DEVICE CONFIGURATION THROUGH MODBUS RTU
MODBUS RTU INTERFACE
The Modbus Remote Terminal Unit (RTU) data link protocol uses EIA-485 as a two-wire, daisy chain network. A
branch is a discrete chain of devices connected to a controller. The max number of devices per segment is (32), as
per the Modbus specifications. 4000 ft (1219.2 m) is the maximum recommended length for a segment, which
includes all devices from the controller to the last device in the daisy chain.
Each branch must have all devices connected with (A) connected to (A) and (B) connected to (B). If a shielded
cable is used, this is not to be connected to the devices. The shield cable should only be connected on one end to
earth ground, usually at the controller.
Each device must be configured for the correct baud rate and have a unique address in each branch. The baud
rate for the branch is set by the controller.
EOL Termination Resistance Selection
RS-485 requires that the last device in a chain have a termination resistor. This is controlled using the J8
switch as shown in
FIGURE 7
.
When the jumper is set to ON(Enabled), a 120 Ohm resistance is added in
parallel to the data line. When the jumper is set to the left(Disabled), the resistance is not added. By default,
the jumper is placed in the disabled position.
MODBUS MESSAGE
Automation Components, Inc.
2305 Pleasant View Road | Middleton, WI 53562
Phone:
1-888-967-5224 |
Website:
workaci.com
Page 4
CRC Check
16 bits
Data
N x 8 bits
Address
8 bits
Function
8 bits
Start
≥3.5 char
End
≥3.5 char
A typical Modbus RTU mode message frame is shown below. In the Modbus RTU mode, the messages
between frames are separated by at least 3.5 characters time’s silent interval. If the silent interval between
two characters is more than 3.5 characters time, the former character was transferred successfully, and the
current character’s transmission starts.
ADDRESS SELECTION
A typical Modbus RTU mode message frame is shown below. In the Modbus RTU mode, the messages
between frames are separated by at least 3.5 characters time’s silent interval. If the silent interval between
two characters is more than 3.5 characters time, the former character was transferred successfully, and the
current character’s transmission starts.
BAUD RATE, DATA BITS, PARITY, AND STOP BITS SELECTION
Baud rate is set to 9600, but can also be configured to 4800 via the register.
The device that requests information is called the Modbus Master and the devices giving the information
are Modbus Slaves. The Modbus sensors are slave devices and the number of Data Bits needs to be the
same as in the Master device configuration. ACI’s Modbus RTU sensors utilize 8 data bits during
communication exchange.
Parity default setting is NONE. Stop bit default setting is 1. Both settings can adjusted via the register.
FUNCTION
The function code is the second data in the frame. Valid function codes are from 0~127 (01H~7FH). See the
relevant Modbus standard. It supports 03H/06H function codes, shown as the following Modbus Poll
software. The detail register addresses are in: 6 General registers table.
BROADCAST MODE TO WRITE DATA TO SLAVE
Using broadcast mode, customer can write data to all slaves connected to the network. Address of
broadcast mode to write data is 0.
For example: change slave address with broadcast mode, customer can set a new slave address. Note: since
this operation will modify all the addresses of the slavers to the same address, it is NOT applicable for
network of more than one slave.
SPECIAL MODE TO READ DATA FROM SLAVE
With the special mode, customer can read the register data under the circumstance of NOT knowing the
slave address.
Address of special mode read data: 255(0xFF)
Note: this operation is applicable for ONLY ONE slave in the network.
Version: 2.0
I0000939
