Apogee Instruments S2-141 Manuale d'uso - Pagina 12

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Each register in a sensor represents a value in the sensor, such as a measurement or a configuration parameter.

Some registers can only be read, some registers can only be written, and some can be both read and written. Each

separate address than the sensor address, but can be easily confused with the sensor address.

However, there are two different indexing schemes used for Modbus sensors, though translating between them is

simple. One indexing scheme is called one-based numbering, where the first register is given the index of 1, and is

thereby accessed by requesting access to regis er 1. The other indexing scheme is called zero-based numbering,

where the first register is given the index 0, and is thereby accessed by requesting access to register 0. Apogee

Sensors use zero-based numbering. However, if using the sensor in a system that uses one-based numbering, such

as using a CR1000X logger, adding 1 to the zero-based address will produce the one-based address for the register.

According to the Modbus protocol specification, Holding Registers (the type registers Apogee sensors contain) are

defined to be 16 bits wide. However, when making scientific measurements, it is desirable to obtain a more precise

value than 16 bits allows. Thus, several Modbus implementations will use two 16-bit registers to act as one 32-bit

754 floating point numbers.

Apogee Modbus sensors also contain a redundant, duplicate set of registers that use 16-bit signed integers to

represent values as decimal-shifted numbers. It is recommended to use the 32-bit values, if possible, as they

contain more precise values.

Communication Parameters:

Apogee Sensors communicate using the Modbus RTU variant of the Modbus protocol. The default communication

parameters are as follows:

Slave address: 1

Baudrate: 19200

Data bits: 8

Stop bits: 1

Parity: Even

Byte Order: Big-Endian (most significant byte sent first)

The baudrate and slave address are user configurable. Valid slave addresses are 1 to 247. Since the address 0 is

reserve as the broadcast address, setting the slave address to 0 will actually set the slave address to 1. (This will

also reset factory-calibrated values and should NOT be done by the user unless otherwise instructed.)