2.4 (3/05)
Honeywell Analytics CM4 Protocol Manual
Checksum (Check Character)
During transmission of the packet, an error could change
the value of the data. Suppose in our example, for instance,
the packet 0x03, 0x42, 0x41, 0x44 is actually 0x03,
0x42, 0x30, 0x44. How can you determine that the numbers
you get are the same as the numbers sent? Using a
check-character is a method of assigning a value to the
packet to check if any of bytes have been modified.
If all the data bytes are added together and this sum made
into a byte, that byte could be called a check character, or
more commonly referred to as a checksum. For the CM4
instrument, the checksum is the negated sum of all the bytes
in the packet. In our example, the packet is 0x03, 0x42,
0x41, 0x44, 0x36. For this packet, the sum modulo 0x100
of all the bytes added to the checksum must equal zero (0).
Any other result indicates there is an error with the data.
The slave's data contained within the packet is interpreted in
a somewhat different manner than our example. The data is
composed of two sections, a command and one or more
parameters. The command indicates what type of
information is being transmitted in the packet.
The parameters contain specific arguments or data values
to be interpreted. Parameters for most CM4 instrument's
packet require at least four bytes for the Date and Time
stamps. This information is important to provide a date and
time reference for each communication from the instrument.
You should ensure that the date and time have been set
accurately in each CM4 monitor.
Every packet sent by the CM4 instrument also contains an
address, a length, a command and its associated
parameters, and a checksum. The CM4 instrument assumes
that the master's address is 0 (zero). You assign a unique
address to each slave which communicates with
the master.
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