Modbus Protocol Description; Master/Slave Principle; Tru Transmission Mode - JUMO mTRON T Manual

3.1

Master/slave principle

Communication between a master (e.g. PC, notebook or central processing unit) and a slave
device (e.g. measuring and control system) and a Modbus takes place according to the master-
slave principle, in the form of data request/instruction - response.
Slave 1
The master controls the data exchange, the slaves only have a response function. They are
identified by their device address.
NOTE!
The central processing unit can be operated as a Modbus master as well as a Modbus slave.
The master and slave function can also be used in parallel.
This makes it possible to transmit the external analog, integer, and digital inputs and texts (vari-
ables) both from a master device to the central processing unit (slave) as well as actively have
the central processing unit (master) import them from one or several slaves.
If the central processing unit is operating as a master, the corresponding Modbus and device
addresses must be assigned using the setup settings of the Modbus frames.
3.2

TRU transmission mode

In addition to Modbus/TCP, RTU mode (Remote Terminal Unit) can also be used as the transfer
mode. The data is transmitted in the binary format with 8 or 16 bits for integer values and 32 bits
for float values. The most significant bit (msb) is transmitted first. The ASCII operating mode is
not supported.
Data format
The data format describes the structure of a character transmitted.
Data format
(configuration)
8 - 1 - no parity
8 - 1 - odd parity
8 - 1 - even parity

3 Modbus protocol description

Master
Slave 2
Start bit Data bits
1 8
1 8
1 8
Slave n
Parity bit Stop bit
0 1
1 1
1 1
Number of
bits
10
11
11
25