User's Manual

Protocol Operation
Crescendo UHF Half-Duplex User Manual Page 39 of 78
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. Note that the broadcast
address 0 and addresses 248 to 255 are not supported.
 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. Note in the Modbus standard the CRC is in little endian format where the rest of the
Modbus packet is big endian.
6.6.1 Modbus Queries
When Modbus Query has been enabled and a Modbus Address has been configured in the protocol settings,
the Crescendo supports a limited set of Modbus queries for configuration and diagnostics. The following
Modbus function codes are supported:
 Read holding registers (0x03): This function is used to read 1-125 contiguous holding registers
from the device.
 Write holding registers (0x06): This function is used to write a single register in the device.
It should be noted that the radio address and Modbus Address are not the same configuration setting.
However, it is encouraged that the radio address and Modbus Address are set to be the same where ever
possible.
Packets received over the serial port or RF with a Modbus address that matches the radios configured
Modbus Address are processed and the response is returned over the same interface. Packets that have a
Modbus address that does not match the configured address are routed as normal.
For a Modbus register map explaining how Crescendo configuration and diagnostic parameters map to
Modbus registers please contact RF Innovations.
For a more detailed explanation of Modbus function codes, please refer to the Modbus Application Protocol
Specification, www.modbus.org, rev 1.1a.
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