Using RS-485 data connection

To change the RS-485 settings, establish a maintenance connection with RS-232. See Connecting to AQT400 with RS-232 (CLI).

If you are using MOG100, you do not need to adjust the settings.

The RS-485 interface is used mainly for the data connection.

The interface supports the following data transfer protocols: Modbus ASCII and Modbus RTU.

When using the RS-485 interface, use a protected and twisted pair cable with both ends terminated with a 120 Ω termination resistor. AQT400 is terminated by default. To remove the termination, contact Vaisala technical support.

Using a terminal program, define the settings for the RS-485 interface.

To use the RS-485 port in Modbus ASCII mode, type the following:
```
set rs485_baud=9600
set rs485_mode=1
set rs485_parity=e
set rs485_databits=7
write --really
reboot --really
```
- Bit rate: 9600 bps
- Mode: ASCII
- Port settings: 7E1 (7 data bits, even parity, one stop bit)
To use the RS-485 port in Modbus RTU mode, type the following:
```
set rs485_baud=9600
set rs485_mode=4
set rs485_parity=e
set rs485_databits=8
write --really
reboot --really
```
- Port settings: 8E1 (8 data bits, even parity, one stop bit)
Modbus RTU mode may give communication errors. This does not affect data transfer or the availability and accuracy of measurement data. You may safely ignore the errors.

The Modbus modes are:

0 = ASCII CSV
1 = Modbus ASCII
4 = Modbus RTU

Using RS-485 data connection - AQT400

AQT400 Series Configuration Guide