Honeywell CM4 - Protocol Packet Definition; Packet Format; Start Code; Receiver Address

8 Rev. 2.4 (3/05)
Honeywell Analytics CM4 Protocol Manual
Protocol Packet Denition
Packet Format Two similar protocols are supported by CM4 software.
The original protocol with a minimum packet length of ve
bytes is included for compatibility with previous software.
Additionally, a new protocol with a minimum packet length
of six bytes is included for greater robustness.
Examples of packets using both protocols are included at
the end of this book.
The format of packets using original version 1 protocol is
as follows:
The format of packets using the new version 2 protocol
is similar:
Start Code Size: 1 byte. Always 0x40
Receiver Address Size: 1 byte
For responses from slave to master,
this is always 0. For inquiries from
master to slave, this must match the
slave’s address as set with menu
function 3.3.1.
Transmitter Address Size: 1 byte
For inquiries from master to slave,
this is always 0. For responses from
slave to master, this will match the
slave’s address as set with menu
function 3.3.1.
Length Size: 1 byte
The length of the total packet
from start code to checksum.
Command Size: 1 byte
This is 0x28 to 0x69 as dened in
the remainder of this book.
Data Size: 0-250 byte(s)
This section varies according to the
command chosen and is often
empty.
Checksum Size: 1 byte
The two’s complement of the sum of
all the previous bytes in the packet.
This makes the sum of the entire
packet a multiple of 0x100.
start code
receiver
address
length command data checksum
start code
receiver
address
transmitter
address
length command data checksum