Read Coil Status (Function Code 01) - ProSoft Technology CompactLogix MVI69E-MBS User Manual

MVI69E-MBS ♦ CompactLogix™ Platform
Modbus Serial Enhanced Communication Module

8.2.4 Read Coil Status (Function Code 01)

Query
This function allows you to obtain the ON/OFF status of logic coils (Modbus 0x range)
used to control discrete outputs from the addressed slave only. Broadcast mode is not
supported with this function code. In addition to the slave address and function fields, the
message requires that the information field contain the initial coil address to be read
(Starting Address) and the number of locations that are interrogated to obtain status
data.
The addressing allows up to 2000 coils to be obtained at each request; however, the
specific slave device may have restrictions that lower the maximum quantity. The coils
are numbered from zero; (coil number 1 = zero, coil number 2 = one, coil number 3 =
two, and so on).
The following table is a sample read output status request to read coils 0020 to 0056 (37
coils) from slave device number 11.
Note: This is the structure of the message being sent out to the Modbus network. The byte values below are
in hexadecimal display
Function
Node
Code
Address
01
0B
Response
An example response to Read Coil Status is as shown in the table below. 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 filled in 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.
Func
Node
Code
Address
01
0B
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Data Start Data Start
Point High Point Low
00 13
Byte Data Coil Data Coil
Count Status 20 Status 28
to 27 to 35
05 CD 6B
Number of Number of
Points High Points Low
00 25
Data Coil Data Coil Data Coil
Status 36 Status 44 Status 52
to 43 to 51 to 56
B2 OE 1B
Reference
User Manual
Error Check
Field (2 bytes)
CRC
Error Check
Field
(2 bytes)
CRC
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