CONTREX CX-1200 - Manual (Page 3)

Each parameter value is transmitted over the network as 6 bytes.
The first 4 bytes (2 words) contain a signed long integer.
The next byte, the 5th byte, is a resolution byte. It represents the number of digits after
the decimal point. Here are some examples:
Real value Signed integer Resolution byte
1000 1000 0
12.34 1234 2
1.50 150 2
-200 -200 0
The 6th byte is the Definition byte. It is NOT important. Treat it as a placeholder only.
It can be used for obscure purposes in other types of serial communications, but it was
included in the DeviceNet protocol only to create an even number of bytes. PLC's often
assign variable names, and memory addresses, by the word (2 bytes) It is useful to keep
CX1200 parameters in "even byte" groups.
Shortcut tip:
Most users do NOT need to dynamically monitor decimal location. Since you are writing
and reading carefully selected and known parameters, you already know where the
decimal points are. If a follower setpoint ratio is always near 1.000, then you know the
format will be x.xxx. You would permanently set the resolution byte for writing a
setpoint ratio to the value 3. Then, to send a ratio of 1.123, assign the 1st 4 bytes a value
(long integer) of 1123, and append a resolution byte of 3, and a definition byte of zero,
for a total of 6 bytes. If you are reading a setpoint ratio, you can ignore the resolution
byte (you know it will be 3), and ignore the definition byte (not important). You would
receive a long integer (1st 4 bytes) of 1123, and you would routinely divide it by 1000 to
place the decimal correctly. The rest of your PLC code would then accept this as a real
value of 1.123.
Since you will be ignoring incoming resolution and definition bytes, you only need to act
upon the 1st 4 bytes (2 words) of any parameter in the read list.
Since the resolution byte can be predetermined (decimal places), and the definition byte
is not important, you can easily pre-establish the 2 bytes to append to the outgoing
"write" parameters. If you treat these as 1 word, or as a 2 byte integer, the following
values will
place the correct resolution byte (decimal places) in the "high" byte of this word or
integer: