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Modbus/TCP & Modbus RTU Function Codes
Function Code 02 - Read Digital Input Status
24 HC900 Hybrid Controller Communications User Guide Revision 4
9/03
4.3 Function Code 02 - Read Digital Input Status
Description
Function code 02 (1X references) is used to read a digital input’s ON/OFF status using 16 bit addressing
for DI access and data is returned in a binary format mapped into bytes.
Broadcast is not supported.
Query
The query message specifies the starting input and the quantity of inputs to read. The DI address in the
message is based on the slot and channel number of the digital input being read.
Example:
Read inputs for channels 1 to 16 in Rack #1, Slot 1, from the controller with slave address 1.
Query message format for function code 02
Slave
Address
(00 for TCP)
Function
Code
Starting
Address
High
Starting
Address
Low
Number
Inputs
High
Number
Inputs Low
CRC
(RTU)
CRC
(RTU)
TCP Example 00 02 00 00 00 10
RTU Example 01 02 00 00 00 10 CRC CRC
Response
The input status in the response message is packed as one input per bit of the data field. Status is indicated
as: 1 = ON; 0 = OFF. The LSB of the first data byte contains the input addressed in the query. The other
inputs follow toward the high order end of this byte, and from low order to high order in subsequent bytes.
If the returned input quantity is not a multiple of eight, the remaining bits in the final data byte will be
padded with zeros (toward the high order end of the byte). The byte count field specifies the quantity of
data bytes returned. Table 4-5 shows the Modbus Comm Digital I/O Channel to Address Mapping.
Example
: Inputs for channels 2 and 6 in Rack #1, Slot 1 are on, all others are off.
Response message format for function code 02
Slave
Address
(00 for TCP)
Function
Code
Byte Count Data Data CRC
(RTU)
CRC
(RTU)
TCP Example 00 02 02 22 00
RTU Example 01 02 02 22 00 CRC CRC
In the response the status of inputs 1 - 8 is shown as the byte value 22 hex, or 0010 0010 binary. Input 8 is
the MSB of this byte, and input 1 is the LSB. Left to right, the status of input 6 through 1 is: OFF-OFF-
ON-OFF-OFF-OFF-ON-OFF. The status of inputs 9-16 are shown as 00 hex, or 0000 0000 with the same
bit ordering.