CAI 700M - Page 180

700M HFID 6-2-15
180
Error
Function Code
1 Byte
Function code + 0x80
Exception Code
1 Byte
01 or 02 or 03 or 04
Here is an example of a request to read discrete outputs 20–38:
Request
Response
Field Name
(Hex)
Field Name
(Hex)
Function
01
Function
01
Starting Address Hi
00
Byte Count
03
Starting Address Lo
13
Status of Outputs 27-20
CD
Quantity of Outputs Hi
00
Status of Outputs 35-28
6B
Quantity of Outputs Lo
13
Status of Outputs 38-36
05
The status of outputs 27–20 is shown as the byte value CD hex, or binary 1100 1101. Output
27 is the MSB of this byte, and output 20 is the LSB.
By convention, bits within a byte are shown with the MSB to the left and the LSB to the right.
Thus the outputs in the first byte are 27-20 from left to right. The next byte has outputs 35-28 left
to right. As the bits are transmitted serially, they flow from LSB to MSB: 20 . . . 27, 28 . . . 35,
and so on.
In the last data byte, the status of outputs 38-36 is shown as the byte value 05 hex, or binary
0000 0101. Output 38 is in the sixth bit position from the left, and output 36 is the LSB of this
byte. The five remaining high-order bits are zero filled.