NORD USS and Modbus RTU Manual
62 Subject to technical amendments BU 0050 GB-3111
2.3 Telegram Structure
The address field consists of eight Bits, which represent the address of the recipient. In its response, the Slave
returns this address to the Master, so that the master can identify the response. The function field consists of 8
Bits. These encode how the content of the data field is to be interpreted. If the Slave has received the query
from the Master correctly, it responds with the same function code. The structure of the data field is explained
in detail in the section "Function Codes". Finally, a 16Bit CRC checksum is transmitted.
2.4 RTU Frames
In RTU mode the start of transmission is not marked by control codes, but rather via a break in transmission at
least 3.5 characters long. The length of the transmission break therefore depends from the transfer speed. The
end of the message is also marked by a break in transmission which is at least 3.5 characters long.
2.5 Function Codes
Function codes specify the required action associated with the transmission of the telegram. The following
codes are supported:
Reading access to all IN & OUT bits on the the bus
Writing access to individual bus IN bits
Simultaneous writing access to all bus IN bits
Reading access to parameters
Writing access to an individual parameter (max 16Bit)
Writing access to 32Bit parameters or several PZD data
Data
Variable = N x 8 bits
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