Burkert Type 8717 - Manual (Page 66)

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Starting up the Modbus
8. STarTing up The MODBuS
8.1. general information
The MFC supports the Modbus communication protocol as of firmware A.00.90 for devices with digital set-point
setting (version RS485 e.g. 8713).
Firmware versions higher than A.00.96 are support the Modbus communication protocol for analog devices.
The Modbus operates according to a master-slave method. In this case the MFC is designed as the slave.
Adjustable addresses are 1 to 32.
The BUS address of the devices can be set either with the Bürkert configuration tool MassFlowCommunicator in
the view “HART / Modbus COM Settings” or directly via the Modbus master. If an address change is set via the
Modbus master, the new address is not valid until the next commands are issued.
The communication is monitored by a timeout detection. If a timeout occurs, the device is set to a safe state (set-
point value is set to 0, causing the valve to close).
For analog devices the setpoint from analog input is active after a timeout.
The timeout can be specified via the holding register Timeout detection Time , the default value is 60 (seconds).
The timeout detection can be deactivated by a value of 0. For analog devices the timeout detection cannot be
deactivated.
Communication is via Modbus RTU. The preset communication parameters are:
Transfer rate: 9600 baud
Start bit: 1
data bits: 8
Stop bits: 1
Parity: none
8.2. Modbus in general
The Modbus protocol was developed by Modicon for programmable controllers and has evolved into a widely used
communication protocol in the industry.
A Modbus master can address individual slaves. The slaves send back a telegram (reply) on request which was
individually addressed to them. The Modbus protocol defines the format for the request from the master by entering
in the protocol the device address, a function code for specifying the requested action, all data to be transmitted and
a checksum. The reply telegram of the slaves is also specified with the aid of the Modbus protocol. It includes fields
for acknowledgement of the implemented action, for all data to be sent back and for a checksum. If an error occurs
on receipt of the telegram or if the slave cannot execute the requested action, the slave sends back an error telegram.
The following diagram shows the structure of a command:
request from master reply telegram from slave
device address device address
Function code Function code
• data • data
Checksum Checksum
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MFC Family