Control Techniques Commander SE Advanced User's Manual page 13

1.3.2 Writing Data
To write data (i.e. a value) to a drive parameter (i.e. host writing data to a drive), the message format from the host is as
follows:
CC
EOT GA
Where:
CC = Control Character
GA = Group address
GU = Unit address
[M1][M2] = Menu number
[P1] [P2] = Parameter number
[D1][D2] ...[Dn] = Data characters including decimal point and sign where necessary
BCC = Block checksum (calculated as described below)
The data field can be of a variable length with a maximum length of 7 characters.
Following the 'write' message, the drive will respond with a single control character, as follows:
Control
Character
NAK
ACK
An example of a host writing a value of +76.4 to parameter 1.25 of a drive that is addressed as unit 6 of group 2, the host
message is:
CC Address
EOT 2
The BCC in this example is calculated by the host as follows:
Calculating the block checksum (BCC)
The block checksum is calculated by applying an exclusive 'or' function (XOR) to all of the characters of a message after the
'STX' control character.
XOR truth table
A
0
0
1
1
For the 'write' message example above:
The first character of the BCC calculation is 0 (0011 0000 in binary), the value of which is taken as a starting or result value.
The next character is 1 (0011 0001 in binary), which now has the exclusive 'or' (XOR) operator act upon it. With the previous
result value, a new result occurs of (0000 0001) in binary.
The complete calculation is show in the table below:
Character
0
1
2
5
+
7
6
.
4
ETX
The final value is the BCC, provided that its equivalent decimal value exceeds 31 (or 1FH) (ASCII characters from 0 to 31 are
used as control codes).
When the final XOR result produces a decimal value less than 32, then 32 is added. In this example, 0011 0101 is 53
Commander SE Advanced User Guide
Issue Number: 4
Address CC
GA UA UA STX
Meaning
Message invalid.
Data is too long or out of range, parameter
is invalid, parameter is read-only, or the
BCC is incorrect.
Acknowledge — Message is valid and has
been understood and implemented.
CC Parameter
2 6 6 STX 0 1
B Out
0 0
1 1
0 1
1 0
Binary Value XOR result
0011 0000 0011 0000
0011 0001 0000 0001
0011 0010 0011 0011
0011 0101 0000 0110
0010 1011 0010 1101
0011 0111 0001 1010
0011 0110 0010 1100
0010 1110 0000 0010
0011 0100 0011 0110
0011 0101 =
0000 0011
BCC
Parameter
M1 M2 P1 P2 D1
Control Character on
Computer Keyboard
ASCII Character
Control-U
Control-F
Data
2 5 + 7 6 .
Data CC BCC
D2 .... Dn ETX BCC
CC BCC
4 ETX 5
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