Vertiv Liqui-tect LP6000, Installer/User Manual

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6 MODBUS COMMUNICATION PROTOCOL
This section describes the Modbus protocol supported by LP6000 for use when configuring
communication via Modbus network.
LP6000 communicates via the half-duplex EIA-485 serial-communication standard. Liqui-tect LP6000
may be configured as a master or slave device with multiple devices on a multi-drop network.
6.1 Transmission Modes
LP6000 supports only RTU mode of transmission (does not support ASCII) with 8 data bits, no parity, and
1 stop bit.
Each packet consists of the following four fields:
• Slave Address Field: 1-byte length. Identifies the slave device in the transaction. Set on EIA-485
Port/Modbus Configuration on page 48.
• Function Field: 1-byte length. Indicates the function to perform. Supported functions are 03
(Read 4xxxx output registers), 04 (Read 3xxxx input registers), 06 (Preset single register) and
16 (Preset multiple registers).
• Data Field: variable length. 16-bit registers, transmitted high-order-byte first (big-endian).
• Error Check (Checksum) Field: Identifies transmission errors. Uses a 16-bit cyclic redundancy
check (CRC-16).
6.1.1 Exception Responses
Exception responses are generated as a result of invalid commands from the Modbus master or an
attempt to read and invalid register. The high-order bit of the function code is set to 1. The data field
contains the exception error code, described in the following table.
CODE NAME DESCRIPTION
01 Illegal Function Function code is not supported.
02 Illegal Data Address Attempt to access an invalid address.
03 Illegal Data Value Attempt to set a variable to an invalid value.
Table 8.1 Exception Codes
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