Modbus Driver

The VOLTTRON Modbus Driver interface allows the Platform Driver Agent to act as a client in communication with a ModbusTCP server. First released in 1979 for use with PLCs, Modbus has become a common protocol for interacting with remote devices. More information on the Modbus Protocol can be found here

Attention

VOLTTRON’s modbus driver supports the ModbusTCP protocol only. For ModbusRTU support, see VOLTTRON’s Modbus-TK driver <Modbus-TK-Driver>.

Modbus Device Configuration

The Modbus device configuration file follows the device configuration file convention. All devices using the Modbus interface should use “modbus” as the value of the driver_type key. Reference device configuration file convention for information on other standard device configuration keys. Modbus specific configurations are contained in driver_config, a key-value dictionary used to establish communication with a ModbusTCP server: The driver_config dictionary accepts three keys:

device_address:

IP Address of the device. (required)

port:

Port the device is listening on. (Defaults to 502 — the standard Modbus port)

slave_id:

Slave ID of the device. (Defaults to 0. Use 0 for no slave.)

A typical Modbus device configuration will have the form:

{
    "driver_config": {"device_address": "10.1.1.2",
                      "port": 502,
                      "slave_id": 5},
    "driver_type": "modbus",
    "registry_config":"config://registry_configs/hvac.csv",
    "interval": 60,
    "timezone": "UTC",
    "heart_beat_point": "heartbeat"
}

This sample Modbus device configuration file can also be found in the volttron-lib-modbus-driver repository here

Modbus Registry Configuration File

The driver’s registry configuration file specifies information related to each point on the device, in a CSV or JSON file. More detailed information of driver registry files may be found here. The driver’s registry configuration must contain the following items for each point:

Volttron Point Name:

The name by which the platform and agents running on the platform will refer to this point.

Units:

Used for meta data when creating point information on the historian.

Modbus Register:

A string representing how to interpret the binary format of the data register. The string takes two forms:

• “BOOL” for coils and discrete inputs.

• A format string for the Python struct module. See the Python3 Struct docs for full documentation. The supplied format string must only represent one value. See the documentation of your device to determine how to interpret the registers. Some Examples:

  • “>f” - A big endian 32-bit floating point number.

  • “<H” - A little endian 16-bit unsigned integer.

  • “>l” - A big endian 32-bit integer.

Writable:

Either TRUE or FALSE. Determines if the point can be written to.

Point Address:

Modbus address of the point. Cannot include any offset value, it must be the exact value of the address.

Mixed Endian:

(Optional) Either TRUE or FALSE. If true, this will reverse the order of the Modbus registers that make up this point before parsing the value or writing it out to the device. This setting has no effect on bit values.

Default Value:

The default value for the point. When the point is reverted, it will change back to this value. If this value is missing it will revert to the last known value not set by an agent.

Any additional fields will be ignored. It is common practice to include a Point Name or Reference Point Name to include the device documentation’s name for the point and Notes and Unit Details for additional information about a point.

The following is an example of a Modbus registry configuration file (as CSV):

Catalyst 371

Reference Point Name	Volttron Point Name	Units	Units Details	Modbus Register	Writable	Point Address	Default Value	Notes
CO2Sensor	ReturnAirCO2	PPM	0.00-2000.00	>f	FALSE	1001		CO2 Reading 0.00-2000.0 ppm
CO2Stpt	ReturnAirCO2Stpt	PPM	1000.00 (default)	>f	TRUE	1011	1000	Setpoint to enable demand control ventilation
Cool1Spd	CoolSupplyFanSpeed1	%	0.00 to 100.00 (75 default)	>f	TRUE	1005	75	Fan speed on cool 1 call
Cool2Spd	CoolSupplyFanSpeed2	%	0.00 to 100.00 (90 default)	>f	TRUE	1007	90	Fan speed on Cool2 Call
Damper	DamperSignal	%	0.00 - 100.00	>f	FALSE	1023		Output to the economizer damper
DaTemp	DischargeAirTemperature	F	(-)39.99 to 248.00	>f	FALSE	1009		Discharge air reading
ESMEconMin	ESMDamperMinPosition	%	0.00 to 100.00 (5 default)	>f	TRUE	1013	5	Minimum damper position during the energy savings mode
FanPower	SupplyFanPower	kW	0.00 to 100.00	>f	FALSE	1015		Fan power from drive
FanSpeed	SupplyFanSpeed	%	0.00 to 100.00	>f	FALSE	1003		Fan speed from drive
HeatCall1	HeatCall1	On / Off	on/off	BOOL	FALSE	1113		Status indicator of heating stage 1 need
HeartBeat	heartbeat	On / Off	on/off	BOOL	FALSE	1114		Status indicator of heating stage 2 need

Another example registry file can also be found in the volttron-lib-modbus-driver repository here.