Modbus Communication - Siemens SINAMICS V20 Operating Instructions Manual

Communicating with the PLC

7.2 MODBUS communication

7.2 MODBUS communication
Overview
In MODBUS, only the master can start a communication and the slave will answer it. There
are two ways of sending a message to a slave. One is unicast mode (address 1 to 247),
where the master addresses the slave directly; the other is broadcast mode (address 0),
where the master addresses all slaves.
When a slave has received a message, which was addressed at it, the Function Code tells it
what to do. For the task defined by the Function Code, the slave may receive some data.
And for error checking a CRC code is also included.
After receiving and processing a unicast message, the MODBUS slave will send a reply, but
only if no error was detected in the received message. If a processing error occurs, the slave
will reply with an error message. The following fixed framing characters in a message cannot
be altered: 8 data bits, 1 parity bit, and 1 or 2 stop bits.
Supported Function Codes
The SINAMICS V20 supports only three Function Codes. If a request with an unknown
Function Code is received, an error message will be returned.
FC3 - Read Holding Registers
When a message with FC = 0x03 is received, then 4 bytes of data are expected, that is, FC3
has 4 bytes of data:
● 2 bytes for the starting address of register
● 2 bytes for the number of registers
Byte 1 Byte 2
Address FC (0x03)
Inverter response
Byte 1 Byte 2
Address FC (0x03)
170
Byte 3 Byte 4
Start address
High Low
Byte 3 Byte 4 Byte 5
Number Register 1 value
of bytes
High Low
Byte 5 Byte 6
Number of registers
High Low
... Byte N*2 - Byte N*2
1
... Register N value
High Low
Operating Instructions, 11/2016, A5E34559884-006
Byte 7 Byte 8
CRC
High Low
Byte N*2 + Byte N*2 +
1 2
CRC
High Low
SINAMICS V20 Inverter