Honeywell SMV800, User Manual

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Page 106 SMV800 Modbus User's Guide Revision 1.0
3.2.3. Baud Rate (Software Configurable)

Default baud rate: 9600

Available baud rate: 1200, 2400, 4800, 9600, 19200
Make sure the RS-485 network is only terminated twice on the entire bus. Best practice
would suggest this be done once on each end. Termination at multiple point on the bus will
hamper communication.
The format for both query and response frames is as follows:
For communication to specific SMV800 Modbus, the address field contains the slave’s
polling address. The function field contains a function code. Which indicates the read, write
or diagnostic command to be performed as part of query. When the SMV800 modbus
device responds to a query, the function field will either verify the device’s response
provide an exception that explains any errors encountered while processing the command.
Table 3-1 provides an overview of these response codes. When transmitters receive a
query, it will not respond until the command has been completed. No subsequent
commands will be processed until the first command is finish.
Table 3-1: Exception Response Codes
Exception
Response Description Explanation
01 Illegal function The received message function is not an
allowable action for the transmitter.
02 Illegal data address The address referenced in the data field is not an
allowable address for the memory location.
03 Illegal data value The value referenced in the data field is not
allowed in the addressed memory location.
04 Save device failure An unrecoverable error occurred while the slave
was attempting to perform the requested action.
06 Slave device is busy The slave is engaged in processing a long
duration command. The host should retransmit
the message later when the slave is free.
The data field contains information that is specific to each individual function.
The error check field contains a 16-bit CRC checksum that is used to verify the integrity of
the message frame.
Device Address Function Code
Data Bytes Error Check