Modbus Communication - Siemens SINAMICS V20 Series Operating Instructions Manual

Parameter Function
r2024[0] USS / MODBUS error statistics
...
r2031[0]
r2018[0...7] CO: PZD from USS / MODBUS on
RS485
P2019[0...7] CI: PZD to USS / MODBUS on RS485
6.2

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 ... 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 can
not be altered: 8 data bits, 1 parity-even bit and 1 stop bit.
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 = 0 x 03 is received, then 4 bytes of data are expected, that is,
FC3 has 4 bytes of data:
● 2 bytes for the starting address
● 2 bytes for the number of registers
Byte 1 Byte 2
Address FC (0 x 03)
SINAMICS V20 Inverter
Operating Instructions, 07/2012, A5E03728167
Byte 3 Byte 4
Start address Start address
(MSF) (LSF)
Setting
The state of the telegram information on RS485 is reported
regardless of the protocol set in P2023.
Displays process data received via USS / MODBUS on
RS485.
Displays process data transmitted via USS / MODBUS on
RS485.
Byte 5 Byte 6
Number of Number of
registers (MSF) registers (LSF)
Communicating with the PLC
6.2 MODBUS communication
Byte 7 Byte 8
CRC CRC
109