Modbus Support¶

Modbus is an alternative data communication protocol commonly used by certain pieces of equipment like electric meters. Unlike BACnet, it has very limited support for complex data types or automatic discovery; generally, a device profile is required in order to interpret the data from a Modbus device.

A device profile specifies the data type for each Modbus register as well as provides a useful name. In order to integrate a Modbus device using Normal, you must first create a profile; then, you may create one or more connections to different devices using that profile.

Creating a Modbus Profile¶

A device profile contains a list of register addresses along with data types and associated human-readable names for these registers. Profiles may be created directly using the Modbus API; but most users will probably use nfcli, which supports importing profiles from csv files.

A profile is simply a CSV file with the required columns, as shown below. Additionally, some configuration variables such as the endianness may be set in the header.

# this file is named skylogger.csv
# uuid: d4a12bx8-03a9-11ed-a97c-a30ea19188b1
# profileName: 170321_skytron_ModbusList
# endianness: ENDIAN_BIG
# highWordFirst: true
Register Type,Name,Address,Data Type,Unit
3,Temp. Channel 1 "Module temperature",10,FLOAT32,°C
3,Temp. Channel 2 "Ambient temperature",12,FLOAT32,°C
3,Radiation Channel 1,20,FLOAT32,W/m2, 1
3,Radiation yield Channel 1,FLOAT32,Wh/m2

$ nfcli create-modbus-profile skylogger.csv

Once the profile has been created, you may create a connection to it; you can also see the currently installed profiles using nfcli list-modbus-profiles.

Devices usually come with documentation including the modbus register map for your device. You may also be able to fine one someone else has created and convert it to this format; for instance, Contemporary Controls publishes many maps in a similar format.

Connecting to a Modbus device¶

Once you have created a profile for device, you must create a connection for each instance of a device.

$ nfcli create-modbus-connection -p d4a12bb8-03a9-11ed-a97c-a30ea19188b1 -a tcp://192.168.103.190:502 -n "test"

In this example, the UUID must be the ID of a profile you have importted; and the url string is the address of the Modbus device. You may also provide a human readable name to help identify the connection.

Profile Format¶

A profile file consists of two sections: a header with commented lines; and a body consisting of a list of registers. The header allows configuration of several device parameters. Although the profiles are usually loaded to and from CSV files, the format is defined by CreateDeviceProfile.

The body consists of a header, followed by zero or more registers. The possible column headers are: