BAS Integration Manual

© 2015 680_B - 10/15
5 of 8
A Watts Water Technologies Company
Communication Protocol Modbus over RS-485
Physical Layer RS-485 two-wire plus signal ground
Baud Rate 9600, 19200, 38400, 768000 (default: 9600 bps)
Recommended Cable 18 AWG shielded, twisted pair (STP)
Transmission Mode RTU or ASCII (default: RTU)
Maximum Cable Length 76800 bps: 580 ft. (177 m)
57600 bps: 1,158 ft. (353 m)
19200, 9600 bps: 3,280 ft. (1,000 m)
3,280 ft (1,000 m) for all baud rates if 120 Ohm terminating resistors used
Start Bit 1 bit
Data Length 8 bits for RTU mode
7 bits for ASCII mode
Parity None (2 stop bits)
Even (1 stop bit)
Odd (1 stop bit)
(default: Even)
Addressing 1 to 247
(default: 1)
Modbus Specification
Modbus Connection
Use a top or back knock-out to bring a shielded twisted pair wire from the nearest Modbus RS485 connection point into
the wiring chamber. Wire the A, B & G conductors from the Modbus point to the 680 A, B & Gnd terminals.
Gnd should not be connected to the ground screws in the 680.
Modbus Troubleshooting
If there is no communication, check the following:
• Check that the polarity on the Modbus A & B terminals
is correct.
• Check that Modbus GND terminal is securely
connected.
• Check that the baud rate is the same on both devices.
If the communication is intermittent, check the
following:
• Check that the communication cable is of the twisted
pair type.
• Reliable communication depends on the cable length
and baud rate used. Long cable length may require a
lower baud rate.