Control Keypad
77(96)
7
7.4.8 Modbus interface
NXL has a built-in Modbus RTU bus interface. The signal level of the interface is in accordance with
the RS-485 standard.
Protocol:
Modbus RTU
Baud rates:
300, 600, 1200, 2400, 4800,
9600, 19200, 38700, 57600 (bit/s)
Signal level:
RS-485 (TIA/EIA-485-A)
Input impedance:
2 k
Ω
7.4.8.1
Modbus RTU protocol
Modbus RTU protocol is a simple but effective fieldbus protocol. Modbus network has a bus
topology, where every device has an individual address. With the help of the individual bus
addresses the commands are directed to the single devices within the network. Modbus supports
also broadcast -type messages, which are received by every device of the bus. Broadcast messages
are sent to the address ’0’ which is reserved for these messages.
The protocol includes CRC error detection and parity check for preventing the handling of messages
containing errors. In Modbus the data is transferred in hex mode asynchronically and a break of
approximately 3.5 characters is used as an end character. The length of the break depends on the
used baud rate.
Function
code
Function name
Address
Broadcast
messages
03
Read Holding Register
All ID numbers
No
04
Read Input Register
All ID numbers
No
06
Preset Single Register
All ID numbers
Yes
16
Preset Multiple Register
All ID numbers
Yes
Table 7-12. Modbus commands supported by NXL
RS-485 Transceiver
Control terminal
RS-485 A
RS-485 B
Terminal resistor 120
Ω
X7
-> MF2-MF3
X9
-> MF4-MF6
