Modbus Interface
WirelessHART Fieldgate SWG70
116
E Hauser
Registers
Modbus specifies four different types of register:
• Discrete input registers contain the discrete input values and possibly status
• Input registers contain analog input values and status
• Coil registers contain discrete output values and possibly status
• Holding registers contain analog output values and status
In addition, it is possible to "pack" discrete inputs and outputs into words, which are then
stored as appropriate in the input or holding registers.
Fig. 16-1 overleaf gives an overview of the register and reference address ranges used for
each register type.
For Fieldgate, the holding registers are a read only image of the input registers which can
be accessed when the Read Modbus Registers Mode is set to "Input and Holding". See
HART implementation
The Modbus implementation in Fieldgate SWG70 maps HART dynamic process variables
over Modbus input registers and also uses input registers to provide device-related status
information. The Modbus implementation is based on the following assumptions:
• The HART Command 3 dynamic variables are used for automatic Modbus mapping.
• For Modbus access, publishing must be enabled for all relevant devices.
• The Fieldgate caches the relevant information. Modbus commands will therefore access
the internal Fieldgate memory.
• Modbus input registers are only supported if they are associated with HART dynamic
process variables.
• A device can either be a WirelessHART device or a wired HART device connected to a
WirelessHART Adapter (SWA70).
• WirelessHART and wired HART devices are mapped independently within the Modbus
input registers.
