Honeywell NXL - Manual (Page 77)

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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 kΩ
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 120Ω
X7 -> MF2-MF3
X9 -> MF4-MF6