92 Rockwell Automation Publication 5069-UM003A-EN-P - May 2018
Appendix B Master Command List
Response
An example response to Read Coil Status is as shown in the following table.
The data is packed one bit for each coil. The response includes the slave
address, function code, quantity of data characters, the data characters, and
error checking. Data is packed with one bit for each coil (1 = ON, 0 = OFF).
The low-order bit of the first character contains the addressed coil, and the
remainder follows. For coil quantities that are not even multiples of eight, the
last characters are completed with zeros at high-order end. The quantity of data
characters is always specified as quantity of RTU characters, that is, the number
is the same whether RTU or ASCII is used.
Because the slave interface device is serviced at the end of a controller's scan,
data reflects coil status at the end of the scan. Some slaves limit the quantity of
coils provided each scan; thus, for large coil quantities, multiple PC
transactions must be made using coil status from sequential scans.
The status of coils 20…27 is shown as CD (HEX) = 1100 1101 (Binary).
Reading from left to right, this status shows that coils 27, 26, 23, 22, and 20 are
all on. The other Data Coil Status bytes are decoded similarly. Due to the
quantity of coil statuses that are requested, the last data field, which is shown
1B (HEX) = 0001 1011 (Binary), contains the status of only five coils
(52…56) instead of eight coils. The 3 left most bits are provided as zeros to fill
the 8-bit format.
Node Address Function Code Byte Count Data Coil
Status 20…27
Data Coil
Status 28…35
Data Coil
Status 36…43
Data Coil
Status 44…51
Data Coil
Status 52…56
Error Check
Field
(2 bytes)
0B 01 05 CD 6B B2 OE 1B CRC
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