Sensaphone Sentinel Pro, Инструкция по установке и настройке, Страница: 35 / 44

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ChaPTer 4: moDBuS
Byte order
: Modbus registers that are multiple bytes in size can be
arranged in either Big Endian or Little Endian format. Big endian is some-
times referred to as network byte order and little endian is sometimes
referred to as Intel byte order. If Big-Endian is specified, the Sentinel Pro
will always interpret the first byte sent in a Modbus data packet as the most
significant byte and the last byte sent as the least significant byte. If Little-
Endian is specified, the first byte read in each register will be interpreted as
the least significant and the last byte read will be interpreted as the most
significant byte.
Register Order
: In cases where the size of the data type being read is great-
er than the register size, multiple registers must be read to form a complete
value. For example, if one wishes to read a floating point value at address
location 1 from a device with 16-bit modbus registers, the registers at
addresses 1 and 2 must be combined to form the complete value. The reg-
ister order specifies how these registers are ordered to create the final
value. If “Big Endian” is specified, the first register read (lowest address) by
the Sentinel Pro is interpreted as the highest order value and the last regis-
ter read (greatest address) is interpreted as the lowest order value. Little
endian specifies that the first register read contains the least significant
byte(s) and the last register contains the most significant byte(s).
Data Representation
: Values read from Modbus devices can be processed
by the Sentinel Pro using the following options:
Sign: Select “Signed” if the value you are reading uses 2’s compliment
notation to represent negative values. Select “Unsigned” if no sign data
included in the value.
Scaling Operand: (Integer values only) Use this option to have the
value read multiplied or divided by a specified factor. For example,
many Modbus devices used a fixed-point integer notation to represent
a value. Use Divisor operand to display the value with the decimal
point in the correct place.
Scaling Offset: This function allows for the read value to be offset by a
fixed amount. Note that the offset is applied after the scaling operation
(if applicable).
ConneCtIng Modbus rs-485 deVICes to your sentInel pro
RS-485 is a standard communication interface that consists of a differential pair of wires and an optional
ground lead. Multiple slave devices can be connected on the same pair of communication wires to form
a communication bus with a length up to 4,000 feet (1,200M). A maximum of 255 uniquely addressed
slave devices can be connected to the same bus. However, due to hardware limitations usually it is not
advisable to exceed more than 32 devices on the same network. This is not a hard limit and varies based
upon the load applied to the Sentinel Pro by the third party devices and how well the physical network is
set up. The Sentinel Pro itself presents a 1/8 load to the network.
When planning an RS-485 network, several design guidelines should be considered in order to ensure
an optimal signal between your Sentinel Pro and Modbus slave devices on the RS-485 bus. Following the
guidelines described below is especially important for long signal runs and/or with many devices con-
nected to the same bus.