Phoenix Contact RAD-900-DAIO6, User Manual, Page: 99 / 140

Addressing I/O extension modules and RAD-900-DAIO6 with Modbus RTU
3827_en_B PHOENIX CONTACT
97/138
9 Addressing I/O extension modules and RAD-900-DAIO6
with Modbus RTU
9.1 Modbus function codes
In the Modbus protocol, the function codes define which data is to be read or written. With
a single request, the registers 1 ... 123 can be read or written.
Table 9-1 Supported Modbus function codes
9.2 Modbus protocol
The data is transmitted using the Modbus/RTU (Remote Terminal Unit) protocol. Commu-
nication takes place according to the master/slave method. The Modbus master initiates
communication with a request to the slave. If the slave detects that its address has been ac-
cessed by the master, the slave always sends a response.
Only the master is able to initiate communication. The slaves are not able to initiate commu-
nication and do not communicate with each other.
The connected extension modules write the analog or digital input and output values to an
internal register. The Modbus master (e.g., a PLC) can read the individual registers using
the Modbus address of the slaves. The data to be transmitted is always included in a de-
fined frame. The frame is referred to as telegram.
The Modbus protocol defines the format of the telegrams. If an error occurs when the tele-
gram is received on the slave side, or if the slave is unable to carry out the master request,
an error telegram is sent back to the master.
Code number Function code Description
fc 03 Read Holding Register Read process output data
(address area 40010 ... 40999)
fc 04 Read Input Register Read process input data
(address area 30010 ... 30999)
fc 16 Write Multiple Registers Write multiple output registers word by word
Other function codes exist in the Modbus protocol, but they are not supported.