
VOC Pro Transmitter • 35
Modbus Address Setting
Modbus is the leading industrial open control protocol. Modbus is available in several
different types, depending upon the media over which it is transmitted. Like most
communication protocols, Modbus uses a master/client type behavior. The master sends a poll
request for information to the client, the client decodes the request, and then sends a response
with the requested data back to the master.
A Modbus message includes a Modbus address, commonly referred to as a unit ID. The
Modbus address is used to identify the server address in RS-485 networks. Each server is
assigned an address and listens for messages which contain this number in the Modbus
address field.
The VOC Pro uses the original Modbus RTU over the RS-485 link. RS-485 Modbus has 255
addresses, ranging from 1 to 255. Eight of the addresses are used for internal system settings,
leaving addresses 1 to 247 available for your device.
The RS-485 Modbus communication parameters used in the VOC Pro is 8 data bits, no parity,
and 1 stop bit; these parameters are fixed and cannot be changed. The floating point data
values are presented with the least significant bytes first.
When using Modbus over a RS-485 network, the communication parameters MUST be set
correctly for all devices. For multiple devices using Modbus, ensure that no two units are
assigned the same address. A duplication of addresses could cause errors in the transmission
of data. Modbus addresses can be assigned sequentially or another appropriate address
scheme for the specific network setup.
The factory default setting on the VOC Pro for the Modbus address setting is 1.
1. If necessary, enter the Product Settings and Configuration menu by pressing and holding
the MENU button for 6 seconds.
2. Press and release the MENU button until the Modbus Address screen appears.
3. Use the ADD and SUB buttons to increase and decrease the Modbus address number,
respectively.
4. Press and release the MENU button to move through the Product Settings and
Configuration menu or to exit to the normal operating mode screen.