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Modbus Communications

1.5.2 Function Code

The function code field tells the addressed slaves what function to perform. Function codes are

designed to invoke a specific action by the slave device. The function code ranges from 1 to 127.

Liebert IntelliSlot 485 Modbus server supports the following Modbus function codes.

Table 3 Supported Modbus function codes

Code

Function

Description

01 Read Coils

Read from 1 to 2000 contiguous status of coils managed by the server. Coils

in the response message are packed as one per bit of a byte, 1=On and

0=Off. If the requested quantity of coils is not a multiple of 8, zeros are padded

in the final byte.

02 Read Discrete Inputs

Read from 1 to 2000 contiguous input status managed by the server. Discrete

inputs in the response message are packed as one per bit of a byte, 1=On

and 0=Off. If the requested number of inputs is not a multiple of 8, zeros are

padded in the final byte.

03 Read Holding Registers

Read the contents of contiguous block of 1 to 127 holding registers. Data are

packed as two bytes per register; the first byte contains the high order bits.

04 Read Input Registers

Read the contents of contiguous block of 1 to 127 Input registers. Data are

packed as two bytes per register; the first byte contains the high order bits.

Write Single Coil

Write a single output to either On (1) or Off (0) mapped in coil section.

06 Write Single Register Write a value into a single holding register;

Write Multiple Coils

Force each coil in a sequence of coils to either On or Off.

Write Multiple Registers

Write values into a block of contiguous registers (1 to 120)

1.5.3 Data Fields

The data field length varies, depending on whether the message is a request or a response to a

packet. This field typically contains information required by the slave device to perform the

command specified or to the response to a data request from the master device.

1.5.4 Error Check Field

The Error Check Field consists of a 16-bit (2 byte) Cyclical Redundancy Check (CRC16). It allows the

receiving device to detect a packet that has been corrupted by transmission errors.

1 2 3 4 5 6 7 8 9 10 11 12 ... 99 100

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Emerson Liebert FPC Manual de usuario Pagina 7