Modbus Information
In a Modbus network there are two types of devices: Masters and Slaves.
Master Devices
These devices ask other devices for information
Slave Devices
These devices provide information, for example energy meters
Modbus also has two serial transmission modes: RTU and ASCII.
All Modbus devices on a network must have the same transmission mode and all Modbus devices have to implement RTU and this is the default mode of devices.
Modbus Data
Modbus defines 4 different types of data:
Discrete Input (Input status)
Single Bit
Read only
Coil (Coil status)
Single Bit
Read and Write
Input Register
16 Bits
Read only
Holding Register
16 Bits
Read and Write
Modbus defines a basic data type (word) as a 16 bit value. To read 32 bit values, two consecutive 16 bit values can be read.
Reading Modbus Data
Step 1 — Adding a Modbus network
Open the modbusAsync palette.
Drag and drop a ModbusAsyncNetwork into the Drivers section of your station.
Right-click on the ModbusAsyncNetwork and select Views > Property Sheet.
Expand the Serial Port Config property.
Enter COM2 as the Port Name (the default RS485 connection is COM2). Depending on your Modbus equipment you may need to change the other properties here (see manufactures website).
Step 2 — Adding a Modbus device
Drag and drop a ModbusAsyncDevice into the ModbusAsyncNetwork.
Enter the Modbus address into the Device Address field.
Right-click on the ModbusAsyncDevice and select Actions > Ping.
As Modbus does not have a Ping protocol, if any data is received from the device (even if its an Modbus error) the Health property will read OK.
You can specify a Ping Address which is a Modbus register address, but it is not necessary.
Ping failure
If the Ping fails this means no data has been received from the device. Possible failures:
- Wires not connected in the right order
- Terminating resisters might be needed at the end of the bus
- Incorrect COM port selected
Step 3 — Adding a Modbus point
Expand the Client Points folder in the modbusAsync palette and drag and drop a point into the Points object.
Expand the Data Address property and enter the address of the Modbus register you want to get data from.
NB: Some Modbus registers are writable and some are not (see above) use the correct point type from the palette.
The Address Format specifies how the address will be interpreted. Modbus addresses are quoted in the manufacturers data sheet, addresses are usually in hexadecimal or integer formats.
HAWK data points
A HAWK has the following numeric data types:
Name, Data size, Fractional Part, Signed
Integer
16 Bits
No
No
Signed Integer
16 Bits
No
Yes
Long
32 Bits
No
Yes
Floating point
32 Bits
Yes
Yes
As Modbus data points are only 16 bits, a HAWK will automatically read more than one register in a row and combine the data.
The following points can be added to a ModbusAsyncDevice:
ModbusClientRegisterBitPoint***
Single Bit
Read
ModbusClientRegisterBitWritable***
Single Bit
Read and Write
ModbusClientBooleanPoint
Single Bit
Read
ModbusClientBooleanWritable
Single Bit
Read and Write
ModbusClientNumericPoint*
32/64 Bits
Read
ModbusClientNumericWritable*
32/64 Bits
Read and Write
ModbusClientNumericBitsPoint**
1–16 Bits (Variable)
Read
ModbusClientNumericBitsWritable**
1–16 Bits (Variable)
Read and Write
* Number of bits depends on numeric type selected (see table above)
** You can select the start bit and the number of bits to read
*** You can select a particular bit of a register to read/write
Reading 8 bit values
If you need to read integers that are 8 bits long use a ModbusClientNumericBitsPoint. Set the Beginning Bit to 0 and the Number Of Bits to 8.
Troubleshooting
If you can connect to a device, but cannot get the addresses correct, try adding or subtracting one from the address. For example if the address in the data sheet is 75, try setting the address to 74 or 76 (some Modbus devices list their registers off by one).
