Honeywell HC900 User Manual page 14

Introduction
Modbus RTU RS232/RS485 Communication Ports
General Modbus RTU message format
Query message format
[Slave Address, Function Code, Function code dependent data, CRC 16]
Response message format
[Slave Address, Function Code*, Function code dependent data, CRC 16]
* If an error is detected in a valid message the response function code is modified by adding 80 (hex) and the
function code dependent data is replaced by an exception response code as described in 5.
Exception Codes
Between messages, the RS-485 link is in a high impedance state. During this time receiving devices are
more susceptible to noise generated false start of messages. Although noise-generated messages are rejected
due to address, framing, and CRC checking, they can cause the loss of a good message when they are
included in the message stream. In the slave the transmitting device enables its transmitter line diver and
forces an idle line state onto the link for three character time slots prior to transmitting. This forces
termination of any noise generated messages and improves message frame synchronization.
Modbus RTU Data Layer
The data layer includes:
 Diagnostic loopback,
 Function code recognition / rejection,
 Busy / repoll,
 Data error code generation
Errors detected by the data layer are rejected and the slave responds to the polling device with a Modbus-
type status exception error. A summary of the Modbus status exception codes is listed in Section 5. Modbus
RTU Exception Codes.
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HC900 Process Controller Communications User Guide
Modbus RTU
Revision 13
April 2017