User`s manual

MGate 4101-MB-PBS UM Quick Configuration
A-7
Slaves are Identified by ID
Each Modbus slave in a system is assigned a unique ID between 1 and 247. Whenever a master makes a
request, the request must include the ID of the intended recipient. Master devices themselves have no ID.
0 1~247 248~255
Broadcast address Slave individual address Reserved
Communication is by Request and Response
All Modbus communication is by request and response. A master sends a request and a slave sends a response.
The master will wait for the slave’s response before sending the next request. For broadcast commands, no
response is expected. This is illustrated by three scenarios as follows:
Normal
The master sends a request to the slave. The slave sends a response with the requested information.
Exception
The master sends a request to the slave. The slave may not support the command or an error is detected, so
it sends an exception to the master.
Broadcast
The master sends a broadcast command, such as a reset command. Every slave on the network complies with
the command, and no response is sent to the master.
Requests Need a Time Limit
The original Modbus protocol was not designed for simultaneous requests or simultaneous masters, so only one
request on the network can be handled at a time. When a master sends a request to a slave, no other
communication may be initiated until after the slave responds. The Modbus protocol specifies that masters use
a response timeout function to identify when a slave is nonresponsive due to device or line failure. This function
allows a master to give up on a request if no response is received within a certain amount of time. This is
illustrated as follows: