Modbus Interface
Page 44
May 2020
DSY-PM-01378-EN-08
MODBUS INTERFACE
Communications
Protocol
Modbus RTU
Interface
RS485, 2-wire and ground
Data transmission
Half-duplex
Baud rates
9600 (default), 19200, 38400, 57600 and 115200
Word length
8-bits
Parity
None
Stop bits
1
Max� devices on
network
127
Address range
1…127
Cable
Shielded twisted pair with ground wire minimum 24 awg
Battery Life
9600 Baud
Up to 6 years with Modbus enabled and no loop power
All other Baud rates
Up to 1 year with Modbus enabled and no loop power
RS485 standards state that a daisy-chained topology is recommended with stubs being
as short as possible (much shorter than the main bus length)� Use a shielded twisted-pair
cable no less than 24 awg for connecting devices on a RS485 network�
The B2900 monitor is rated as a 1/8 unit load device (input impedance equal to 96 kΩ)� The
RS485 specification states it is capable of supporting 32 standard unit loads (1 standard unit
load equals 12 kΩ)� In order to determine the maximum number of devices on a network,
the user must identify the unit load rating of each device on the network�
The maximum common input voltage range of the B2900 monitor is –7…10V� This differs
from the RS485 standard of –7…12V� To make sure this range is achieved, the RS485 ground
connection must be tied together in a daisy-chained fashion� The shield of the cable used
should be tied to chassis or earth ground on only one end of the network� See
for an example configuration and description�
Use a termination resistor of 120 Ω at the end of the bus�
A subset of the standard Modbus commands is implemented to provide access into
the data and status of the B2900 monitor� The Modbus commands and their limitations
supported by the B2900 monitor can be found in
