Modbus RTU
January 2023
•
Rev. 0
• Standard Devices User Manual
33
Modbus RTU Communication
Modbus RTU can be used to read and log sensor data, switch between analog and digital control modes, adjust device settings,
and control the device.
Establishing Communication
The device can communicate via the input/output port on
the device.
•
When using a USB cable to connect your device
to a Windows 10 computer, it should recognize
your USB as a virtual COM port automatically.
•
After physically connecting your device to a
Windows PC, you can check which COM port
number it uses by opening the Windows Device
Manager and expanding "Ports (COM & LPT)".
•
The default Modbus configuration
has the following settings:
• Baud Rate:
19200
• Data Bits:
8
• Stop Bits:
1
• Parity:
None
• Flow Control:
None
Multidrop Information
Devices can operate on networks with other devices. When
installing devices in an RS-485 network with multiple types
of devices, confirm that the total load of all devices does not
exceed 32 units on an unrepeated network segment. Consult
the EIA-485 standard for more information.
Modbus RTU Serial Protocol
Alicat uses the Modbus standard of offsetting registers by 1
from addresses, meaning register 1 is equivalent to address
0. However, some systems expect data to be mapped as
0-indexed addresses. Different Modbus control systems may
refer to registers, offsets, or addresses in their documentation
without clarifying their meaning. If your control system uses
a 0-indexed numbering scheme then decrement all register
references in this manual by 1.
If you are unsure of which addressing scheme your control
system uses, perform a test read of register 1200. If the device
responds with Error code 2: “Illegal Data Address”, then your
system is using the standard 1-indexed numbering system
and the values in this manual can be used as-is. If the device
returns a value of 0 instead of an error, decrement all registers
listed in this section by 1 to arrive at the correct offset.
Process Data
Alicat devices make no distinction between “Input” and
“Holding” registers. Modbus function codes FC03 and FC04
can be used interchangeably to read data from the device.
Sensor and process values are stored as big-endian, 32-bit
IEEE-754 floating point numbers spanning two registers. Your
control system will need to chain these into a single value to
interpret them correctly.
