Scaime eNod4-B - Modbus RTU; 3.1 Physical Interfaces; 3.3 Modbus RTU Supported Functions; 3.4 Frames Structure

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User manual SCAIME: NU-soft-eNod4B-E-0716_236704-C.doc
3 MODBUS RTU
3.1 Physical interfaces
Modbus RTU communication protocol can be used either through eNod4 USB port, AUX port. Modbus RTU or Profibus
only depending on version on DB9 connection.
USB port behaves as a full duplex interface whereas the DB9 and AUX ports support half-duplex RS485
communication. Supported baud rates are 9600, 19200, 38400, 57600, and 115200.
For a complete description of the recommendations about eNod4 RS485 connection, please refer to the user manual
“characteristics and functioning” of the eNod4.
Note: using eNod4 through USB requires installing first the necessary USB drivers available on the website
http://www.scaime.com.
3.2 Byte format
Data transmitted to eNod4 thanks to Modbus RTU communication protocol must respect following format:
• 1 start bit
• 8 data bits
• no parity
• 2 stop bits
Every Modbus RTU frame is ended by a CRC-16 2-bytes code whose polynomial generator is
(cf. CRC-16 calculation algorithm).
3.3 Modbus RTU supported functions
As a Modbus RTU slave, eNod4 supports following Modbus RTU functions:
Function
Code
read N registers*
03
H
/ 04
H
write 1 register*
06
H
write N registers*
10
H
* 1 register = 2 bytes, maximum admitted value for N is 30.
Note: Broadcast addressing is not allowed by eNod4.
3.4 Frames structure
During a read or write transaction, the two bytes of a register are transmitted MSB first then LSB.
If a data is coded on 4 bytes (that means it requires two registers), the two LSB are stored in the low address
register and the two MSB are stored in the high address register.
3.4.1 Function (03H/04H) – read N input registers (N = 30 max)
• request command sent to the slave :
slave address
03
H
or 04
H
starting register
offset
N registers
CRC16
1 byte
1 byte
2 bytes
2 bytes
2 bytes
• slave response :
G(x) = x
16
+ x
15
+ x
2
+ 1