3 Modbus protocol description
16
Modbus-RTU/ASCII via TCP/IP
With Modbus-RTU/ASCII via TCP/IP the MBAP header is omitted, the entire Modbus telegram (with
CRC) is transmitted via TCP.
3.5 Device address (RS485)
The device address can be set to between 1
DEC
and 254
DEC
. Each Modbus station must have a unique
device address.
The following data exchange variants are available for accessing the connected stations:
Query
This is a data request/instruction from the master to a slave via the corresponding device address
(1 to 254). The addressed slave responds.
Broadcast
The broadcast is an instruction from the master to all slaves via the device address 0 (for example, to
transmit a certain value to all slaves).
The connected slaves do not respond. In such a case, the correct acceptance of the value by the slaves
should be checked by a subsequent readout at each individual slave. A data request using the device
address 0 is meaningless.
NOTE!
A maximum of 31 slaves can be accessed via the RS485 interface.
The device address 0 is reserved as a Modbus broadcast address:
an instruction from the master to address 0 is executed by all slaves, however, none of the slaves re-
spond (since it would otherwise lead to a data collision).
The address is given in binary format in the transmission protocol.
3.6 Function codes
Function overview
The functions described in the following are available for the readout of measured values, device data,
and process data, as well as for writing specific data.
NOTE!
A hexadecimal number is marked by a preceding "0x".
Example: 0x0010 (= 16
DEC
)
NOTE!
If the device does not respond to these functions or generates an error code, these can be evaluated.
chapter 3.9 "Error messages", Page 23
Function code Function Limit
0x03 or 0x04 Reading n words Max. 127 words (254 bytes)
0x06 Writing one word Max. 1 word (2 bytes)
0x10 Writing n words Max. 127 words (254 bytes)
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