SMAX MODBUS®
6 Modbus® interface
Lika SMAX Modbus series linear encoders are Slave devices and implement the
Modbus application protocol (level 7 of OSI model) and the “Modbus over Serial
Line” protocol (levels 1 & 2 of OSI model).
For any further information or omitted specifications please refer to “Modbus
Application Protocol Specification V1.1b” and “Modbus over Serial Line.
Specification and Implementation Guide V1.02” available at www.modbus.org.
6.1 Modbus Master / Slaves protocol principle
The Modbus Serial Line protocol is a Master – Slaves protocol. One only Master
(at the same time) is connected to the bus and one or several (247 maximum
number) Slave nodes are also connected to the same serial bus. A Modbus
communication is always initiated by the Master. The Slave nodes will never
transmit data without receiving a request from the Master node. The Slave
nodes will never communicate with each other. The Master node initiates only
one Modbus transaction at the same time.
The Master node issues a Modbus request to the Slave nodes in two modes:
•
UNICAST mode
: in that mode the Master addresses an individual Slave.
After receiving and processing the request, the Slave returns a message
(a “reply”) to the Master. In that mode, a Modbus transaction consists of
two messages: a request from the Master and a reply from the Slave.
Each Slave must have a unique address (from 1 to 247) so that it can be
addressed independently from other nodes. Lika devices only implement
commands in “unicast” mode.
•
BROADCAST mode
: in that mode the Master can send a request to all
Slaves at the same time. No response is returned to “broadcast” requests
sent by the Master. The “broadcast” requests are necessarily writing
commands. The address 0 is reserved to identify a “broadcast” exchange.
NOTE
Lika devices do not implement commands in “broadcast” mode.
MAN SMAX MB E 1.2
Modbus® interface
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