Allen-Bradley 5069-SERIAL, User Manual, Page: 91 / 110

Rockwell Automation Publication 5069-UM003A-EN-P - May 2018 93
Master Command List 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