ProSoft Technology MVI56E-MCMR User Manual
Page 180
Reference
MVI56E-MCMR ♦ ControlLogix Platform
User Manual
Modbus Communication Module with Reduced Data Block
Page 180 of 225
ProSoft Technology, Inc.
May 13, 2014
6.6.2 Read Coil Status (Function Code 01)
Query
This function allows the user to obtain the ON/OFF status of logic coils 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 will be
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 from Slave device number 11.
Adr
Func
Data Start Pt Hi Data Start Pt Lo Data # Of Pts Ho Data # Of Pts Lo
Error Check Field
11
01
00
13
00
25
CRC
Response
An example response to Read Coil Status is as shown in Figure C2. 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
will be 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 follow. For coil
quantities that are not even multiples of eight, the last characters will be 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 will reflect coil status at the end of the scan. Some Slaves will 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.
Adr Func
Byte
Count
Data Coil
Status 20 to
27
Data Coil
Status 28 to
35
Data Coil
Status 36 to
43
Data Coil
Status 44 to
51
Data Coil
Status 52 to
56
Error
Check
Field
11
01
05
CD
6B
B2
OE
1B
CRC