5 Modbus Connection
5-10
NB-series Programmable Terminals Host Connection Manual (V108)
5-6 Modbus Protocol
Modbus functions as a request-response protocol which provides the service specified by function
codes.
Modbus protocol such as ASCII, RTU, and TCP does not specify the physical layer, but defines the
message frame and is available to communicate with the Controllers through varied networks. In ASCII
mode and RTU mode, the protocol is composed of the each combination of message and data, or
command and response. The data communications of Modbus adopt master-slave mode. In the mode,
first of all, the master sends a message and the slave receives the message, followed by the slave
replies the data to the request by the master.
As the master can also send the data directly to the slave to change the data of the slave, that means
the master and the slave can perform bidirectional read and write.
When the PTs of NB series performs the communications following the Modbus protocol, it is
performed in accordance with Modbus communications procedure.
Common format of message frame is shown as follows.
Address code
Data field of the message frame is two characters when the transmission mode is ASCII and eight
bits when it is RTU. Addresses of the slave are ranged from 0 to 247 (written by decimal) and
addresses of each device are allocated from 1 to 247. The master device select the slave device
through the data field of the message in which the addresses of the slave communicating with the
master are stored. When the slave replies the message, in order to recognize which slave replied to
the message, the addresses replied from the slave are stored in the data field of the response mes-
sage. Address 0 is used as a broadcast address which can be recognized by all the slave devices.
Function code
5-6-1 Introduction to Modbus Protocol
Address code Function code Data area Error checking code
8 bits 8 bits N × 8 bits 16 bits
Modbus Function
Code
Name Function
Corresponding
Address Type
01 Read coil status Reads the bit data (N bits) 0x
02 Read input discrete
value
Reads the bit data 1x
03 Read multiple registers Reads the integer type/character type/status
word /floating-point data (N words)
4x
04 Read input registers Reads the integer type/status word/
floating-point type data
3x
05 Write single coil Writes the bit data (one bit) 0x
06 Write single register Writes the integer type/character type/status
word /floating-point data (one word)
4x
15 Write multiple coils Writes the bit data (N bits) 0x
16 Write multiple registers Writes the integer type/character type/status
word /floating-point data (N words)
4x
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