User's Manual

Protocol Operation
Crescendo VHF Half-Duplex User Manual Page 38 of 74
6.6 Modbus RTU Protocol
When using the Modbus RTU protocol, the Crescendo expects Modbus RTU packets on the serial port in the
following format.
Start
T1-T2-T3-T4
Address
(1 byte)
CRC
(2 bytes)
Data
(0…251 bytes)
Function
(1 byte)
Figure 8: Datagram protocol packet format
The fields of a Modbus packet are:
Start: A silent interval of at least 3.5 character times.
Address (8-bit): The address of the slave unit in the Modbus transaction.
Function (8-bit): The Modbus function indicating the function to perform, or response to a function
request.
Data: Any additional information required for the action that was specified in the function field.
CRC: A 16-bit cyclic redundancy check over the address, function, and data using the polynomial
0xA001.
6.7 Distributed Network Protocol (DNP)
The Crescendo supports the Distributed Network Protocol (DNP) as defined by the DNP Users Group
(
http://www.dnp.org/). The packet structure for a DNP packet expected on the serial port is shown in Figure
9.
…
Start
0x0564
Destination
(16)
Source
(16)
CRC
(16)
Data
(0…16)
Len
(8)
Ctrl
(8)
CRC
(16)
Figure 9: DNP packet format
Start (16-bit): A fixed start of packet marker, always 0x0564.
Len (8-bit): The length of the packet, including all fields except Start, Length, and CRC.
Ctrl (8-bit): Defines the control information for the packet.
Destination (16-bit): The 16-bit destination address. This can be any singlecast address, or the
broadcast address.
Source (16-bit): The 16-bit source address. When sending a packet to a radio, this field does not
need to be set.
Data: The fixed length header block is followed by optional data blocks of up to 16 bytes.
CRC (16-bit): A 16-bit CRC value calculated over the previous fields, excluding the CRC fields.
The CRC uses the polynomial 0xA6BC.
Main -> Protocol -> Modbus
Main -> Protocol -> DNP
Main -> Protocol -> Datagram