Command Structure; Modbus Rtu/Ascii - Algodue ELETTRONICA UPM309 Manual Del Usuario

2. COMMAND STRUCTURE

The master communication device can send reading or writing commands to the slave (instrument). The
structure for reading and writing commands is following described according to the used communication
protocol (RTU/ASCII or TCP).

2.1 MODBUS RTU/ASCII

In this section, the tables describe the reading command structure (Query) and the writing command structure.
Both commands are followed by a response sent by slave.
These tables refer to a master-slave communication in MODBUS RTU.
READING COMMAND STRUCTURE (function code $03/$04)
The master communication device can send commands to the instrument to read its status, setup and the
measured values. More registers can be read, at the same time, sending a single command, only if the registers
are consecutive (see chapter 4). Values contained both in Query and Response messages are in hex format.
Structure
Slave address
Function code
Starting register
Words to be read
CRC
Query example: 0103000000020BC4
Structure
Slave address
Function code
Data bytes
Requested reading data
CRC
Response example: 010304000392109F66
54
Example Byte
01 -
03 -
00 High
00 Low
00
High
02 Low
0B High
C4 Low
Example Byte
01 -
03 -
04
-
00 High
03 Low
92 High
10 Low
9F High
66 Low
MODBUS RTU/ASCII, TCP