Description - Chauvin Arnoux ULYS FLEX Modbus Notice De Fonctionnement

1. DESCRIPTION

MODBUS RTU/ASCII is a master-slave communication protocol, able to support up to 247 slaves connected in
a bus or a star network.
The protocol uses a simplex connection on a single line. In this way, the communication messages move on a
single line in two opposite directions.
Master-slave messages can be:
• Reading (Function code $03 / $04): the communication is between the master and a single slave. It allows
to read information about the queried instrument.
• Writing (Function code $10): the communication is between the master and a single slave. It allows to
change the instrument settings.
• Broadcast: the communication is between the master and all the connected slaves. It is always a write
command (Function code $10) requiring MODBUS address $00, and has no response by slaves. This
functionality can be used only with register $2040.
In a multi-point type connection (MODBUS RTU/ASCII), slave address (called also MODBUS address) allows
to identify each instrument during the communication. Each instrument is preset with a default slave address
(01) and the user can change it.
COMMUNICATION FRAME STRUCTURE
RTU mode:
Bit per byte: 1 Start, 8 Bit, None, 1 Stop (8N1)
Name
START FRAME
ADDRESS FIELD
FUNCTION CODE
DATA FIELD
ERROR CHECK
END FRAME
ASCII mode:
Bit per byte: 1 Start, 7 Bit, Even, 2 Stop (7E2)
Name
START FRAME
ADDRESS FIELD
FUNCTION CODE
DATA FIELD
ERROR CHECK
END FRAME
MODBUS RTU/ASCII
Length Function
4 chars idle At least 4 character time of silence (MARK condition)
8 bits Instrument MODBUS address
8 bits Function code ($03 / $04 / $10)
n x 8 bits Data + length will be filled depending on the message type
16 bits Error check (CRC)
4 chars idle At least 4 character time of silence between frames
Length Function
1 char Message start marker. Starts with colon ":" ($3A)
2 chars Instrument MODBUS address
2 chars Function code ($03 / $04 / $10)
n chars Data + length will be filled depending on the message type
2 chars Error check (LRC)
2 chars Carriage return - line feed (CRLF) pair ($0D & $0A)
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