Control functions
7.8 Communicating with the PLC
SINAMICS V90, SIMOTICS S-1FL6
266
Operating Instructions, 04/2019, A5E36037884-007
7.8.2
Modbus communication
7.8.2.1
Overview
The SINAMICS V90 servo drive can communicate with the PLC through an RS485 cable by
using the standard Modbus communication protocol. There are two ways of sending a
message to a slave.
●
Unicast mode (addresses 1 to 31): The master sends a message to a sepcific slave
directly.
●
Broadcast mode (address 0): The master sends a message to all slaves at the same time.
The broadcast mode cannot be used for an error request because 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
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Byte 7
Byte 8
Address
FC (0x03) Start address
Number of registers
CRC
High
Low
High
Low
High
Low
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
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Byte 7
Byte 8
Address
FC (0x06) Start address
New register value
CRC
High
Low
High
Low
High
Low
