RevPi as Modbus RTU Master

The RevPi base module can act as a Modbus RTU Master, enabling you to connect and communicate with various Modbus RTU slaves such as temperature sensors, push-buttons, and motion detectors. This guide walks you through setting up and configuring your RevPi as a Modbus RTU Master for robust industrial communication.

Prerequisites #

Hardware

✓ RevPi base module (e.g. RevPi Connect 4)
✓ Modbus slave devices (e.g. sensors, actuators)
✓ USB-RS485 converter (if using USB interface)
OR
✓ Built-in RS485 Terminal (on RevPi Connect 4)

Software

✓ Web browser (e.g., Google Chrome or Mozilla Firefox).

Preparation #

For USB Connection (RevPi Core or RevPi Connect)

▷ Connect a USB port of the RevPi base module to the adapter.

▷ Use a D-Sub 9 socket to connect the adapter to the slave devices.

▷ Specify the address of the slaves and make a note of it.
If necessary, you can find information on this in the manufacturer’s operating instructions.

Note	If you are using several slaves, you have to make sure that each slave has its own address. If two or more slaves have the same address, communication errors occur.

Signal	Sub-D Pin
RS485-	1
RS485+	2
GND	5
+5V	6

Some USB serial adapters might only have RS485 data signal lines (D+ and D-).

For RS485 Terminal (RevPi Connect/Connect 4)

RevPi Connect models come equipped with an RS485 screw-type terminal for direct connection. The terminal features:

A (+): positive RS485 line.
B (-): negative RS485 line.
GND: ground.
Functional Earth: ensures signal integrity.

▷ Ensure correct wiring for robust and cost-effective integration of RS485 devices.

The socket has common A/B line terminals (better marked than D+ and D-) and reference terminals (internal GND and functional earth via 1 MOhm RC network).

Wiring example for RevPi Connect:

Wiring example for RevPi Connect 4

Step 1: Configuring Hardware Connections #

▷ Wire the A (+) and B (-) lines from the RevPi RS485 terminal (or USB-RS485 adapter) to the slave devices.

▷ Assign Unique Slave Addresses:
Refer to the device’s manual to configure the address.

Step 2: Configuring Modbus RTU Master in PiCtory #

▷ Start pictory.

▷ Add the base module and Add the virtual Modbus RTU Master to your configuration, see Arrange Devices.

▷ Drag the base module from the Device Catalog onto the virtual DIN rail.

▷ Open the folder Virtual Devices in the Device Catalog.

▷ Drag Modbus RTU Slave to the base module on the virtual DIN rail.

❯ The Modbus RTU Master now appears in the configuration

▷ Configure Modbus RTU Master, see Configuring Inputs and Outputs

▷ Select the Modbus RTU Master in the configuration.

▷ Set the following parameters in the Value Editor:

Parameter Description

Parameter	Description
Input	Configure up to 32 input values (16 bits each)
Output	Configure up to 32 output values (16 bits each)
device_path	Path to the Linux device file (default: `/dev/ttyRS485`)
baud_rate	Speed of the serial connection (default: `19200`)
parity	Parity bit configuration: None, Even (default), Odd
data_bits	Number of data bits (default: `8`)
stop_bits	Number of stop bits (default: `1`)

Input

Configure up to 32 input values (16 bits each)

Output

Configure up to 32 output values (16 bits each)

device_path

Path to the Linux device file (default: /dev/ttyRS485)

baud_rate

Speed of the serial connection (default: 19200)

parity

Parity bit configuration: None, Even (default), Odd

data_bits

Number of data bits (default: 8)

stop_bits

Number of stop bits (default: 1)

Note	Configure `udev rules` for consistent device file paths when using multiple devices of the same type.

▷ Save the configuration and restart the driver to apply the changes.

Step 3: Configuring Modbus Commands #

▷ Open the context menu of Modbus RTU Master in the configuration and select Extended Data.

▷ In the input mask, define commands to communicate with the slave:

Parameter	Description
Slave Addr.	Unique address of the slave device. May be assigned only once.
Function Code	Modbus function (e.g., READ_HOLDING_REGISTERS, WRITE_SINGLE_REGISTER).
Register Address	Address of the Modbus register or bit (check slave documentation).
Quantity	Number of registers/bits to read or write.
Action Interval	Interval between commands (in milliseconds).
Device Value	Variable name in the RevPi process image for the first word/bit of the command.

Parameter

Description

Slave Addr.

Unique address of the slave device. May be assigned only once.

Function Code

Modbus function (e.g., READ_HOLDING_REGISTERS, WRITE_SINGLE_REGISTER).

Register Address

Address of the Modbus register or bit (check slave documentation).

Quantity

Number of registers/bits to read or write.

Action Interval

Interval between commands (in milliseconds).

Device Value

Variable name in the RevPi process image for the first word/bit of the command.

▷ Ensure commands do not overlap in the process image.

Note	For devices using 0-based addressing, increment the register address by 1 during configuration.

▷ Select File › Save.

▷ Select Tools › Reset Driver to apply the configuration.

Troubleshooting and Error Codes #

If errors occur, check the Modbus_Master_Status register for the error code:

Error Code Meaning

Error Code	Meaning
`0x10`	Device not found. Check wiring.
`0x11`	Device does not respond. Verify configuration and register address.
`1`	ILLEGAL FUNCTION: Invalid function code.
`2`	ILLEGAL DATA ADDRESS: Invalid or write-protected register address.
`12`	INVALID CRC: Corrupted packet. Check wiring and connections.
`110`	CONNECTION TIMED OUT: Slave did not respond.

0x10

Device not found. Check wiring.

0x11

Device does not respond. Verify configuration and register address.

1

ILLEGAL FUNCTION: Invalid function code.

2

ILLEGAL DATA ADDRESS: Invalid or write-protected register address.

12

INVALID CRC: Corrupted packet. Check wiring and connections.

110

CONNECTION TIMED OUT: Slave did not respond.

▷ To reset an error, manually write 1 to the Master_Status_Reset or Action_Status_Reset register.

Note	Use a higher Action Interval for heavy system loads to prevent communication delays.

Note	Extend Modbus RTU Master capacity by adding more masters for additional values.