Rockwell Automation Publication 5069-UM003B-EN-P - January 2020
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Modbus Command Codes
Appendix B
Read Input Status (Function Code 02)
Query
This function allows you to obtain the ON/OFF status of discrete inputs
(Modbus 1x range) in the addressed slave. PC 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 input address to be
read (Starting Address) and the number of locations that are interrogated to
obtain status data.
The addressing allows up to 2000 inputs to be obtained at each request; however,
the specific slave device can have restrictions that lower the maximum quantity.
The inputs are numbered form zero; (input 10001 = zero, input 10002 = one,
input 10003 = two, and so on, for a 584).
The following table is a sample read input status request to read inputs 10197 to
10218 (22 coils) from slave number 11.
Response
An example response to Read Input Status is as shown in the following table. 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 completed 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.
TIP
This is the structure of the message being sent out to the Modbus network.
The following byte values 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
