Honeywell DCP 50 Product Manual page 43

G. Error and Exception Responses
If the controller receives a message which contains a corrupted character (parity
check fail, framing error etc), or if the CRC16 check fails, the controller ignores the
message. If the message is otherwise syntactically flawed (e.g. the byte count or
word count is incorrect) the controller will also not reply.
However, if the controller receives a syntactically correct message which nonetheless
contains an illegal value, it will send an exception response, consisting of five bytes
as follows:
The Function Number field consists of the function number contained in the message
which caused the error, with its top bit set (i.e. function 3 becomes 0x83), and the
Exception Number is one of the codes contained in the following table:
Code Name
1 ILLEGAL FUNCTION
2 ILLEGAL DATA ADDRESS
3 ILLEGAL DATA VALUE
4 DEVICE FAILURE
5 ACKNOWLEDGE
6 BUSY
7 NEGATIVE ACKNOWLEDGE
H. CRC16 Calculation
This is a 16-bit cyclic redundancy checksum. It is calculated in accordance with a
formula which involves recursive division of the data by a polynomial, with the input
to each division being the remainder of the results of the previous one.
The formula specifies that input is treated as a continuous bit-stream binary number,
with the most significant bit being transmitted first. However, the transmitting device
sends the least significant bit first.
According to the formula, the dividing polynomial is 2
but this is modified in two ways:
Function Number out of range
Parameter ID out of range or not supported
Attempt to write invalid data/action not carried
out
N/A
N/A
N/A
N/A
16
Cause
+ 2 + 2 + 1 (Hex 18005),
15 2
5-5