Vaisala HUMICAP MMT162 Guia do Utilizador descarregar pdf (Página 47)

Chapter 4 _________________________________________________________________ Operation

VAISALA ________________________________________________________________________ 45

Configuration Commands Related to Modbus

In addition to enabling the Modbus protocol in the device, Modbus over

RS-485 needs the following software settings: serial bit rate, parity,

number of stop bits, and Modbus device address.Use the service

commands listed in Table 10 below to configure the Modbus RTU using

the USB service cable.

Table 10 Configuration Commands for Modbus RTU

Service Command

Description

SMODE MODBUS

Enable Modbus communication protocol

SERI b p 8 s

Change baud rate and parity

b = baud rate (9600, 19200, 38400, 57600, 115200)

p = parity (E, N, or O)

8 = number of data bits must be 8

s = number of stop bits (2 if parity is N, 1 otherwise*)

Bit rates less than 9600 b/s are not supported with

Modbus.

ADDR a

Set Modbus address

a = new address (0…255)**

SDELAY d

Add extra Modbus response delay***

d = new delay in units of 1/250 seconds (0…255)

* Modbus specification defines that two stop bits must be used if parity is N.

** Setting address to zero switches the instrument offline from the Modbus bus.

Addresses 248 … 255 are not supported by the Modbus standard but MMT162 works

with them.

*** Extra response delay can be used, for example, to avoid problems caused by

direction-switching delays in bus converters.

NOTE

The instrument must be switched off and on before the configuration

changes made with the service cabl

e take effect.

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Vaisala HUMICAP MMT162 Guia do Utilizador Página 47