Modbus Communication - Siemens SINAMICS V90 Operating Manual

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4.6.2

Modbus communication

The SINAMICS V90 servo drive can communicate with the PLC through an RS485 cable
with the standard Modbus communication protocol. There are two ways of sending a
message to a slave. One is unicast mode (addresses 1 to 31), where the master addresses
the slave directly; the other is broadcast mode (address 0), where the master addresses all
slaves. The broadcast mode cannot be used for an error request since all slaves cannot
respond at once.
For the Modbus data format, V90 supports Modbus RTU while Modbus ASCII is not
supported. Registers of the servo drive can be read by Modbus function code FC3 and
written via Modbus function code FC6 (single register) or FC16 (multiple registers).
Supported function codes
The SINAMICS V90 supports only three function codes. If a request with an unknown
function code is received, an error message is returned.
FC3 - Read holding registers
When a message with FC = 0x03 is received, then four bytes of data are expected, that is,
FC3 has four bytes of data:
● Two bytes for the starting address
● Two bytes for the number of registers
Byte 1
Address
FC6 - Write single register
When a message with FC = 0x06 is received, then four bytes of data are expected, that is,
FC6 has four bytes of data:
● Two bytes for the register address
● Two bytes for the register value
Byte 1
Address
FC16 - Write multiple registers
When a message with FC = 0x10 is received, then 5 + N bytes of data are expected, that is,
FC16 has 5 + N bytes of data:
● Two bytes for the starting address
● Two bytes for the number of registers
134
Byte 2 Byte 3
FC (0x03) Start address
High
Byte 2 Byte 3
FC (0x06) Start address
High
Byte 4 Byte 5
Number of registers
Low High
Byte 4 Byte 5
New register value
Low High
Operating Manual, 04/2017, A5E36618042-003
Byte 6 Byte 7
CRC
Low High
Byte 6 Byte 7
CRC
Low High
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Byte 8
Low
Byte 8
Low