How to change the parity of a WTSC MODBUS RTU device via modpoll

Most of the time, the parity can be changed for the serial port in a Gateway, but sometimes it is required to change the parity of a Modbus RTU device directly.

We will use the software "modpoll" for this purpose, available for Windows and Linux.

In this example we are changing the parity of a WTSC 485 (Wiegand To RS-485 Serial Converter - Link) on a Linux system.

Install modpoll onto your system, which you are using to connect to the Modbus RTU device via a serial interface

We will now have to check the manual of the W2SC to see, what kind of register structure the device has

Register	Type	Value
2000	Slave ID	42
2001	Modbus mode	1 = ASCI 2 = RTU
2002	Parity	1 = None 2 = Odd 3 = Even (default)
2003	Baud rate	default 9600 Baud
2005	Command register	0x42 = write configuration to EEPROM 0x24 = reset the device

As in the documentation described, changing the parity requires the using the register 2002

First we read from the device using the following command

root@ubuntu:/# modpoll -a 42 -b 9600 -d 8 -p even -s 1 -t 4 -0 -r 2002 -c 1 /dev/ttyUSB0
modpoll 3.4 - FieldTalk(tm) Modbus(R) Master Simulator
Copyright (c) 2002-2013 proconX Pty Ltd
Visit http://www.modbusdriver.com for Modbus libraries and tools.

Protocol configuration: Modbus RTU
Slave configuration...: address = 42, start reference = 2002 (PDU), count = 1
Communication.........: /dev/ttyUSB0, 9600, 8, 1, even, t/o 1.00 s, poll rate 1000 ms
Data type.............: 16-bit register, output (holding) register table

-- Polling slave... (Ctrl-C to stop)
[2002]: 3

The device is thus correctly set in parity even, now we will change it to parity none, by adding a 1 to the command line used previously

root@ubuntu:/# modpoll -a 42 -b 9600 -d 8 -p even -s 1 -t 4 -0 -r 2002 -c 1 /dev/ttyUSB0 1
.
.
.

Protocol configuration: Modbus RTU
Slave configuration...: address = 42, start reference = 2002 (PDU), count = 1
Communication.........: /dev/ttyUSB0, 9600, 8, 1, even, t/o 1.00 s, poll rate 1000 ms
Data type.............: 16-bit register, output (holding) register table

Written 1 reference.

To write the changes to the device, we need to use the register 2005

root@ubuntu:/# modpoll -a 42 -b 9600 -d 8 -p even -s 1 -t 4 -0 -r 2005 -c 1 /dev/ttyUSB0 0x42
.
.
.

Protocol configuration: Modbus RTU
Slave configuration...: address = 42, start reference = 2002 (PDU), count = 1
Communication.........: /dev/ttyUSB0, 9600, 8, 1, even, t/o 1.00 s, poll rate 1000 ms
Data type.............: 16-bit register, output (holding) register table

Written 1 reference.

And finally we will reboot the device, for the changes to take effect

root@ubuntu:/# modpoll -a 42 -b 9600 -d 8 -p even -s 1 -t 4 -0 -r 2005 -c 1 /dev/ttyUSB0 0x24
.
.
.

Protocol configuration: Modbus RTU
Slave configuration...: address = 42, start reference = 2002 (PDU), count = 1
Communication.........: /dev/ttyUSB0, 9600, 8, 1, even, t/o 1.00 s, poll rate 1000 ms
Data type.............: 16-bit register, output (holding) register table

Written 1 reference.

Now we can check if the changes were done correctly

root@ubuntu:/# modpoll -a 42 -b 9600 -d 8 -p none -s 1 -t 4 -0 -r 2002 -c 1 /dev/ttyUSB0
.
.
.

Protocol configuration: Modbus RTU
Slave configuration...: address = 42, start reference = 2002 (PDU), count = 1
Communication.........: /dev/ttyUSB0, 9600, 8, 1, even, t/o 1.00 s, poll rate 1000 ms
Data type.............: 16-bit register, output (holding) register table

-- Polling slave... (Ctrl-C to stop)
[2002]: 1

Use the modpoll help for more information regarding the commands available. Here in this example we've used the following

-a : slave address; -b : baud rate; -d : databits; -p : parity; -s : stopbits; -t : register data type; -0 : PDU addressing;

-r : reference; -c : number of values to read

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How to change the parity of a WTSC MODBUS RTU device via modpoll

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