7 Modbus® interface
Lika DRIVECOD positioning units are Slave devices and implement the Modbus
application protocol (level 7 of the OSI model) and the “Modbus over Serial Line”
protocol (levels 1 & 2 of the OSI model).
For any further information or omitted specifications please refer to the
“Modbus Application Protocol Specification V1.1b” and “Modbus over Serial Line.
Specification and Implementation Guide V1.02” available at www.modbus.org.
7.1 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 (up to 247) 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 the 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.
Lika devices do not implement commands in “broadcast” mode.
MAN RD6 MB E 1.1
Modbus® interface
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RD6 Modbus®
