User Guide
Modbus Implementation
The Modbus implementation on the Advisor has several objectives.
It offers the following services to the Modbus Master (PLC, PC or similar):
• Monitor the process variable measured by the instrument
• Identify the status of the instrument (Alarms status, Tare Activation status,…)
• Configure the instrument remotely
The Modbus interface is available as an optional module on the Advisor A90. The supported protocol is
Modbus RTU (Modbus over RS485). Note that the ASCII version of the protocol has not been implemented.
The A90 operates only as a slave on the network, processing requests from a remote Master. Multidrop mode is
supported in that there can be more than one device (A90 or others) on the bus.
Exclusions
It is not possible to configure the product locally and remotely at the same time. If a user is navigating through the
menus, a Modbus Busy exception will be raised to the Master.
The Master cannot be used to override the internal logic of the instrument. For example, it is not possible to directly
control the Alarm Outputs, read the keypad buttons or take control of the displayed value via Modbus.
The Master cannot override operations which are intended to take place locally. For example actions such as Silencing
Alarms, Input Calibration, Temperature Trimming, Input Taring are not available via Modbus.
The Input type is not writeable by the Master as changing the input type has the effect of resetting the product back to
defaults, causing significant problems.
Hardware
The physical hardware layer is a 2 wire RS485 interface. The A90 will see the all requests from the master and every
reply from any other devices on the bus. These are ignored unless the request is specifically addressed to the unit.
The RS485 communication settings can be amended locally on the instrument by going into the “Ser” submenu or via
the dedicated holding registers. The following settings can be changed :
• Baud Rate in kbaud : can be 9.6, 19.2, 38.4, 57.6, 115.2
• Parity: Even, None or Odd
• Number of Stop Bits: 1 or 2
The default values are 19.2kbaud, Even Parity, 1 Stop bit
Setting the Address
The Modbus Slave Address can only be amended locally on the instrument via the “5Er” submenu.
The slave address can range from 1 to 247. The default value is 001.
Note: Address 0 is reserved for broadcast messages. The A90 accepts the broadcast for writing functions, however no
response is returned to the master.
Reset to Defaults
Changing the instrument input type (locally via the keypad) or resetting the configuration to defaults will have the effect
of reverting the Modbus communication settings and slave address to their default values.
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