ProSoft MVI69E-MBS - Page 108

Reference MVI69E-MBS ♦ CompactLogix Platform
User Manual Modbus Serial Enhanced Communication Module
Page 108 of 154 ProSoft Technology, Inc.
June 28, 2017
The following table is a sample read input status request to read inputs 10197 to
10218 (22 coils) from slave 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.
Node
Address
Function
Code
Data Start
Point High
Data Start
Point Low
Number of
Points High
Number of
Points Low
Error Check
Field (2 bytes)
0B
02
00
C4
00
16
CRC
Response
An example response to Read Input Status is as shown in the table below. The
data is packed one bit for each input. 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 input (1=ON, 0=OFF). The lower
order bit of the first character contains the addressed input, and the remainder
follows. For input 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 a 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,
the data reflect input status at the end of the scan. Some slaves limit the quantity
of inputs provided each scan; thus, for large coil quantities, multiple PC
transactions must be made using coil status for sequential scans.
Node
Address
Func
Code
Byte
Count
Data Discrete
Input 10197 to
10204
Data Discrete
Input 10205 to
10212
Data Discrete
Input 10213 to
10218
Error Check
Field
(2 bytes)
0B
02
03
AC
DB
35
CRC
The status of inputs 10197 to 10204 is shown as AC (HEX) = 10101 1100
(binary). Reading left to right, this show that inputs 10204, 10202, and 10199 are
all on. The other input data bytes are decoded similar.
Due to the quantity of input statuses requested, the last data field which is shown
as 35 HEX = 0011 0101 (binary) contains the status of only 6 inputs (10213 to
102180) instead of 8 inputs. The two left-most bits are provided as zeros to fill
the 8-bit format.